Quantitative Precipitation Forecasts (QPFs)
WPC QPF Program Overview Webinar
Forecasters at the WPC and its predecessor organizations have been making Quantitative Precipitation
Forecasts since 1960. Quantitative Precipitation Forecasts, or QPFs, depict the amount of liquid
precipitation expected to fall in a defined period of time.
In the case of snow or ice, QPF represents the amount of liquid that will be measured when the precipitation
is melted. Precipitation amounts can vary significantly over short distances, especially when thunderstorms
occur, and for this reason QPFs issued by the WPC are defined as the expected "areal average"
(on a 20 x 20 km grid) in inches.
Methods for producing QPFs are similar to other meteorological forecasts. First, meteorologists
analyze the current state of the atmosphere. Then they use model forecasts of pressure systems, fronts,
jet stream intensity, etc., to form a conceptual model of how the weather will evolve.
The WPC has unique access to the full suite of operational and ensemble model guidance from modeling centers
in the U.S., Canada, and Europe (the foreign models are global models, so they also make predictions over
the U.S.), including many high-resolution or convection-allowing models that come close to depicting
individual thunderstorm cells (or mountaintops in the case of orographic precipitation.
WPC also stores output from several consecutive runs of all of these models,
allowing for trend analysis of model QPFs. And watching every model every day across the entire continental
U.S. domain, WPC forecasters become very attuned to the strengths, weaknesses, and biases of each model.
During a given cycle, forecasters determine which models are showing a reasonable
amount of precipitation in roughly the correct place and time. Those choices serve as the starting
point for QPFs, but forecasters also make manual adjustments based on their experience.
WPC forecasters often engage in discussion with some of the 122 local National Weather Service Forecast
Offices, 12 River Forecast Centers, and other national centers such as the Storm Prediction Center
and National Hurricane Center. In fact, per a 2005 agreement, the WPC provides the rainfall forecast
(known as a rainfall statement) that the National Hurricane Center inserts into each tropical cyclone
advisory it issues for the Atlantic and eastern Pacific basins. The WPC is also co-located with NOAA's
National Environmental Satellite,
Data, and Information Services (NESDIS) Synoptic Analysis Branch (SAB). The SAB provides information
on satellite trends which helps refine short range QPFs. All of these factors make WPC forecasts
generally more accurate than any individual model, and consecutive versions of WPC forecasts do not
differe as much as consecutive model forecasts sometimes do, although the very fine scale detail at
any given point may vary more than anticipated - owing to automated techniques that scale the WPC
forecast to the very local level.
Technical Description of QPFs:
The QPF contours (isohyets) are drawn to encompass areal average amounts of 0.01, 0.25 inch, 0.50 inch,
1 inch, 1.50 inches, and 2.00 inches. Any values greater than 2.00 inches are drawn in one-inch
increments. In addition, the location of QPF maxima are indicated on the chart by an "X", with the associated maximum
value printed underneath.
It is important to note the valid time period when viewing each product.
Specifically, for the Day 1, 2, and 3 forecasts, QPFs are manually created for 6-hour periods and an
accumulated 24-hour total QPF is also issued. For the Days 4/5 and Day 6/7 QPF, forecasters manually
create a 48-hour accumulation of areal average rainfall. Computer programs then take advantage of
model forecasts of the timing of precipitation to break the WPC forecast down into 6-hourly QPFs.
Forecasts issued on the day shift correspond to periods ending at 0000Z (for issuance times
between 1815Z and 2215Z), and those issued on the night shift correspond to periods ending at 1200Z
(for issuance times between 0615Z and 1015Z).
Multi-day Accumulation Products:
Through simple addition of the various 24-hour and 48-hour QPFs, WPC is able to provide multi-day
accumulation forecasts. Days 1-3, Days 1-5, and Days 1-7 accumulations are made available,
and can be powerful planning tools for many applications.
