Author: nwjunker

Greater San Francisco cold season multiday (72 hr) rainfall events

This website has been constructed to provide the synoptic patterns and normalized anomaly patterns of geopotential heights, 850 temperatures, precipitable water and 850-hPa moisture flux for the 20 greatest 72-hour rainfall events for the greater San Francisco and Sacramento areas since 1949 based on the CDC .25 x .25 degree unified precipitation data set. A summary of the event, it’s synoptic pattern, and selected normalized anomalies are provided for each event. The idea behind the site is that most of the extreme rainfall events in northern and central California are associated with strong atmospheric rivers or moisture plumes. Rare events are not that common and the normalized anomalies can provide a rough estimate how often a similar moisture plume would typically occur which might provide a forecaster with some idea of the rarity of the event. A forecaster or user looking at model forecasts of the synoptic pattern and associated normalized anomalies associated with a potential event could then compare model forecasts or ensemble mean model forecasts of anomalies with the historical major events and the patterns associated with them. Model forecasts with normalized anomalies are available at http://www.hpc.noaa.gov/training/SDs/. Farther down the road, the hope is to developing a method for identifying possible analogs.

Link to the introduction for The top 20 Greater San Francisco and Sacramento California cold season :
This link provides the rational for using normalized anomalies and discusses how they might be used in the forecast process. It is meant as an overview of the patterns associated with major California rainfall events.

Click on each link for the discussion of the events and to view the synoptic and normalized anomaly patterns.

1) December 1964: This link provides a discussion of the event. It was the top ranked event and produced a major flood in northern California with an estimated return period of over 50 years.

2) Jan-Feb 1963: Second ranked event, one that produced major flooding across the Carson, Feather American and Cosumnes River Basins.

3) December 1955: A major event, produced a greater than 50 year flood event.

4) Dec 2002: The 3-day period ending at 1200UTC 16 December ranked fourth on our list which may reflect somewhat that the data coverage in 2002 was better than during some of the earlier events.

5) New Years 2006: Ranked number 5 which may be misleading because of improved Quantitative precipitation. This was a major event with significant flooding, the heaviest since 1997.

6) February 1986: Ranked 6th on the list an was a major event that produced a record flood on the American River Basin and major flooding along other basins nearby. The return frequency for the 10-day rainfall that encompasses this event has been estimated at 1000 years

7) January 1974: ranked 7th, most of the rain fell north of San Francisco. Significant flooding occurred along California’s North Coast.

8) March 1995: major flooding along the Pajaro River Basin in Santa Cruz and Monterey counties. These areas were declared Federal Disaster areas.

9) January 1995: a major event, 42 counties in California were included in the Federal Disaster Declaration due to this extended rainfall event.

10) November 1950: This event and another in early December led to significant flooding in Yuba and Tulare Counties and led to flooding of the Tule River. Roos has listed it as one of the nine big floods of note for California (2007).

11) January 1966 event: The bulk of the rainfall also fell well north of San Francisco. The Modesto-Bee reported flooding near Eureka that sent 1000 people fleeing from their homes. This was associated with the weakest atmospheric river of the 20 events.

12) January 1969 event: The flooding was included in the Roos (2007) listing of major California flooding events. Significant flooding occurred over the Central Valley rivers, the reformation of Tulares Lake in San Joaquin Valley.

13) December 1981: The Syracuse Herald-American had a story from the morning of December 20th stating that the stormkilled 3 people, raised 4 rivers to flood stage and produced mud slides. However, compared to many of the events listed, the flooding during this case was relatively minor.

14) November 1973: No reports of serious flooding could be found from this 3-day rainfall event despite its ranking within the top 20 events within the data set.

15) Feb 1960: No reports of serious flooding could be found from this 3-day rainfall event despite its ranking in the top 20 events within the data set. This suggests that antecedent conditions are an important player in determining whether major flooding will occur.

16) Jan 1970: This event was a rather minor one in terms of significant flooding. The only reports of flooding that could be found on the Internet were a report of flooding on Cache Creek of the Copay valley and an allusion to two boys being missing due to flooding Jan. 26 issue of the European Stars and Stripes.

17) New Years 1997: This event probably should rank higher, the three days totals implied by the HPC 24-hr precipitation analyses were higher than for the 1995 events and the amounts were more impressive than any other in a 10-year period ending in 2000. Major flooding occurred with the flood areas being declared a Federal Disaster. Flooding associated with the event may have rivaled the legendary flood of 1862 (Roos 1997).

18) December 1995: The weather prior to the event had been dry which minimized the run off problems (Reynolds 1996). As noted by Reynolds, synoptically the pattern was very similar to the pattern that was present during the January and March 1995 events.

19) February 2000: December 1999 was the third driest December on record to south central California which probably accounts for the lack of major flooding. Minor flooding was reported

20) March 1974: . An Army Corps of Engineer report was written that included flooding in March along the Russian River in northern California. This was not a major flood event.

21) January 1980: This event helped set stages for a later more significant flood event in February in southern California. The synoptic pattern for this later SoCal event is shown.

References

Acknowledgements: This web page could not have been constructed without help from Rich Grumm and Mike Bodner.

Greater San Francisco and Sacramento California cold season
multiday extreme rainfall events

3-day rainfall events have been ranked based on the magnitude of the maximum rainfall at any grid point in the area enclosed by the red box on Figure 1. The precipitation data used for ranking was the CDC .25 x .25 degree longitude U.S. grid. According to the CDC documentation of the data, the dataset was derived from 3 sources: “NOAA's National Climate Data Center (NCDC) daily co-op stations (1948-...), CPC dataset (River Forecast Centers data + 1st order stations - 1992-...), and daily accumulations from hourly precipitation dataset (1948-...). Nationally, there are about 13000 station reports each day for 1992-..., and about 8000 reports before that yielding about 3x the reports of any existing historic and operational analyses as of 2000. The data were QC-ed to eliminate duplicates and overlapping stations, and standard deviation and buddy checks were applied..”
The area was chosen to investigate was based on trying to identify the rainfall events that would, if they occurred today, have the biggest impact on the high population areas around San Francisco and Sacramento. The area also was chosen because it includes the area being studied during the Hydrometeorological Test Bed West field experiment.

The ranking identifies the most significant cool-season rainfall events and major floods since 1948 but the ranking does not always coincide with which events produced the greatest run-off. Also, the resolution of the data is coarse which sometimes results in the analysis missing significant small scale maxima so the precipitation maximum listed for each event probably underplays the maximum precipitation during most events. Three-day total precipitation analyses are provided along with selected 24-hour daily precipitation during the heaviest period.

For the more recent years where other data is available, the higher resolution analyses are provided for the 24 hour periods of heaviest rainfall. Occasionally, the differences can be marked. Users are encouraged to compare the other high and low resolution analyses to get a feel for the typical differences between the two. Note the differences in the precipitation observed north of San Francisco between the two image below

24-hour .25 x .25 precipitation analysis ending 1200 UCT 09 Nov. 2002 (in mm)

For each event one or two 4-panel charts will be shown. For those looking for additional of these 4-panel charts see http://www.hpc.ncep.noaa.gov/ncepreanal/

The smaller scale maps are also included that show the precipitable water (PW in mm) and its normalized anomaly (see figure above) and display the 850-hPa wind and the normalized anomaly of the 850-hPa moisture flux (MF). These maps are meant to show the anomalous nature of the atmospheric rivers/moisture plumes associated with these events. The also can be compared to forecast normalized PW and moisture flux anomalies for storms being forecast to impact the area.

The normalized departure of any meteorological variable can be defined by
N=(X-m)/s
where X is the value of the variable (e.g., 500-hPa height, u- and v-component of the 850-hPa wind, or moisture flux, etc), m is the daily mean value (based on 21 day running mean) for that grid point, and s is the standard deviation (based on the 21-day running mean) for each grid point.
The beauty of using of normalized anomaly is that it provides you with an estimation about how frequently a similar value of the whatever parameter you are looking at is observed. The higher the normalized anomaly the rarer the event is likely to be. Several studies have documented that normalized anomalies provide a way to objectively rank synoptic events (Hart and Grumm 2001) and can help identify significant weather events (Grumm and Hart 2001, Stuart and Grumm 2006, Junker et al. (in press). The two figures below show the return frequency associated with normalized 850-hPa MF (left) and PW (right) anomalies of various magnitudes for a point just off the California coast near San Franciso. Note that a Normalized PW anomaly of 4s has a return frequency of around 24 months but that the rainy season only lasts 6 months which means that the effective return frequency for a similar moisture plume is more like 4 years rather than the two shown on the figure (below left).

