Quantitative Precipitation Forecasts (QPFs)

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 32 x 32 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.).  The WPC also stores output from several consecutive runs of all of these models, allowing for trend analysis of model QPFs.  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.  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.  Together, the SAB and Day 1 QPF desk at the WPC are known as the National Precipitation Prediction Unit (NPPU).  This collaborative process makes WPC forecasts generally more accurate than any individual model, and consecutive versions of WPC forecasts do not differ as much as consecutive model forecasts sometimes do.
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.  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:
Valid Period
Issuance Time 
Preliminary     Final
Valid Period
Length of Valid Period
Night Shift*




Day 1
0600 Z

06 Z Today (Day 1) – 12 Z Tomorrow (Day 2)
30 hours
Day 1

1000 Z
12 Z Today (Day 1) – 12 Z Tomorrow (Day 2)
24 hours
Day 2
0600 Z
1000 Z
12 Z Tomorrow (Day 2) – 12 Z Day 3
24 hours
Day 3
N/A
0800 Z
12 Z Day 3 – 12 Z Day 4
24 hours
Days 4 and 5
N/A
0900 Z
12 Z Day 4 – 12 Z Day 6
48 hours
Days 6 and 7
N/A
0900 Z
12 Z Day 6 – 12 Z Day 8
48 hours
Day Shift




Day 1
1800 Z

18 Z Today (Day 1) – 00 Z Tomorrow Evening (Day 2)
30 hours
Day 1

2200 Z
18 Z This Evening (Day 1) – 00 Z Tomorrow Evening (Day 2)
24 hours
Day 2
1800 Z
2200 Z
00 Z Tomorrow Evening (Day 2) – 00 Z the Evening of Day 3
24 hours
Day 3
N/A
2000 Z
00 Z Day 3 – 00 Z Day 4
24 hours
Days 4 and 5
N/A
0900 Z
00 Z Day 4 – 00 Z Day 6
48 hours
Days 5 and 6
N/A
0900 Z
00 Z Day 6 – 00 Z Day 8
48 hours
* 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


The Excessive Rainfall Outlooks provide a forecast of the risk of flash flooding across the continental United States.  As of October 5, 2004, the graphics display the probability that precipitation will exceed the flash flood guidance values issued by the River Forecast Centers (RFCs).  A closed contour with an arrowhead delineates the probability forecasts, with risk areas defined to the right of the direction of the arrowhead.
 
On June 29, 2006, the probability categories were changed due to calibration studies conducted at WPC. The calibration for the excessive rainfall graphics are based on the frequency of events for which observed rainfall exceeded flash flood guidance values for a given risk category.  It is important to note that the probabilities assigned to each risk category are small numbers because the risk of flash flooding at any single location is relatively low, even when heavy rain is expected to fall somewhere in the vicinity.  When forecasters outline risk areas, however, they are expecting greater organization of excessive rainfall than would be observed under average conditions. As confidence of excessive rainfall increases the category respectively evolves from Slight to Moderate to High.

Risk Category
Probability of Rainfall Exceeding Flash Flood Guidance at a Point
Slight (SLGT) 
5-10%
Moderate (MDT) 
10-15%
High (HIGH) 
>15%