Schedule of QPF Issuance:
||06Z Today (Day 1) - 12Z Tomorrow (Day 2)
||12Z Today (Day 1) - 12Z Tomorrow (Day 2)
||12Z Tomorrow (Day 2) - 12Z Day 3
||12Z Day 3 - 12Z Day 4
|Days 4 and 5
||12Z Day 4 - 12Z Day 6
|Days 6 and 7
||12Z Day 6 - 12Z Day 8
||18Z Today (Day 1) - 00Z Tomorrow Evening (Day 2)
||00Z This Evening (Day 1) - 00Z Tomorrow Evening (Day 2)
||00Z Tomorrow Evening (Day 2) - 00Z the Evening of Day 3
||00Z Day 3 - 00Z Day 4
|Days 4 and 5
||00Z Day 4 - 00Z Day 6
|Days 6 and 7
||00Z Day 6 - 00Z Day 8
* Forecasts made on the night shifts are verified versus observed rainfall. The valid time of 1200z to 1200z for
QPFs issued by the night shift coincides with the 24-hour observation cycle for National Weather Service
Cooperative Observers and many other rainfall networks such as CoCoRaHS.
All Times shown are in Zulu (Z), or Greenwich Mean Time, and correspond to the time in Greenwich, England,
as seen on a 24-hour clock.
Excessive Rainfall Outlooks
In the Excessive Rainfall Outlooks, the Weather Prediction Center (WPC) forecasts the probability that rainfall will exceed flash flood guidance (FFG)
within 40 kilometers (25 miles) of a point. Gridded FFG is provided by the twelve NWS River Forecast Centers (RFCs) whose service areas cover the lower 48 states.
WPC creates a national mosaic of FFG, whose 1, 3, and 6-hour values represent the amount of rainfall over those short durations which it is
estimated would bring rivers and streams up to bankfull conditions. WPC estimates the likelihood that FFG will be exceeded by assessing
environmental conditions (e.g. moisture content and steering winds), recognizing weather patterns commonly associated with heavy
rainfall, and using a variety of deterministic and ensemble-based numerical model tools that get at both the meteorological and hydrologic factors
associated with flash flooding.
The risk of excessive rainfall is expressed both probabilistically and categorically according to the table below.
||Probability of Rainfall Exceeding Flash Flood Guidance Within 40 km (25mi) of a Point
*Note that the High Risk is only available to forecasters on Day 1 and Day 2. There is, however, ongoing research to improve tools and understanding of flood events so that
High Risk can become skillful enough to be issued at 3-day lead times.
Flash floods are rare events at any one specific location, and, therefore, the probability of a flash flood is low even when forecasters are confident that flash flooding will
occur within the region. When forecasters declare risk areas by placing a contour on an Excessive Rainfall Graphic they are expecting at least some flash flooding to occur, and are
sometimes expecting very
organized heavy rainfall and flash flooding to affect numerous locations. In an ideal case, Slight, Moderate, or High Risk may be advertised 2 to 3 days in advance of an event.
In other cases, as confidence increases (usually as lead time decreases)
the category may be updated from Marginal to Slight and so forth. In other cases, usually involving small or less well-defined weather systems,
risk areas may be introduced quite suddenly and with short lead time - flash flood forecasting is still one of the most difficult aspects of meteorology.
On the Excessive Rainfall Outlook graphics a closed contour with an arrowhead delineates the probability forecasts, with risk areas defined to the right of the direction of the arrowhead.
If conditions are not expected to become favorable for flash flooding anywhere in the lower 48 states then text across the center of the graphic will read,
"THE PROBABILITY OF RAINFALL EXCEEDING FLASH FLOOD GUIDANCE IS LESS THAN 5 PERCENT"
Note that FFG may be difficult to apply to a Day 2 or Day 3 forecast (or even late in Day 1 if it is already raining). Forecasters must make an educated guess as to how FFG
will change in response to exiting precipitation systems (FFG recovery) or incoming precipitation systems (lowering of FFG). Increased variability in model guidance and the
inability of the models to resolve mesoscale features results in greater uncertainty in forecasting excessive rainfall for the Day 2 and Day 3 periods. The product definition,
"probability of rainfall exceeding flash flood guidance," does represent our best attempt to forecast flash flooding from heavy rainfall, which is almost always driven
by convective rain rates or persistent orographic lift. Some events, however, do border on (or transition to) inundation flooding or main stem river flooding occurring
when the causative rain event is of longer than a 6-hour duration and/or is not especially heavy except when summed over longer periods of time. Forecasts of main stem river
flooding are made by the RFCs out to 5 day lead time, and are packaged by WPC into a national mosaic in the Flood Outlook Product.