Expected return frequency as a function of 850-hPa MF at a point just upstream from the Sierra Nevada mountains. X-axis is the magnitude of the normalized anomaly, the y-axis is the return frequency in months.

Expected return frequency as a function of PW at a point just upstream from the Sierra Nevada mountains. X-axis is the magnitude of the normalized anomaly, the y-axis is the return frequency in months. The dotted lines highlight the return frequency of a normalized anomaly with a magnitude of 4 and shows the expected return period of 24 months.

Normalized anomaly fields of 850-hPa MF and PW are attractive fields to use when trying to anticipate extreme precipitation events where terrain plays a significant role in providing lift. Ralph et al. studied 7 floods on the Russian River of California and documented that all were associated with atmospheric rivers which contained anomalously high PW and MF. The importance of the pineapple express or atmospheric rivers to the moisture transport associated with heavy orographic rainfall events along the west coast has been documented in a number of studies (Lackmann and Gyakum 1999, Ralph et al 2005, Neiman et al. 2007, Junker et al. 2007). A discussion of the pattern associated with northern California heavy rainfall events is offered here. Scroll down on the page until you get to the section entitled “ Using climatological anomalies as a forecast tool along the west coast.” An example of how you might use normalized anomalies as a forecast tool is also offered in the same training module.
The Neiman et al. and Junker et al. studies found that the strength of the moisture plume that impinges on the coastal or Sierra Nevada ranges of northern California modulates the maximum amount of precipitation that is observed. The Junker et al. study also noted the similarities in the patterns of 500-hPa height, 850-hPa moisture flux and precipitable water and their anomaly fields for 3 extreme multi-day rainfall events.
A number of investigators have noted that forecasters often rely on pattern recognition and three– or four-dimensional conceptual models
when attempting to interpret output from numerical models as they are limited in the amount of data that they can assimilate (Morrs and Ralph 2007, Stuart et al 2007,Doswell 1992 and 2004). This heuristic approach and knowledge of the model forecast biases are an integral part of the quantitative forecast process (Funk 1991).
The Second Panel on the Future role of the Human in the Forecast Process emphasized the need to identify the important forecast problem of the day and noted that forecasters needed to be able to indentify anomalous weather Situations (Stuart 2007). Normalized anomaly fields along with pattern recognition and knowledge of model biases have the potential to help identify relatively rare events.

24-hr CDC 12.2 km RFC precipitation analysis ending 1200 UTC 09 Dec. 2002 (in mm)

The top left-panel of these charts depicts the 250-hPa heights and isotach analysis, the top right-panel displays the 500-hPa height analysis and normalized 500-hPa height anomaly field, the left bottom-panel provides the 850-hPa height analysis and the normalized anomaly of the 850-hPa temperature and the bottom right-panel shows the 1000-hPa height analysis and the normalized precipitation analysis. The large area maps are provided so forecasters can the synoptic pattern associated with the extreme precipitation event with the pattern being forecast by the operational models or ensemble forecast system. In addition to the large area maps that are posted for each event, selected smaller area anomaly maps are posted, usually of the normalized PW and 850 moisture flux anomalies. For the selected times, additional anomaly maps are also available for selected times at http://eyewall.met.psu.edu/rich/CaliforniaRain/images/. Eventually, we hope to develop a method for objectively identifying the closest matches to anomaly patterns so forecasters can look at them to see how closely they match the current operational forecast fields.

December 1964 extreme rainfall event

The 1200 UTC 20 Dec-1200 UTC 23 December 1964 ranked had the highest ranking for a 3 day period with a maximum grid point value of 432.15 mm. Its maximum rainfall remained primarily to the north of San Francisco. However, the associated flooding ranked in the top 10 in the 1949-1998 period with damages of 1.13 billion dollars (in 1998 dollars). The event produced significant flooding along the North Coast of California with 34 California counties being declared disaster areas.
The event also contributed to significant flooding in Idaho where the Payette, Big Wood, Little Wood, Portneuf, Clearwater and Boise River drainages. Over 100 houses were damaged and thousands of acres of farmlands were flooding. The flooding on the Payette was the largest ever recorded. The estimated cost of the flooding was 21 million dollars

72-hr CDC QPE (mm) from 1200UTC 20 Dec. to 1200 UTC 23 Dec. 1964

Precipitation analysis of the 24 hours (in inches) ending at 1200 UTC 21 Dec 1964 from the .25 by 2.5 deg. unified data set.

Precipitation analysis of the 24 hours (in inches) ending at 1200 UTC 22 Dec 1964 from the .25 by 2.5 deg. unified data set.

Precipitation analysis of the 24 hours (in inches) ending at 1200 UTC 23 Dec 1964 from the .25 by 2.5 deg. unified data set.

The map above is based on (Paulson et al, U.S. Geological Survey Water-Supply Paper 2375, 591 ) and shows the areas in California were flooding was located along with an estimate of the recurrence interval of a flood of that magnitude.

The synoptic pattern associated with this particular event is a blueprint for the majority of the other ranked events. Most 500-hPa analysis during these events had a high latitude positive anomaly in the vicinity of Alaska and the Bering Strait, a negative height anomaly to its south and then a another positive anomaly to the south of the negative (see top right panel of the figure below). The juxtaposition of the negative anomaly with the southern positive anomaly provides a strong geopotential height gradient suggesting strong geostrophic winds. Above normal 850 temperatures were also present which led to high snow levels.

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 0000 UTC 22 Dec. 1964. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

Another common trait was a strong low-level pressure or geopotential height gradient that suggest strong west-southwesterly or southwesterly geostrophic winds. Note the strong gradient at 1000-hPa and 850-hPa on the figure above and the strong 850-hPa winds on the figure below. Other commonalities were the anomalously high PW and 850-hPa moisture flux. The maximum precipitable water (PW) within the atmospheric river was over 30 mm which classifies to river as a strong one (Neiman et al 2007). During the 1964 case the normalized anomaly for PW exceeded 3s and the moisture flux anomaly exceeded 5s (see figures below). The normalized MF anomaly of over 5 suggests a return frequency of at least 18 months (about 3 years) suggesting that the atmospheric river of such strength is a relatively rare occurrence. The extended period of such anomalous MF would probably be even rarer. It’s also important to note that anomalies of greater than 6s are not depicted but might still be present within the area of greater than 5s departures.

PW (mm) and normalized PW anomaly (magnitude is given by the color fill from the bar at the bottom of the figure) valid 1800 UTC 21 Dec 1964 Top) and 1200 UTC 22 December 1964 (bottom)

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 1800 UTC 21 Dec. 1964 Top) and 1200 UTC 22 December 1964 (bottom)

Note that the axis of strongest moisture flux also seems to relate to the area of heaviest rainfall and the area where the return frequency for flooding was considered a greater than 50 year event.

December 1955 Flood

Like most of the heavy rainfall events in California, this event was a multi-day event and ranked second on the list. The heaviest 3-day period was from 1200 UTC 21 December-1200 UTC 24 December. However, heavy rainfall occurred on the 20th over northern California with 15.33 inches reported in Shasta County. Near record flows were reported for central valley California Rivers. The flooding resulted on a dam-break on the Feather river near Yuba City which flooded 100,000 acres. Flooding killed 74, 34 of which died along the Feather River (Roos, 2007, Null website). Intense flooding also occurred along the Carson River Basin in Nevada with the flooding comparable to that of the 1997 flood. This event also produced major flooding in Oregon.

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 24 Dec 1955.

The map above right is based on (Paulson et al, U.S. Geological Survey Water-Supply Paper 2375, 591 ) and shows the areas in California were flooding was located along with an estimate of the recurrence interval of a flood of that magnitude.

