WPC Mission

The Weather Prediction Center (WPC) provides forecast, guidance, and analysis products and services to support the daily public forecasting activities of the NWS and its customers, and provides tailored support to other government agencies in emergency and special situations. 

We are here to assist and be a resource for you. We are available 24 hours a day, 7 days a week to alert you to the potential for significant weather events dealing with heavy rainfall or snowfall, to discuss quantitative precipitation forecasts and model differences relating to general weather and precipitation forecasts, and to provide forecast guidance into the medium range period (days 3 to 7). Most of the forecasters at the WPC have extensive experience at quantitative precipitation, heavy snow and medium range forecasting. 

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 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, the National Water Center, 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 differ 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. 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 Days 4-7, forecasters manually create QPFs for 24-hour periods (for a total of four forecasts corresponding to Day 4, 5, 6, and 7, respectively). 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, WPC is able to provide multi-day precipitation 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	0530Z		06Z Today (Day 1) - 12Z Tomorrow (Day 2)	30 hours
Day 1		1000Z	12Z Today (Day 1) - 12Z Tomorrow (Day 2)	24 hours
Day 2	0530Z	1000Z	12Z Tomorrow (Day 2) - 12Z Day 3	24 hours
Day 3	0530Z	1000Z	12Z Day 3 - 12Z Day 4	24 hours
Day 4	N/A	0400Z	12Z Day 4 - 12Z Day 5	24 hours
Day 5	N/A	0400Z	12Z Day 5 - 12Z Day 6	24 hours
Day 6	N/A	0400Z	12Z Day 6 - 12Z Day 7	24 hours
Day 7	N/A	0400Z	12Z Day 7 - 12Z Day 8	24 hours
Day Shift
Day 1	1730Z		18Z Today (Day 1) - 00Z Tomorrow Evening (Day 2)	30 hours
Day 1		2200Z	00Z This Evening (Day 1) - 00Z Tomorrow Evening (Day 2)	24 hours
Day 2	1730Z	2200Z	00Z Tomorrow Evening (Day 2) - 00Z the Evening of Day 3	24 hours
Day 3	1730Z	2200Z	00Z Day 3 - 00Z Day 4	24 hours
Day 4	N/A	1600Z	00Z Day 4 - 00Z Day 5	24 hours
Day 5	N/A	1600Z	00Z Day 5 - 00Z Day 6	24 hours
Day 6	N/A	1600Z	00Z Day 6 - 00Z Day 7	24 hours
Day 7	N/A	1600Z	00Z Day 7 - 00Z Day 8	24 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 (EROs)

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 ERO is a highly collaborative product and benefits from the input of meteorologists and hydrologists among the WFOs, RFCs, and National Water Center.

The risk of excessive rainfall is expressed both probabilistically and categorically according to the table below.

Risk Category	Probability of Rainfall Exceeding Flash Flood Guidance Within 40 km (25mi) of a Point
Marginal (MRGL)	At Least 5%
Slight (SLGT)	At Least 15%
Moderate (MDT)	At Least 40%
High (HIGH)	At Least 70%

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.

The ERO is now being experimentally issued for Days 4 and 5, and those outlooks only include contours at the Slight and Moderate Risk levels. The risk categories are produced and labeled in ways that are consistent with the Day 1 to 3 ERO, with the same definitions, colors, and nomenclature. When no risk category is assigned for a given Day 4 or Day 5 outlook, there will be a text label placed on the map that says "The probability of rainfall exceeding flash flood guidance is less than 15 percent".

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.

Valid Period	Issuance Time	Valid Period
Day 1	0100Z	01Z - 12Z Day 1 (11 hours)
Day 1	0830Z	12Z Day 1 - 12Z Day 2 (24 hours)
Day 1	1600Z	16Z Day 1 - 12Z Day 2 (20 hours)
Day 2	0830Z	12Z Day 2 - 12Z Day 3
Day 2	2030Z	12Z Day 2 - 12Z Day 3 (an update to the earlier Day 2 forecast)
Day 3	0830Z	12Z Day 3 - 12Z Day 4
Day 3	2030Z	12Z Day 3 - 12Z Day 4 (an update to the earlier Day 3 forecast)
Day 4	0830Z	12Z Day 4 - 12Z Day 5
Day 4	2030Z	12Z Day 4 - 12Z Day 5 (an update to the earlier Day 4 forecast)
Day 5	0830Z	12Z Day 5 - 12Z Day 6
Day 5	2030Z	12Z Day 5 - 12Z Day 6 (an update to the earlier Day 5 forecast)

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 National Hurricane Center (NHC), National Water Center (NWC), and WPC, tropical rainfall and flooding statements for all public advisories for tropical cyclones, and key messages, are collaborated between WPC and NWC and provided to NHC. The statements are produced for active tropical systems that threaten land masses in the Atlantic and eastern Pacific basins, and they describe 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, along with potential impacts from the rainfall.

QPF 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.

Storm Summaries

Storm summaries are issued for significant large-scale storms which:

• 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

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Page last modified: Wednesday, 24-Jul-2024 12:01:03 UTC