Schedule of Excessive Rainfall Outlook Issuance:
Day 1 Excessive Rainfall Outlooks (graphic and associated discussion) have scheduled issuances three times per day:
01, 09, and 15 UTC.
Day-2 and Day-3 versions are issued twice daily. The valid times vary as noted in the table below.
Unscheduled, event-driven updates may be issued as needed.
|| 01Z - 12Z Day 1 (11 hours)
|| 12Z Day 1 - 12Z Day 2 (24 hours)
|| 15Z Day 1 - 12Z Day 2 (21 hours)
|| 12Z Day 2 - 12Z Day 3
|| 12Z Day 2 - 12Z Day 3 (an update to the earlier Day 2 forecast)
|| 12Z Day 3 - 12Z Day 4
|| 12Z Day 3 - 12Z Day 4 (an update to the earlier Day 3 forecast)
Forecast Discussions/Text Products:
National Weather Service forecast discussions have been used for decades to summarize our assessment of the upcoming weather pattern and associated impacts. Discussions are also
used to convey potential worst case scenarios and the forecasters' confidence in both the available model output and the manual forecast. There is a lot of good information in
the discussion products which can be difficult to depict in a gridded or graphical product. Reading the WPC discussions is just about the closest thing to calling up the forecaster
on the phone and asking what they think is going to happen.
Discussions related to excessive rainfall and QPF include the following:
- Excessive Rainfall Discussion (QPFERD) (Day 1; Day 2; Day 3 all available)
- Days 4/5 and 6/7 QPFs are produced by the WPC medium range desk which issues a medium range forecast discussion (PMDEPD).
- Mesoscale Precipitation Discussion (FFGMPD) – see full description below
Tropical Rainfall Statement
Per an agreement between the WPC and the NHC dated March 13, 2005, the WPC is responsible for providing NHC with the
wording for rainfall statements in all public advisories for tropical cyclones. The WPC provides rainfall
statements for active tropical systems that threaten land masses in the Atlantic and eastern Pacific basins.
The Tropical Rainfall Statement describes specific islands or continental regions expected to be impacted by heavy rain.
The statements include a range of expected areal average rainfall as well as the maximum amounts expected at isolated locations.
6-hourly precipitation forecasts are verified using a point (station) method while 24-hour forecasts are verified
using an areal method. Current graphs depicting WPC verification scores are available on the
WPC Verification page. For more details on
the verification of WPC precipitation forecasts, read the
by Olson, Junker and Korty in Weather and Forecasting.
Volume 10, 1995, pgs. 498-511.
Mesoscale Precipitation Discussions
On April 9, 2013, WPC began providing short term guidance during
heavy rain events leading to a threat of flash flooding to the National Weather Service (NWS) Weather Forecast Offices (WFOs),
River Forecast Centers (RFCs), the media, emergency managers and interested partners. Guidance is given in the form of Mesoscale
Precipitation Discussions (MPDs), that are ideally issued 1-6 hours ahead of time, averaging an area equal to roughly half the
size of the state of Kansas. Each MPD consists of a graphic indicating the area of concern and any pertinent meteorological
features as well as a brief text discussion focused on the mesoscale features supporting the anticipated heavy rainfall.
The potential for flash flooding within the area of concern will be highlighted by one of three headlines:
FLASH FLOODING LIKELY High confidence exists that environmental conditions are favorable, or will become favorable, for heavy rainfall
that will result in flash flooding.
FLASH FLOODING POSSIBLE Environmental conditions are favorable, or will become favorable, for
heavy rainfall, but there are questions about how the event will evolve and/or whether
flash flooding will occur.