Precipitation analysis of the 24 hours (in inches) ending at 1200 UTC 22 Dec 1955 from the .25 by 2.5 deg. unified data set.

Precipitation analysis of the 24 hours (in inches) ending at 1200 UTC 23 Dec 1955 from the .25 by 2.5 deg. unified data set.

Precipitation analysis of the 24 hours (in inches) ending at 1200 UTC 23 Dec 1955 from the .25 by 2.5 deg. unified data set.

Picture of the 1955 flood from Roos (2007)

One thing to note is how similar the Dec. 1955 pattern is to the 1964 extreme event pattern (compare 500-hPa anomaly pattern below with the one here). At 500-hPa, both have anomalous high heights in the vicinity of Alaska and the Bering Strait and the positive tilting negative anomaly to its south and east. This negative coupled with a second positive height anomaly to its south ensures a long westerly to west-southwesterly fetch. Also, like the 1964 event, this event was associated with a strong atmospheric river that contained anomalously high PWS and moisture flux.

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 0000 UTC 22 Dec 1955. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

During the single heaviest day of rainfall over central California, the normalized PW anomaly exceeded 4 standard deviations (see below) and the normalized 850-hPa moisture flux anomaly exceeded 5. The pattern of anomalies was signaling a major rainfall event. The magnitude of the PW anomaly suggests a return frequency of at least 24 months. Since the major rainfall events all occurred during the 6-month cool season, the 24 months probably translates to about a 4-year return frequency. Both axis of high moisture flux and PW remains over the same area for an extended period which raised the systems rainfall potential and probably made it even more anomalous than the implied by simply looking return period on the return period figures shown here.

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 0000 UTC 22 December 1955 (top) and 1200 UTC 22 December 1955 (bottom).

850-hPa winds and normalized moisture flux anomaly at 0000 UTC 22 December 1955 (top) and 1200 UTC 22 December 1955 (bottom). (magnitude of the anomaly scale is shown on the scale at the bottom of the figure)

Feb 1986 event

The February 1986 event had the 5th highest 72-hour rainfall but it’s ranking is probably misleading. It had sustained rainfall during a protracted period that extended for longer than 72 hours. Note that it had two significant 72 hour rainfall periods back to back which produced in excess of 450mm in the vicinity of the Feather, American and Cosumnes River Basins and it produced the largest flood volume of any flood during the period between 1950 to 2007 on the American River Basin and the second largest on both the Feather and Cosumnes basins (Roos 2007). A return period of 1000 years was estimated for the 10-day precipitation totals in the central Sierra range. The flooding and levee breaks led to 1.5 billions of dollars in property damage (http://water.ca.gov/levees/history/floods.cfm).
The heaviest rainfall occurred on February 17th when 17.6 inches of rain was reported at Four Trees on the Feather River Basin. 29 inches of rain were reported at Calistoga and Blue Canyon, respectively, in a 10-day period. 13 people were killed due to the flooding. The extreme rainfall was not limited to California. Carson City received over 10 inches or rain during the 10-day period which is about 83 percent of the yearly normal. Flood waters virtually surrounded Reno. The flooding was declared a Federal Disaster for 5 Nevada counties and cause 17 millions of dollars in damages. Flooding from heavy rains also occurred in Box Elder County, Utah.

Heaviest 3-day rainfall analysis (mm) using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 17 Feb. 1986

Heaviest 3-day rainfall analysis (mm) using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 20 Feb. 1986

Picture of the Linda levee break of 1986 (from Roos, 2007)

Precipitation analysis for the 24 hours (in inches) ending at 1200 UTC 17 Feb.

The map above is based on (Paulson et al, U.S. Geological Survey Water-Supply Paper 2375, 591 ) and shows the areas in California were flooding was located along with an estimate of the recurrence interval of a flood of that magnitude.

The synoptic pattern was cut from the same mold as most of the other events. A block with anomalously high 500-hPa heights was present over Alaska and the Bering Sea. A negative height anomaly was located to its south. This negative anomaly and another positive height anomaly to its southeast helped maintain a long southwesterly fetch which tapped the subtropics and helped pull a strong atmospheric river northeastward into California.

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 0000 UTC 17 Feb. 1986 (top panel), 1200 UTC 17 Feb. 1986 (middle panel) and 1200 UTC 19 Feb 1986 (bottom panel).

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 0000 UTC 17 Feb. 1963 (top panel), 1200 UTC 17 Feb. 1963 (bottom panel).

An anomalously strong atmospheric river extended across California for an extended period of time. Normalized PW anomalies grew to above 3 over CA by 1200 UTC 17 Nov and remained over 3 through 1200 UTC 19 Nov. Normalized MF were above 3 from 0000 UCT 17 Feb through 1200 UTC 19 Feb and with the values exceeded 4 during most of the period. The slow movement of the moisture plume led to two consecutive 3 day periods of heavy rainfall.

Jan-Feb 1963 event

The late January-early February 1963 rainfall event produced the third largest gridpoint total. The event produced severe flooding along the Carson River Basin with a number of gage reports suggesting the flooding was 25-50 year event (FEMA). The event ranked as one of the 9 biggest floods since 1909 on the the American, Feather and Cosumnes River Basins(Roos 2007).
The strong atmospheric river associated with the California flooding also helped produce severe flooding along the Clearwater, Boise, Payette, Weiser, Portnuef and Snake River basins. Eight Idaho counties were included in a federal disaster declaration.

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 29 Jan 1963-1200 UTC 01 Feb 1963.

Precipitation analysis during the 24 hours (in inches) ending at 1200 UTC 30 Jan 1963 from the .25 by 2.5 deg. unified data set.

Precipitation analysis during the 24 hours (in inches) ending at 1200 UTC 31 Jan 1963 from the .25 by 2.5 deg. unified data set.

Precipitation analysis during the 24 hours (in inches) ending at 1200 UTC 01 Feb 1963 from the .25 by 2.5 deg. unified data set.

The synoptic pattern is again very similar to the pattern of three preceding events. Each has strong positive anomaly in the vicinity of Alaska and the Bering Sea at 500-hPa with a negative height anomaly to its south with a long southwesterly fetch (see below). The combination produces a strong geopotential height gradient across northern California. A anomalous plume of moisture also extends from the sub-tropics northeastward into California indicating a strong atmospheric river is present.

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 0000 UTC 01 Jan. 1963. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

Within the atmospheric river, normalized anomalies of precipitable water exceeded 4 standard deviations (henceforth 4 sigma) just off the California coast by early Jan 31 with the PW anomaly building to over 5 sigma by 0000 UTC 1 Feb. The greater than 5-sigma PW anomaly (moisture within the atmospheric river) has a return frequency of 120 months arguing that the moisture plume associated with this system was extremely anomalous (see expected frequency figure below). . The normalized moisture flux anomalies were even higher. The greater than 5 sigma anomaly extending northward towards Idaho helps explain their severe flooding in that state.

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 0000 UTC 31 Jan. 1963 (top panel), 0000 UTC 01 Feb. 1963 (bottom panel).

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 0000 UTC 31 Jan. 1963 (top panel), 0000 UTC 01 Feb. 1963 (bottom panel).

Note the rarity of having a greater than 5 sigma PW anomaly (see below). When such a high PW anomaly is present, the chances of an extreme event rises . A highly anomalous forecast of a normalized anomaly on an ensemble mean suggests the model is in strong agreement and that it is signally a pretty rare in magnitude moisture plume.

Expected return frequency as a function of PW at a point just upstream from the Sierra Nevada mountains. X-axis is the magnitude of the normalized anomaly, the y-axis is the return frequency in months. The dotted lines highlight the return frequency of a normalized anomaly with a magnitude of 4 and shows the expected return period of 24 months.

December 2002 event

The 3-day period ending at 1200UTC 16 December ranked fourth on our list which may reflect somewhat that the data coverage in 2002 was better than during some of the earlier events. According to the Environmental News Service, California’s northern coast received 26 inches of rain from December 12 through December 16 and more than 20 inches was recorded at Shasta. In parts of the San Francisco Bay region received more than 17 inches in the same period. The flooding in California killed 3 people.
The heaviest rainfall during this period was concentrated over the northern portion of the area (see figure below) which may also explain why it did not have a larger impact on the general public as the most populated areas were south of the heaviest rain.