If the potential exists for precipitation exceeding guidance values, but the expected probability for a point location is less than 5%, WPC will place the words SEE TEXT over the threat area. This area will then be referenced in the Excessive Rainfall Discussion.
In addition, areas where maximum precipitation amounts may exceed five inches will also be indicated by a red contour and hatching.
If conditions are not expected to become favorable for flash flooding then "Rainfall Not Expected To Exceed Flash Flood Guidance" is appended to the center of the graphic.
On June 5, 2007, WPC began producing Day 2 and Day 3 excessive rainfall forecasts. These forecasts were declared operational on February 11, 2008. Aside from the valid periods, there are several differences between these products and the Day 1 forecasts: 
* Only two probability categories are defined - SLGT and MDT.
* Written discussion is provided in the routine Day 2/3 portion of the QPF discussion (QPFERD) rather than in a separate product.  Furthermore, a text-based representation of the threat region is created and can be accessed at the link below the graphic.
* Forecasts are issued only twice per day. See the table below for information regarding issuance and valid times.
Day 2 and Day 3 excessive rainfall forecasts are based on Flash Flood guidance for Day 1 and quantitative precipitation forecasts for Days 2 and 3, respectively.  There is no Flash Flood guidance issued for Days 2 and 3.  Forecasters must make an educated guess as to how the guidance will change (increase or decrease) in response to future precipitation amounts, for instance in the Day 1 period.  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 3 periods.  WPC forecasters examine various deterministic and ensemble models looking for synoptic patterns that favor organized areas of heavy rainfall and compare this to the Day 1 Flash Flood Guidance before depicting a threat area.
Note:  Flash Flood Guidance values incorporate soil type, land coverage, and a host of other factors in an attempt to describe the rain rate necessary to yield significant surface runoff and flash flooding over a given area.  The River Forecast Centers issue guidance values for 1-, 3-, and 6-hour periods.  Flash Flooding is considered to be caused by rainfall occurring in 6 or fewer hours, whereas longer duration rainfall represents areal flooding or inundation. The WPC excessive rainfall products focus specifically on flash flooding. 
Schedule of Excessive Rainfall Outlook Issuance:
Day-1 Excessive Rainfall Outlooks (graphic and associated discussion) are issued four times per day: 03, 06, 15, and 18 UTC. Day-2 and Day-3 versions are issued twice daily.  The valid times varies as noted in the table below. For Day-1 the 03 Z and 15 Z issuances are valid for 21-hour periods, while the 06 Z and 18 Z issuances are valid for 30 hours.  Unscheduled, event-driven updates may be issued as needed.
Valid Period
Issuance Time
Valid Period
Day 1
0300 Z
03 Z – 24 Z (21 hours)
Day 1
0600 Z
06 Z – 12 Z (30 hours)
Day 1
1500 Z
15 Z – 12 Z (21 hours)
Day 1
1800 Z
18 Z – 00 Z (30 hours)



Day 2
0630 Z
12 Z Day 2 – 12 Z Day 3
Day 2
1830 Z
00 Z Day 2 – 00 Z Day 3



Day 3
0830 Z
12 Z Day 3 – 12 Z Day 4
Day 3
2030 Z
00 Z Day 3 – 00 Z Day 4


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 the forecasters’ confidence in both the available model output and the manual forecast.

Schedule of QPF Discussion (QPFPFD) Issuance:

 
Valid Period
Issuance Time 
Preliminary     Final
Night Shift


Days 1-3
0900 Z
1100 Z
Days 4-7*
N/A
0700 Z
Day 1 Excessive Rainfall Discussion

0230 Z, 0700 Z, and unscheduled updates
Day Shift


Days 1-3
2100 Z
2300 Z
Days 4-7*
N/A
1600 Z
Day 1 Excessive Rainfall Discussion

1500 Z, 1900 Z, and unscheduled updates
   

* Days 4/5 and 6/7 QPFs are produced by the WPC medium range desk which issues a medium range forecast discussion (PMDEPD).
Text-based coding of the 24-hour QPF contours
Included at the end of the QPF Discussion for Days 1-3 is a bulletin that describes the shape of the forecast isohyets using latitude and longitude pairs.  The order in which the pairs appear is important as this gives the contour or isohyet a clockwise direction as the points are plotted to create the contour.  The forecast value associated with a given isohyet is valid in the enclosed area to the right of the direction in which the contour is drawn.  The 0.50” contour described in the example below would represent an areal average of 0.50” forecast over parts of northwestern Colorado and southern Wyoming.
Example:  
 0.25 350731 349761 349789 347803 340819 327837 310854 297864 284874 
 0.50 404072 395071 390076 387081 389084 395085 402082 404076 404072 
The first field is the value of the contour; in this case the values are 0.25" or 0.50". The following fields are the latitude/longitude pairs for the contour. The first three digits of the pair are the degrees of latitude (in tenths of degrees North latitude). The last three digits of the pair are the degrees of longitude (in tenths of degrees West longitude).  If the fourth digit is less than 5, a leading "1" is added to indicate longitudes greater than or equal to 100° W. From the above message, the following table gives the decoded lat/lon pairs: 

  
Value: 0.25"
Value: 0.50"
35.0N, 73.1W
40.4N, 107.2W
34.9N, 76.1W
39.5N, 107.1W
34.9N, 78.9W
39.0N, 107.6W
34.7N, 80.3W
38.7N, 108.1W
34.0N, 81.9W
38.9N, 108.4W
32.7N, 83.7W
39.5N, 108.5W
31.0N, 85.4W
40.2N, 108.2W
29.7N, 86.4W
40.4N, 107.6W
28.4N, 87.4W
40.4N, 107.2W


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.
QPF Forecast Verification:
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 article by Olson, Junker and Korty in Weather and Forecasting.Volume 10, 1995, pgs. 498-511