FLASH FLOODING UNLIKELY High confidence exists that environmental conditions are unfavorable, or will become unfavorable, for
heavy rainfall that will result in flash flooding. (typically issued toward the end of an event)
While flash flooding is caused by a variety of factors (e.g., intense rainfall, dam failure, ice jams), WPC's Metwatch desk will only
focus on flash floods triggered by intense rainfall that occur over a sufficient areal coverage. Localized flash flooding is not considered.
The MPD appears under WMO Header: AWUS01 KWNH
and AWIPS header: KWNH FFGMPD
Medium Range/Extended (3-7 days)
The medium range graphical forecast products include:
- Surface pressure patterns, circulation centers, fronts, and 500mb heights
for days 3-7 into the future
- Daily maximum and minimum temperatures and anomalies for days 3-7
- Daily precipitation probabilities for days 3-7
- Two 48-hour QPFs encompassing Days 4-5 and Days 6-7
- 5-Day Total QPF for Days 1-5 and a 7-Day Total QPF for Days 1-7
The surface pressure and fronts graphics are generated three times per day; the 0330 UTC and 1500 UTC issuances display
features over the continental U.S. (CONUS) only, while the 1900 UTC issuance includes fronts and surface pressure fields for much of the Northern Hemisphere.
The 500mb forecasts and Min/Max/PoP graphics are primarily focused on the CONUS and issued twice per day.
In addition to the graphical forecasts, the forecasters prepare two daily written discussions.
They highlight medium-range model differences, provide weather solution preferences, a measure of uncertainty, forecaster reasoning and
highlight any significant weather expected to impact the CONUS during the Day 4-7 time frame. Forecasters also provide a separate discussion
describing guidance differences and preferences across Hawaii by 1230 UTC.
One meteorologist works during the overnight hours (0000-0900 UTC), while two prepare
the forecasts during the day shift (1130-2030 UTC). The overnight forecaster generates
a set of North American 3-7 day pressure systems/fronts and 500mb forecasts, 3-7 day sensible weather grids, two 48-hour QPFs, and discussion.
During the day shift, the two meteorologists collaborate to generate a new set of these same medium range products along with Northern Hemisphere 3-7
day pressure systems/fronts, targeted observation guidance, and the Hawaiian discussion.
They routinely use output from the GFS, ECMWF, and UKMET medium
range models and also consider the Canadian, the Navy's NAVGEM model, and ensemble guidance from
the GFS, ECMWF, Canadian, and North American Ensemble Forecast System (NAEFS).
In addition, during hurricane season, at 1700 UTC on a daily basis since June
1, 1997, the medium range pressure dayshift forecaster also participates in a conference
call with the NHC via the Hurricane Hotline to discuss current and potential
tropical activity in the Atlantic and Eastern Pacific oceans and how the
medium range models are handling the situation.
Alaska Medium Range (Days 4-8)
To accommodate a request for support from the NOAA National Weather Service (NWS) Alaska Region,
the NCEP/Weather Prediction Center's (WPC) Alaska Medium Range Desk is issuing the following
|Day 4-8 Fronts and Pressure Graphics
|Day 4-8 500 hPa Height Graphics
|Alaska Medium Range Discussion
|Day 4-8 Max/Min Temps and Probability of Precipitation Grids
Surface graphics depict surface pressure patterns (from which surface winds can
be inferred), high and low pressure circulation centers and fronts for days 4-8. The
500 hPa height graphics display the general flow pattern forecast for days 4-8, and the gridded
guidance depicts various important meteorological variables for the forecast period.
An WPC meteorologist interprets available deterministic and ensemble model guidance and collaborates with
the WPC contiguous U.S. (CONUS) medium range forecasters. The Alaska forecaster then uses the
available model guidance and meteorological reasoning to depict the most likely scenario for days 4-8.
That meteorologist then composes a forecast discussion outlining deterministic and ensemble model differences,
preferences and trends.
In addition, within the discussion the forecaster communicates confidence level, forecast
uncertainty and any significant weather expected in the forecast period.