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 16 Dec 2002.

Higher resolution analyses of the 3 individual days that were included in the 3-day rainfall event (in mm).

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 1200 UTC 14 Dec. 2002. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

The synoptic pattern showed many of the same characteristics of other major events but also showed one difference. The similarities were the negative height anomaly off the Pacific Northwest coast and the positive anomaly to its south at 500-hPa and presence of an atmospheric river with anomalously high PW within it. Another similarity with other events was the strong pressure or height gradient that extended into northern California which implied strong southwesterly geostrophic winds. The one notable difference was the lack of a positive height anomaly over Alaska or the Bering Sea.

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 1800 UTC 13 Dec. 2002 (top panel), 1200 UTC 14 Dec. 2002 (bottom panel).

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 1800 UTC 13 December 2002 (top panel), 1200 UTC 14 Dec. 2002 (bottom panel).
.

Anomalous PW and 850-hPa moisture flux were present through much of the 3-day event. During the heaviest 24 hour period, the atmospheric river or moisture plume impinged upon northern California though most of the 24-h period. The normalized moisture flux anomaly increases from around 3 early in the period to over 5 sigma by 0600 UTC 14 Dec. (not shown) with the over 5 sigma departure continuing through 1200 UTC 14 Dec. The greater than 42 mm PW maximum and greater than 3 sigma PW departure that was present off the coast at 1800 UTC 13 Dec.. The anomalous PW continued to shift eastward with the greater than 42 mm PW maximum being located just off the California coast and the greater than 3 sigma PW departure having spread across northern California southward to San Francisco by 0600 UTC 14 Dec (not shown). The normalized anomalies decreased somewhat during the 24 hour period after 1200 UTC 14 Dec. before again increasing during the following 24 hours with normalized anomalies of 850-hPa moisture flux again exceeding 5 sigma by 0600 UCT 16 Dec. Such high normalized anomalies suggest a return period for such a strong moisture plume would be around 3 years.

New Years 2006 event

Environmental News Service reported that the flooding along the Russian River was the worst since 1997. The crest associated with the event reach 10 feet above flood state at Guerneville and 5-feet above flood stage in Napa city sending water into the streets of the city. 1000 home were flooded along the Napa river. There was also flooding of the Suisan Marsh area (see picture below). The 3-day period ending at 1200 UTC 2 Jan was the fifth ranked event on our list. The image below shows the areas that were hardest hit by the event were generally north of San Francisco Bay. The California Department of Water Resources provided the following information on the flooding along the Napa and Russian Rivers. The return interval for the flooding on the Napa, Russian and Cosumnes Rivers from the rainfall on December 31 was 1 in 30, 1 in 15, and 1 in 50, respectively. The return period for similar river stages along the upper Sacramento, Feather, American and Yuba rivers for a 3-day event was once every 5 years. Widespread flooding was also reports in Nevada along the Carson River.

Picture of the Suisun Marsh Flood of Jan. 2006 (From Roos 2007).

Picture from California Department of Water Resources briefing slide

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 02 Jan 2006.

Higher resolution analyses of the 3 individual days that were included in the 3-day rainfall event (in mm).

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 0000 UTC 31 Dec. 2005. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

The atmospheric river associated with this event fit the definition of a strong one. However, the normalized anomalies associated with the event were not as high as during many of the extreme rainfall cases discussed within this training module. The normalized PW maximum reached only 3.27 sigma which has only around a 1-yr return frequency where the anomaly was largest and less than that elsewhere. The normalized MF anomaly was more impressive suggesting around a 3-year return period for the moisture flux in the vicinity of the Feather River. Note the greater than 4 sigma normalized MF anomalies on the figures below.

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 1800 UTC 30 Dec. 2005 (top panel), 1200 UTC 31 Dec. 2005 (bottom panel).

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 1800 UTC 30 December 2005 (top panel), 1200 UTC 31 Dec. 2006(bottom panel).
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January 1974 event

This event ranked highly but its effects were well north of San Francisco. The event produce significant flooding on the Klamath and Noya rivers along California’s North Coast. The Noya river streamflow records provide some perspective on the magnitude of the flooding of the rivers along the North Coast. 50% or the time the Noya stream flow is less than 32 cfs but during the flood reached a record crest of 26600 cfs. Major flooding in Oregon, Idaho and Montana were also associated with the event.

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 16 Jan 1974.

Precipitation analysis of the 24 hours (in inches) ending at 1200 UTC 15 Jan 1974 from the .25 by 2.5 deg. unified data set.

Precipitation analysis of the 24 hours (in inches) ending at 1200 UTC 16 Jan 1974 from the .25 by 2.5 deg. unified data set.

The synoptic pattern in most respects is similar to the other events. At 500-hPa, a positive height anomaly is again found in the vicinity of the Bering Sea and negative height anomaly is located to its south (see below). The second more southerly negative height anomaly is a little east and south of many of the other cases. The increase ridging to the east probably helped direct the moisture plume more into California’s North Coast and into Oregon than some of the other events. Like many “pineapple connection” or atmospheric river events, abnormally warm temperature and high snow levels were present so for locations in the Cascade range and northern Rocky Mountains, snow melt and heavy rain contributed to the flooding. The entrance region of any upper level jet streak was also present along the North Coast.

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 1200 UTC 15 Jan. 1974. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

Like during many of the other events, a strong atmospheric river with anomalously high PW and 850-hPa moisture flux was impinging upon the coast during the period of heavy rainfall (see figures below). In this case, the highly anomalous moisture extended northeast across Oregon into Idaho and Montana with the PW anomaly reaching 4 sigma over Idaho and the moisture flux anomaly exceeding 5 sigma. Granted over the Rocky Mountains a forecaster should look at moisture flux anomalies at a higher level than 850-hPa, Still, the analyses suggest the amount or moisture moisture flux during this event were highly unusual.

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 0600 UTC 15 Jan. 1974 (top panel), 1200 UTC 15 Jan. 1974 (bottom panel).

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 0600 UTC 15 Jan. 1974 (top panel), 1200 UTC 15 Jan. 1974 (bottom panel).

March 8-11, 1995 event

The 3-day period ending at 1200UTC 11 March 1998 ranks as number 8. The event produced heavy rain and flooding in the Napa and Sonoma valleys as well as in the Sierra range. The Napa River peaked at 5.3 feet above flood stage in St. Helena which lead to nearly 400 residents having to be evacuated because of the flood waters. At Napa, the river peaked at 30.5 feet, 5.5 feet above flood stage which brought flood waters into the city. However, the biggest impact was along the Pajaro River Basin in Santa Cruz and Monterey counties. The flood control along the river is designed to handle a 19000 cfs discharge (a 25-year event). During the March event, the band of heavy rainfall associated with the storm stalled over Pajaro River Basin. The two red arrow are pointing to small areas where more than 6 inches fell during 24-hours, the HPC analysis indicates over 12 inches of rain fell over the basin. The differences between the 72-hr CDC analysis with a maximum rainfall of slightly more than 150 mm over the basin while the maximum implied by the two HPC analyses would be in excess of 300 mm.
A levee break near the upstream from Watsonville led to flooding of a over 300 acres of prime farmland in Monterey county including the town of Pajaro. The area of severe flooding was declared a Federal Natural Disaster.

Greater than 6 inches

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 08 Mar 1995-1200 UTC 11 March 1995.

HPC rainfall analysis for the 3-heaviest days of the event (in inches).

The synoptic pattern was again very similar to the majority of the other events. Ridging and a relative geopotential height maximum was present over the Bering Sea and western Alaska at 500-hPa with a potent negative height anomaly off the California coast. The anomalously low heights associated with the trough helps maintain a potent geopotential height gradient implying stronger than normal southwesterly geostrophic winds. The deep southwesterly flow tapped into subtropics and pulled a strong atmospheric river northeastward where it impinges upon the California coast. In this case, the left exit region of an upper level jet streak was located where the heaviest rain fell along the coast. 850 temps were anomalously warm, another common trait associated with cases exhibiting strong atmospheric rivers.

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 1200 UTC 10 Mar. 1995. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 0000 UTC 09 Mar. 1995 (top panel), 1200 UTC 09 Mar. 1995 (bottom panel).