- Day 4-8 Surface Fronts and Pressures graphics
- Day 4-8 500 hPa Height graphics
- Alaska Medium Range Forecast Discussion
- Day 4-8 Maximum/Minimum Temperature grids
- Day 4-8 12-hour Probability of Precipitation grids
- Day 4-8 derived Dewpoint Temperature, Cloud Cover, Precipitation Type, and Wind Speed/Direction grids
The WPC model diagnostic meteorologist prepares the Model
Diagnostic Discussion which evaluates the NAM and GFS along with
other operational model and ensemble guidance for each significant system affecting the continental
U.S. through 84 hours from model initialization.
This discussion emphasizes model differences and preferences, with an evaluation of NAM/GFS analyses
if there are significant errors and a review of model trends and biases if appropriate.
There are two issuances during each of the day and night shifts corresponding to the arrival of latest model data.
The following table shows the deadline and content for each issuance.
||Evaluate NAM and GFS initializations
Compare NAM/GFS and other available model/ensemble guidance
Review model trends and biases as appropriate
Discuss model preferences
||Evaluate the ECMWF/UKMET/Canadian global models relative to other current guidance
Finalize model preferences
During the cool season
(Nov 1 - Apr 15), this meteorologist is also primarily responsible for requesting
reconnaissance flights whenever the potential exists for major winter storm
development over the East or Gulf Coast states.
Surface Analysis Products
The following chart indicates the approximate issuance and web posting schedule for the WPC Surface Analysis. This product depicts the analysis of synoptic and sub-synoptic/mesoscale surface features including highs, lows, fronts, troughs,
outflow boundaries, squall lines, and drylines. The analysis domain covers most of North America, the Western Atlantic and Eastern Pacific oceans, and the Gulf of Mexico.
The Pacific Ocean analyses (East and West Pacific)
are prepared by NCEP's Ocean
Probabilistic Heavy Snow/Icing Forecasts
PRODUCT DELIVERY SCHEDULE
The WPC Winter Weather Desk (WWD) is staffed two shifts a day from September
15 through May 15. The WWD forecaster routinely releases updated forecasts twice daily
at 0900Z and 2100Z (4am/pm EST or 5am/pm EDT respectively). Forecasts may be updated if
warranted by rapidly changing situations.
|PROBABILITY GRAPHICS FOR SNOW AND FREEZING RAIN
These graphics indicate the probability (potential)
for a location to receive specific thresholds of accumulated snow or ice.
Snowfall - closed lines represent the probability (slight, moderate,
and high) that enclosed areas will receive equal to or greater than a
specific threshold accumulation (4", 8" or 12") of snowfall in a 24 hour period.
Freezing Rain - depicts the probability in the same manner and time
period as snowfall, but with an accumulation threshold of .25" (one quarter of
an inch) of freezing rain.
Note the 4" threshold on the Snowfall Probability Graphic is drawn only for elevations less than 7,500 feet.
Elevationcriteria is not imparted for the 8" and 12" thresholds.
CAUTION ! The probability contours
may appear to inordinately expand, contract or "jump" geographically
after a scheduled update. This is partially due to the frequency
which the products are updated. The "new" 24 hour period covers
the last 12 hours of the previous issuance AND the next 12 hours.
Additionally two model cycles have passed since the last scheduled issuance.
Specific (deterministic) accumulations for a particular location in the United States
can be obtained via the National Weather Service home page.
Note, at this site you will have to click the GRAPHICAL FORECAST tab prior to clicking a location on the map.
The probabilistic graphics combined with the deterministic forecasts provide
a user both the most likely amount expected from an event and the potential the event will produce
accumulations in excess of specific thresholds.
The probability thresholds used are defined as follows:
At times the forecasters may use only one or two isolines for the forecast.
This simply implies slight or slight to moderate probability for the
SLGT - 10% to 40% chance of occurrence within the outlined area.
MODERATE (MDT) - 40% to 70% chance of occurrence within the outlined area.
HIGH - 70% chance or greater of occurrence within the outlined area.
To gain further insight into this forecast, please read the Heavy
Snow Discussion (HSD) that accompanies these graphical products.
LOW TRACKS GRAPHIC
depicts the forecast location of significant surface lows impacting the
48 contiguous United States in 12 hour increments out to 72 hours into the future. It
is provided in two formats, non-technical and technical.