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 0000 UTC 09 Mar. 1995 (top panel), 1200 UTC 09 Mar. 1995 (bottom panel).

The atmospheric river impinged upon the coast of northern and central California for an extended period on March 9 and 10. During the 48-hour period the normalized PW anomalies exceeded 3 sigma. By 0000 UTC 9 March, a band of 850-hPa that exceeded 3 sigma moved into the region with a small area of greater than 3 being located just on the coast. During the next 12 hours, the 850 moisture flux (MF) increased until the normalized anomaly exceeded 5 sigma. The moisture plume slowly sand southward during the next 24 hours with the magnitude of the normalized MF anomaly decreasing somewhat at 0000 UTC 10 March (not shown) but then re-strengthening again to over 5 sigma by 1200 UTC. The combination of the slow movement of the plume and the very high normalized anomalies within it, are a common trait of these multi-day extreme rainfall amounts.

January 8-11, 1995 event

The 3-day period ending at 1200 UTC 9 Jan ranked ninth on the list but heavy rain was reported across northern California through January 11th.
42 counties in California were included in the Federal Disaster Declaration due to this extended rainfall event. Sacramento and western Placer counties were particularly hard hit as many small streams overflowed their banks with disastrous results to adjacent properties (Curtis and Humphrey 1995). Serious flooding was also reported along the Napa river in St. Helena where the river peaked at 4.5 feet above flood stage. Significant flooding was also reported along the Russian River (California Dept Water Res. Bull 69-95).

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 06 Jan 1995-1200 UTC 09 Feb 1995.

24-h HPC precipitation analysis with contours of 0.50, 1, 2, 3, 4, 5 and 6 inches ending at a) 1200 UTC 09 Jan 1995, b) 1200 UTC 10 Jan 1995, c) 1200 UTC 11 Jan 1995.

The synoptic pattern again was very similar to that of the majority of the cases. At 500-hPa, an impressive negative height anomaly was present off the coast of northern Pacific northwest coast and a positive height anomaly was located to its south. The placement and strength of the negative anomaly is similar to the placement of similar anomaly during the March 1995 event. Such a strong anomaly displaces the polar jet southward leading to cyclones develop farther south than normal. Like the majority of the events, the gradient implies strong southwesterly flow on the south and east side of the cyclone which taps the subtropics and pulls an atmospheric river northward. While the 500-hPa heights show no strong positive anomaly across Alaska or the Aleutian Islands, the heights do imply ridging across the Aleutian Islands. Note on the figure below that the 850-hPa temps were higher than normal suggesting high snow levels.

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 1200 UTC 09 Jan. 1995. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 0600 UTC 09 Jan. 1995 (top panel), 0000 UTC 10 Jan. 1995 (bottom panel).

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 0600 UTC 09 Mar. 1995 (top panel), 1200 UTC 09 Jan. 1995 (bottom panel).

The PW within the moisture plume was high enough for the atmospheric river to the classified as a strong one according to the Neiman et al. (2008)definition. However, the PW and its normalized anomaly values within the plume were lower than during the March 1995 event. During most of the event it exceeded 2 sigma but did not exceed 3. However, the strong pressure gradient and resulting winds still produced highly anomalous 850-hPa moisture flux with an extended period of greater than 4 sigma values being draped across the region by 0000 UTC 9 Jan. The anomaly then increased to over 5 sigma within the next 6 hours before again settling back to just under 5 sigma. The slow movement of the moisture plume and the extended period of very anomalous moisture flux favored an extreme rainfall event.

New Years 1997 event.

The CDC .25 by .25 degree precipitation analysis ranked this as the 17th highest 3-day total which is very misleading as this was the most impressive event during a 10-year study by Junker et al. (in press) that included the 1995 events. The New Years flood produced nearly 2 billion dollars in property damage and forced more than 100,000 people from their homes (Baird and Robles 1997). December 1996 set the stage by being an exceptionally wet month prior to the 3-day event. The flooding associated with the event may have rivaled the legendary flood of 1862 (Roos 1997). The flooding of the Feather and Cosumnes Rivers was the highest on record while the flooding on the American River almost equal to the flooding in 1986 (see figure below).

From Roos 2007

There were numerous levee failures along the San Joaquin and Sacramento valleys. More that 120,000 people were evacuated from their homes. Nine people were killed and 48 counties were declared disaster areas. For additional information on the impact of the floods, click here. High snow levels and rain led to serious flooding in Idaho where the Weiser river exceeded flood stage by 7 feet and in Montana along the Musselshell, Yellowstone and Little Salmon Rivers.

A picture to show the scope of the flooding in California. From Roos 2007

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 02 Jan 1997.

24-h HPC precipitation analysis with contours of 0.50, 1, 2, 3, 4, 5 and 6 inches ending at a) 1200 UTC 31 Dec 1996, b) 1200 UTC 01 Jan 1997, c) 1200 UTC 02 Jan 1997.

The synoptic pattern was very similar to that of the majority of the top ten ranked events. A 500-hPa ridge was located in the vicinity of the Bering Sea with an area of anomalously high heights (see below). A strong negative anomaly with greater than 3 sigma departures from normal was located to its south and east. A positive anomaly was located to its south. The strong negative anomaly west of northern California and the long fetch of deep southwesterly flow helped bring warmer than normal air into California and much of the west with an over 2 sigma temperature departure extending from northern California northeastward into Idaho and Montana. The deep southwesterly flow also tapped a very strong and unusually wide atmospheric river. The synoptic pattern promised to bring anomalous moisture and warmth from California northeastward into Idaho and Montana.

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 1200 UTC 31 Dec. 1997. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 1800 UTC 31 Dec. 1996 (top panel), 1800 UTC 01 Jan. 1997 (bottom panel).

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 1800 UTC 31 Dec. 1996 (top panel), 1800 UTC 01 Jan. 1997 (bottom panel).

The maximum PW within this “river” was the highest found in any plume during a 10 year period (1991-2000). The width of the moisture plume also was among the largest found. Normalized PW anomalies exceeded 3 sigma on all three days included of the HPC precipitation analyses shown above. During January 1st the PW anomaly reached greater than 5 sigma, a return period of more than 120 months which probably translated to over 20 years given the 6-month length of the extreme rainfall threat season. The normalized 850-hPa moisture flux anomalies (NMFA) were also high through the event. At 0000 UTC 30 December the 850-hPa moisture flux exceeded 5 sigma (not shown). The very high NMFA continued on December 31 and increased to over 6 (figure not shown) signaling that the MF within this atmospheric river was rare. The rainfall maximum was generally located where the maximum moisture flux was aimed for the longest time. For a more in depth discussion of the normalized anomalies associated with this event, see Junker et al 2007.

17-20 November 1950 event

The 1950 event was the 10th heaviest on the list. This event and another in early December led to significant flooding in Yuba and Tulare Counties and led to flooding of the Tule River and migrant worker lodging in Porterville (see picture below). Roos has listed it as one of the nine big floods of note for California (2007). The flooding was also serious enough in Nevada to prompt a U.S. Geological Survey report of the flooding. The peak streamflow on the Truckee river was the highest recorded until New Year’s 1997 event.

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 20 Nov. 1950.

The 24 hr period of heaviest rain during the 3 day period. 24-hr CDC QPE ending 1200 UTC 18 Nov. 1950.

Picture of the flooding of the Tule River and migrant worker camp.

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 0000 UTC 18 Nov.
1950. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

The synoptic pattern is a variation of the basic pattern. The 500-hPa height pattern on 0000 UTC 18 Nov. has a positive height anomaly over Alaska and a negative to its south. However, the gradient between the negative height anomaly located off the Pacific northwest the ridge over Baja weaker than during the majority of the cases during the first 24 hours of the event. However, anomalously high PW was present within the atmospheric river.

The 500-hPa continued to evolve during the event and more closely matched the norm later during the event (see figure at left) as another stronger negative height anomaly developed under the block. The resulting deep southwesterly flow implied by the gradient between this negative anomaly an the ridging just west of Baja help maintain the strong atmospheric river across California. with normalized PW anomaly of over 4 sigma present at 0000 UTC 20 November (not shown>

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 0000 UTC 18 Nov. 1950 (top panel), 1200 UTC 18 Nov. 1950 (bottom panel).

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 0000 UTC 18 Nov. 1950 (top panel), 1200 UTC 18 Nov. 1950 (bottom panel).

The normalized moisture flux anomaly during this event was unusually low due to the relatively light 850 winds over California (See above), However, the atmospheric river remained almost stationary through the heaviest day of rain with the PW anomalies building to over 4 sigma by the end of the day. The normalized PW anomaly of 4.95 sigma valid at 1200 UTC 18 Nov. places the return frequency at around 10 years (see below) but given that the heavy rain season is limited to 6 months that places the return frequency even higher, probably closer to 20 years. In this case, the 850 moisture flux anomalies were above normal but were lower than during most of the other 20 extreme events.

02-05 January 1966 precipitation event

The three day rainfall maximum ranked eleventh on the list but the event produced much less flooding in California than some of the other events as its heavy rainfall was much less widespread. The bulk of the rainfall also fell well north of San Francisco. However, the Modesto-Bee reported flooding near Eureka that sent 1000 people fleeing from their homes.

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UTC 05 Jan 1969.

Precipitation analysis of the 24 hours (in inches) ending at 1200 UTC 04 Jan 1966 from the .25 by 2.5 deg. unified data set.

Precipitation analysis of the 24 hours (in inches) ending at 1200 UTC 05 Jan 1966 from the .25 by 2.5 deg. unified data set.

The pattern associated with this event did not follow the template of most of the other top 20 events. Still, it was associated with a positive height anomaly at 500-hPa over Alaska and a strong negative one to its south. However, the negative anomaly was located a little farther to the north and east without having a southwestward extension to the negative anomaly. Also, there was no positive height anomaly to the south of the negative anomaly. Therefore, the pattern showed less of a southwesterly fetch than most of the events (see below) with the strong gradient being confined to the northern part of the state. . The atmospheric river was much weaker than during the other cases and could not be defined as a strong river.

The most unusual thing about this case compared to the others was the lack of strong PW anomalies (see below left). The normalized 850 MF anomaly of 2.5 to 3 across northern CA suggests that a plume of similar magnitude will occur in a matter of a couple of months or sooner. Despite the rather pedestrian normalized anomalies, this event still produced impressive precipitation amounts on the CDC unified data set. This suggests that moisture flux and PWs are not the only modulating factor governing precipitation amounts. This was the weakest “river” of the 20 cases”

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 0000 UTC 04 Jan 1966.

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 0000 UTC 04 Jan 1966.

18-21 January 1969 event

The January flood was primed by a 3 day rainfall event centered around January 12th. The second extended rainfall event that ran from 1200 UTC 18 January-1200 UTC 21 January then produced major flooding . The flooding was included in the Roos listing of major California flooding events. Significant flooding occurred over the Central Valley rivers, the reformation of Tulares Lake in San Joaquin Valley. Even worst flooding occurred farther south in the southern portion of the state where flooding along the Sespe Creek killed six hikers. The figure below indicates the primary areas of flooding. Note that the worst flooding occurred south of the San Francisco area closer to where the axis of strongest moisture flux.

Paulson, R.W., Chase, E.B., Roberts, R.S., and Moody, D.W., Compilers, National Water Summary 1988-89-- Hydrologic Events and Floods and Droughts: U.S. Geological Survey Water-Supply Paper 2375, 591 p.

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 18 Jan 1969-1200 UTC 21 Jan 1969.

Precipitation analysis of the 24 hours (in inches) ending at 1200 UTC 19 Jan 1969 from the .25 by 2.5 deg. unified data set.

Precipitation analysis of the 24 hours (in inches) ending at 1200 UTC 20 Jan 1969 from the .25 by 2.5 deg. unified data set.

Precipitation analysis of the 24 hours (in inches) ending at 1200 UTC 21 Jan 1969 from the .25 by 2.5 deg. unified data set.

The synoptic pattern associated with this case was very typical of the pattern exhibited by most of the events. A closed 500-hPa anticyclone was present in the vicinity of the Aleutians with as associated area of anomalously high heights. A area of below normal heights were found to its southeast and southwest which helped produce a long a positive tilt to the associated trough. A greater than 2 sigma negative height departure was located west of the California coast located far enough south to help tap into a strong atmospheric river.

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 0000 UTC 21 Jan 1969. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

PW (mm) and normalized PW anomaly (top left), 850 winds (standard barbs and flags) and normalized 850 MF (top right), 500-hPa heights and normalized anomalies of the departure from normal (bottom left), and 700-hPa heights and normalized departures from normal avlid 0600 UTC 21 Jan 1969. Magnitude of the normalized anomalies is given by the color fill from the bar at the bottom of each figure)

Note the greater than 3 sigma height departure west of San Diego at 0600 UTC 21 January and note how it helps produce a implied southwesterly geostrophic flow ahead of the trough and how the an anomalously strong MF plume then stretched from off the California coast northeastward into Idaho.

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 1200 UTC 21 Jan. 1969.

The normalized anomaly of MF climbed to above 5 by 1200 UTC 21 Jan across southern California and the PW anomaly (not shown) to greater than 2. The high normalized MF anomaly suggested return frequency of having a similar moisture plume over that location would be over 24 months or given the rainy season is confined to the cold season, more the 4 years.

1200 UTC 18-21 Dec 1981

The 3-day period was the 13th largest grid point maximum on our list. The Syracuse Herald-American had a story from the morning of December 20th stating that killed 3 people, raised 4 rivers to flood stage and produced mud slides. However, compared to many of the events listed, the flooding during this case was relatively minor. The heaviest single day was during the 1200 UTC 18 Dec. to 1200 UTC 19 Dec. period.

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 18 Dec 1981-1200 UTC 21 Dec 1981.

Heaviest 24-hr period ending 1200 UTC 19 Dec 1981.

Like many of the events, this one was associated with an area of anomalously high 500-hPa heights at high latitudes. This time. the strong positive anomaly was over western Canada rather than Alaska or the Aleutian Islands as in most of the cases. Instead of being south of the positive anomaly, the negative anomaly was located to its west. However, south of this negative anomaly was a positive height anomaly. The negative/positive height couplet enhanced the gradient and helped produce a strong upper level jet. Southwesterly 850-hPa winds were located along the exit region of this jet streak. (see figure below).

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 1200 UTC 31 Dec. 1997. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

As with many of the events, a strong atmospheric river was present with anomalously high PW. This river was also associated with an associated area of above normal 850-hPa temperatures (see figure above). With time, the positive anomaly at 500-hPa that was located to the south of the negative elongated eastward while a greater than 2 sigma negative anomaly strengthened/developed west of northern California (not shown). This change gave the 500-hPa anomaly pattern more of a classic look with an implied long fetch of stronger than normal west-southwesterly geostrophic flow.

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 1800 UTC 18 Dec. 1981 (top panel), 0600 UTC 19 Dec. 1981 (bottom panel).

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 1800 UTC 18 Dec. 1981 (top panel), 0600 UTC 19 Dec. 1981 (bottom panel).

The moisture within the atmospheric river was impressive and highly anomalous. Normalized PW anomaly over California on the 19th exceeded 4 standard deviations.

1200 UTC 09 –12 Nov 1973 rainfall event

No reports of serious flooding could be found from this 3-day rainfall event despite its ranking in the top 20 events within the data set. The 72-hr QPE from the CDC .25 by .25 deg. unified data set (below left) ending 1200 UTC 12 Nov. 2973 and the analysis for the heaviest 24-hr period (below right) offer a general feel for the precipitation pattern but likely underdo the magnitude of the heaviest rainfall by quite a bit.

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 12 Nov. 1973.