- The non-technical version depicts the low position and track forecast by the NCEP WPC meteorologist in
white. Each low position is accompanied by a lead time (Eastern Time). The circle around each
low represents a 75% probability the observed low will be located within the circle. Note: The
probability is derived using previous season's verification data. For
reference, existing surface lows are depicted with a red marker without yellow circles.
- The technical version depicts the low position and track forecast by the NCEP WPC meteorologist in black.
Each low is accompanied by a forecast central pressure. Additionally there are no less than 35 different computer model
forecasts of low position for a given lead time available to the WPC
forecaster - these are depicted with symbols. Both the central
pressure and computer model forecasts are color coded according to lead
time (Universal Time). Together, the WPC forecast position of the low and
computer generated position provide a user both the
preferred position and track of the low and a sense of the uncertainty
with the forecast.
Note - winter weather is
not always associated with significant surface lows.
- For reference, existing surface lows are depicted with a red marker.
Surface lows can also be found on the WPC Surface Analysis. One can see the
current location surface lows and the forecast path of both existing surface
lows and surface lows expected to develop within three days time.
Short Range Forecasts
|6 and 12 hour forecasts
||0200Z (Night Shift)
||1400Z (Day Shift)
|18 and 24 hour forecasts
||0430Z (Night Shift)
||1630Z (Day Shift)
|30, 36, and 48 hour forecasts
||0730Z (Night Shift)
||1930Z (Day Shift)
|60 hour forecast
||0800Z (Night Shift)
||2000Z (Day Shift)
||0900Z (Night Shift)
||2100Z (Day Shift)
The short range meteorologist prepares 6 through 60 hour forecasts for
the continental U.S., southern Canada, and northern Mexico.
These products are issued twice daily using numerical model output from
the National Weather Service's (NWS) Global Forecast System (GFS) and North American
Mesoscale model (NAM), as well as guidance from the European Centre for Medium-Range Weather Forecasts
(ECMWF), the United Kingdom's Meteorology Office (UKMET), the Meteorological Service of Canada, including ensembles.
Coordination with the surface analysis, model diagnostics, quantitative precipitation,
winter weather, and tropical forecast desks is also performed during the forecast process.
The short range forecast products include surface pressure patterns (isobars), circulation centers and fronts for
6-60 hours, and a depiction of the types and extent of precipitation that are forecast at the valid time
of the chart. The primary goal is to depict accurately the evolution of major weather systems that will
affect the continental U.S. during the next 60 hours.
In addition, discussions are written on each shift and issued with the forecast
packages that highlight the meteorological reasoning behind the forecasts and significant weather across the continental United States.
Please note that at this time isobars are not included on the 6-hour forecast and precipitation is not included on the
60-hour forecast chart.
Storm summaries provide both a summary of the significant weather which has occurred,
and an WPC general forecast of the storm system over the next 1 to 2 days.
Storm summaries serve as a central source for storm information which would
otherwise have to be gleaned from a number of NWS Forecast Office websites.
Storm summaries are issued for significant large-scale storms which:
If two or more separate storm systems are occurring simultaneously, a storm summary is issued for each individually.
- Affect multiple NWS Forecast Office areas of responsibility
- Are likely to be of media interest
- Impact large population areas, or major transportation systems, or otherwise make a significant impact upon the nation's or a region's commerce
- Are usually snow and/or ice storms, but which may be rainfall events if they are causing widespread flash flooding, mudslides, etc.
Tropical Public Advisories
The WPC will issue public advisories after the National Hurricane Center
(NHC) discontinues its
advisories on subtropical and tropical cyclones that have moved inland,
but still pose a threat of heavy rain and flash floods in the
conterminous United States or adjacent areas within Mexico which affect
the drainage basins of NWS River Forecast Centers. The last NHC advisory
will normally be issued when winds in an inland tropical cyclone drop
below tropical storm strength, and the tropical depression is not
forecast to regain tropical storm intensity or re-emerge over water.
WPC advisories will terminate when the threat of flash flooding has ended.