24-hr rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 11 Nov. 1973 for the heaviest of the 3 days.(in inches)

The synoptic pattern was very similar to the pattern associated with the majority of the 20 events. An area of anomalously high heights was located over much of Alaska including a maximum over the Aleutian Islands. A strong negative anomaly was located to its southeast with still another positive to its east-southeast and east. A strong low was indicated by the 850-hPa height analysis and was located off the coast of California and Oregon. This low and the ridging to the southeast helped provide southwesterly flow that tapped into a strong atmospheric river that intersected the coast (see below)

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 1200 UTC 10 Nov 1973. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

The normalized anomaly PW and 850 moisture flux plots below indicate that within the atmospheric river the PW anomaly exceeded 3 sigma and the normalized 850 MF anomaly briefly reached 4 sigma. They suggest a return frequency for having an atmospheric river of similar magnitude over that location is about once every one or two seasons.

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 0000 UTC 11 Nov. 1973 (top panel), 0600 UTC 11 Nov. 1973 (bottom panel).

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 0000 UTC 11 Nov. 1973 (top panel), 0600 UTC 11 Nov. 1973 (bottom panel).

1200Z 6 Feb 1960-1200 UTC 9 Feb 1960

The 3-day period in 1960 was the 15 highest ranked event. No reports of serious flooding could be found from this 3-day rainfall event despite its ranking in the top 20 events within the data set. This suggests that antecedent conditions are an important player in determining whether major flooding will occur.

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 09 Feb. 1960.

24-hr rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 08 Feb. 1960 for the heaviest of the 3 days.

The heavy precipitation associated with this 3-day event generally occurred north of San Francisco. Again it is worth noting that the actual precipitation was probably quite a bit heaviest than the amounts analyzed using the rather course resolution CDC data set.
The region where the strongest geopotential height gradient was located and where the atmospheric river impinged upon California north of SFO which probably explains why the heaviest precipitation was also positioned north of the city. The synoptic pattern associated with this event had a strong negative height anomaly off the pacific northwest coast with a positive anomaly to its south and southwest at 500-hPa and 700-hpa (not shown). This couplet and the resulting gradient helped produce a long fetch of strong westerly flow. The strong shortwave associated with the negative height anomalies resulted in a strong cyclone which was evident on the 1000-hPa and 850-hPa analyses (see below)

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 1200 UTC 08 Feb 1960. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 0600 UTC 08 Feb. 1960 (top panel), 1200 UTC 08 Feb. 1960 (bottom panel).

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 0600 UTC 08 Feb. 1960 (top panel), 1200 UTC 08 Feb. 1960 (bottom panel.

The PW values within the plume were high enough to classify the atmospheric river a strong one. However, the normalized PW anomaly only reach a little over 3 which puts the return period at around 3 months meaning during any season, a forecaster might see two such plumes intersecting the same region of the coast. The strong 850-hPa winds helped produce a normalized 850 MF of greater than 4 sigma. This suggests for a similar normalized MF anomaly, the return period of having that strong of MF would be around 6 months, or about once during the rainy season.

20-23 January 1970 event

This event was a rather minor one in terms of significant flooding. The only reports of flooding that could be found on the Internet were a report of flooding on Cache Creek of the Copay valley and an allusion to two boys being missing due to flooding Jan. 26 issue of the European Stars and Stripes. Neither source elaborated on the flooding, so the flooding may not have even been associated with this event.

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 23 Jan 1970.

24-hr rainfall analysis (in inches) using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 21 Jan 1970.

24-hr rainfall analysis (in inches) using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 23 Jan 1970.

This event, like most of these cases, was associated with a strong atmospheric river. However, heights over Alaska were not anomalously high even though there was a ridge across western Canada and eastern Alaska at 500-hPa (see below). Initially, the negative height anomaly south of Alaska was farther north than for many of the events. However, with time the anomaly strengthened as shifted south and the pattern became more similar to the patterns associated with the majority of the event. The negative anomaly formed a dipole with a positive height anomaly over much of California. This positive anomaly extended south-westward suggesting a stronger than normal gradient.

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 1200 UTC 21 Jan 1970. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

Same as figure above except for 1200 UTC 23 Jan 1970.

A ridge at 1000 and 850-hPa southwest of California and a cyclone associated with the negative height anomaly to the north helped tap into the atmospheric river. Above normal 850-hPa temps accompanied the “river” as it surged into California. An entrance region of a upper level jet streak may have provided additional lifting across northern California on January 23.

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 0600 UTC 21 Jan 1970 (top panel), 1200 UTC 21 Jan 1970 (bottom panel).

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 0600 UTC 21 Jan 1970 (top panel), 1200 UTC 21 Jan 1970 (bottom panel).

As noted earlier, the event was associated with high enough PW to be considered at strong atmospheric river. 850-hPa MF normalized anomalies approached 3 sigma while the normalized PW anomalies exceeded 3 on the Jan 21. Such anomalies argue to a heavy rainfall event, but not necessarily, a rare one.

1200 UTC 10 Dec through 1200 UTC 13 Dec. 1995 event.

The 3-days period ranked 18th on the list. The weather prior to the event had been dry which minimized the run off problems (Reynolds 1996). As noted by Reynolds, synoptically the pattern was very similar to the pattern that was present during the January and March 1995 events. A strong shortwave at 500-hPa that had anomalously low heights associated with it was off the coast of northern California. A potent low pressure system had developed in response to this feature. This strong low and a ridge to its southeast provided a strong height and pressure gradient implying strong southwesterly low level winds. These southwesterly winds helped initiate the northward movement of a strong atmospheric river that impinged the coast (see below).

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 0000 UTC 12 Dec. 1995. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

Like many atmospheric river or pineapple express cases, this event was associated with above normal 850-hPa temperatures that were drawn northward by the strong low level winds and an anomalously strong v-wind component (not shown). The normalized MF and PW anomalies were impressive and were on the same order as many of the other significant events. The magnitude of the normalized MF anomaly suggests that this event was associated with a rather rare atmospheric river despite the lack of major flooding.

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 0000 UTC 12 Dec. 1995 (top panel), 0600 UTC 12 Dec. 1995 (bottom panel).

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 10 Dec 1995-1200 UTC 13 Dec 1995.

The 24 hr period of heaviest rain during the 3 day period. 24-hr CDC QPE ending 1200 UTC 12 Dec. 1995.

That the CDC 24-h analysis showed a maximum of almost 7.00 inches attests to this being a significant rainfall event as it usually underdoes the maximum. The 24 hour HPC analysis (not shown) for the same period indicated over 5 inches of rainfall just north of San Francisco Bay and another over the Siskiyou Mountains of northern California in the vicinity of the two ovals depicted on the map (top right).

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 0000 UTC 12 Dec. 1995 (top panel), 0600 UTC 12 Dec. 1995 (bottom panel).

1200 UTC 11 Feb. to 1200 UTC 14 Feb 2000 event

December 1999 was the third driest December on record to south central California which probably accounts for the lack of major flooding. However, the Maraposa County Sherriff reported that street flooding had occurred in the San Joaquin Valley and that numerous roads were closed due to creeks flooding roads in the county according to NOAA Weather Service Storm Data

Heaviest 3-day precipitation analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 14 Feb 2000 (in mm).

While the precipitation analyses shown below indicate a very wet three day period. A 10-year study by Junker et al. (in press) suggests that the 24 hour maximum observed on the map ending at 1200 UTC 14 Feb. was exceeded a number of times. That the event made the top 20 3-day total raises the question of whether the list is biased towards recent events because precipitation observational data has increased during the last several decades.

4

24 hr HPC QPE analysis (in inches) ending at 1200 UTC 12 Feb 2000.

24 hr HPC QPE analysis (in inches) ending at 1200 UTC 13 Feb 2000.

24 hr HPC QPE analysis (in inches) ending at 1200 UTC 14 Feb 2000.

The synoptic pattern was a variation on the common pattern associated with most of the ranked cases. A positive (albeit a weak one) and closed 500-hPa high were located in the vicinity of Alaska. There is also a negative to the south of this positive height anomaly (see figure below). However, the magnitude of both the positive and negative anomalies were weaker than during most of other 19 cases. Like with most cases, there was also a second positive height anomaly to the southeast of the negative one.

12Z27MAR1974 12Z30MAR1974

While the most significant flooding during 1974 was during the January storm. An Army Corps of Engineer report was written that included flooding in March along the Russian River in northern California. The rainfall during the 12Z 27 March to 1200Z March 1974 made the list of the top 20 but barely and would not have made the list if 3 day periods like Dec 21-24 1964 or Feb 16-19 1986 had been included. They were not included as one or more of their 24 hour periods were already included in the 72 hour period listed for the event.

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 30 Mar 1997 (in mm).

24-hr rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 29 Mar 1974 (inches)

The heaviest rainfall occurred north of San Francisco where a strong atmospheric river impinged upon the area during March 29 and 30.

24-hr rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 30 Mar 1974 (inches)

The case is unusually in that there was no area of anomalously high 500-hPa heights across Canada, Alaska or the Bering Straits (see below) . Also, the strong negative height anomaly off the west coast of the U.S was a little farther west than during most of the other cases. However, there was a greater than 2s positive height departure from normal south of the negative which helped produce a stronger than normal geopotential height gradient.

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 0000 UTC 29 Mar. 1974. The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

Again, the precipitable water within the atmospheric river was high enough to meet Neiman et al. (2008) criteria for a strong atmospheric river (see the images below). A normalized 850 MF anomaly of greater than 4 was located off the California coast at 1800 UCT 29 March. This moisture plume swung into the coast and brought a greater than 3 sigma PW anomaly into northern California by 0000 UCT Mar 30 and a greater than 3 normalized 850 MF anomaly into the Sierra Nevada by 0600 UTC 30 March (see figures below). The return period for such normalized anomalies is about 3 months suggesting that an atmospheric river of similar strength will impact the coast about twice during a rainy season

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 1800 UTC 29 Mar. 1974 (top panel), 0600 UTC 30 Feb. 1974 (middle panel) and 1200 UTC 30 Feb 1974 (bottom panel).

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 1800 UTC 29 Mar. 1974 (top panel), 0600 UTC 30 Feb. 1974 (middle panel) and 1200 UTC 30 Feb 1974 (bottom panel).

1980 Jan and Feb events

This series of heavy rainfall events produced significant flooding in Central and Southern California which killed 18 people. The worst flooding was and occurred in February as a second series of storms impacted south of San Franscisco. Heavy rain and unusually warm temperature that led to snow melt helped produce significant flooding along the Carson River Valley in California and Nevada.

Heaviest 3-day rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 14 Jan 1980.

24 hr QPE rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 12 Jan 1980.

The synoptic pattern during this event shared many characteristics with other major flood events. A positive height anomaly was present west of Alaska at 500h and to the southeast of this ridge a strong negative height anomaly was present (see below). Another positive anomaly was present to its south leading to a stronger than normal gradient. The event was associated with an atmospheric river that helped pull anomalously warm air north and eastward. Note the above normal 850 temps of greater than 3 sigma off the California coast at 1200 UTC 12 Jan 1980.

24 hr QPE rainfall analysis using the CDC .25 deg by .25 deg unified data set ending 1200 UCT 13 Jan 1980.

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 1200 UTC 12 Jan 1980 (top 4-panel) and valid 0000 UTC 14 Jan 1980 (bottom 4-panel). The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

A strong atmospheric river was present with normalized PW anomalies that for awhile exceeded 4 standard deviations. The river also produced an extended period with anomalously high 850 moisture flux with the normalized anomalies exceeding 4s.

PW (mm) and normalized PW anomaly (magnitude of the anomaly scale is shown on the scale at the bottom of the figure) valid 1200 UTC 12 Jan. 1980 (top panel), 0000 UTC 13 Jan. 1980 (middle panel), and 0600 UTC 13 Jan 1980 (bottom panel).

850-hPa winds (standard barbs and flags) and normalized anomaly of 850-hPa moisture flux (magnitude is given by the color fill from the bar at the bottom of the figure) valid 1200 UTC 12 Jan. 1980 (top panel), 0000 UTC 13 Jan. 1980 (middle panel), and 0600 UTC 13 Jan 1980 (bottom panel).

The second series of heavy rainfall events in 1980 occurred in southern California from a series of rainstorms that occurred February 12-18. There was again anomalously high heights over Alaska, below normal heights off the California coast with a second area of above normal heights south of the negative height anomaly (see figure below). The pattern tapped into a strong atmospheric river with anomalously high PW and MF (not shown). The series of February storms produced over 36 inches of rain at Mount Wilson Observatory (see figure at the bottom of the page). Over 18 people were killed from floods, flash floods and mudslides in southern California and Central Arizona.

Large area 4-panel chart, 200-hPa heights and isotachs (top left), 500-hPa heights and normalized height anomaly (top right), 850-hPa heights and normalized temperature anomaly (bottom left), and 1000-hPa height and normalized PW anomaly (bottom right) valid 1200 UTC 16 Feb. 1980 (top 4-panel) and valid 0000 UTC 14 Jan 1980 (bottom 4-panel). The magnitude of the normalized anomalies are given by the color fill with the scale on the left had side of each panel.

References

Bairds, B. P. and R. R Robles, 1997: Emergency management issues in the California floods of 1997: lessons learned or lessons lost? Available from Specialized Training Institute, P. O. Box 8123, San Luis Obisbo, CA, 50 pp.
Bulletin 69-95: High Water in California, October 2003. State of California, The Resources Agency, Dept. of Water Resources, 193 pp. Available at
http://www.dfm.water.ca.gov/pubs/Bul69-95/Appendix.pdf
Curtis, D. A. and J. H. Humphrey, 1995: Use of radar-rainfall estimates to model the January 9-10, 1995 floods in Sacramento, CA. Available at
http://www.onerain.com/includes/pdf/whitepaper/UseRadarRain_in_1995Flood.pdf
Doswell, C. A. III, 1992: Forecaster workstation design: Concepts and issues. Wea. Forecasting, 7, 398-407.
Doswell, C. A. III, Weather forecasting by humans-Heuristics and decision making. Wea. Forecasting, 19, 1115-1126.
Funk, T.W. 1991: Forecasting techniques utilized by the Forecast Branch of the National Meteorological Center. Wea. Forecasting, 6, 548-564.
Grumm, R. H. and R. Hart, 2001: Standardized anomalies applied to significant cold season weather events. Preliminary findings. Wea, Forecasting, 16, 736-754.
Hart, R. and R. H. Grumm, 2001: Using normalized climatolgical anomalies to rank synoptic-scale events objectively. Mon. Wea. Rev., 129, 2426-2442.
Junker, N.W., R.H. Grumm, R. Hart, L. A. Bosart, K. M. Bell, F. J. Pereira: Use of normalized anomalous fields to anticipate extreme rainfall in the mountains of northern California, accepted Wea. Forecasting, 2007.
Moors, R. E. and F. M. Ralph, 2007: Use of information by National Weather Service Forecasters and Emergency Managers during CALJET and PACJET-2001. Wea. Forecasting, 22, 539-555.
Neiman, P. J., F. M. Ralph, G. A. Wick, J. D. Lundquist, M. D. Dettinger, 2008: Meteorological characteristics and overland precipitation impacts of atmospheric rivers affecting the west coast of North America based on eight years of SSMI/I satellite observations. J. of Hydrology, 9, 22-47.
Ralph, F.M., G.A. Wick, S. I. Gutman, M.D. Dettinger, C. R. Cayan, and A. B. White, 2006: Flooding on California’s Russian River: The role of atmospheric rivers. Geophys. Res. Lett., 33, L13801, doi:10.1029/2006GL026689.
Reynolds, D. W., 1996: Similarities of three major precipitation events in California during 1995. Preprints, 15th Conference on Weather Analysis and Forecasting. August 19-23, Norfolk, Va, J127-J130.
Roos, M., 2007: A half century of watching California Floods. Summer 2007 DWR News, available from the California Dept. of Water Resources, 17-22.
http://www.water.ca.gov/climatechange/docs/Roos-flooding.pdf
Stuart, N. A. and R. H. Grumm, 2006: Using wind anomalies to forecast east coast winter storms. Wea. Forecasting, 21, 952-968.
Stuart, N.A., D. M. Schultz, and G. Klien, 2007: Maintaining the role of humans in the forecast process. Bull. Amer. Meteor. Soc., 88, No. 12, 1893-1898.
U.S. Geological Survey, 1954, Floods of November-December 1950, in western Nevada: U.S. Geological Survey Water-Supply Paper 1137-H, table 2.

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