Product Information
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and others interested in the details of WPC operations. |
WPC MissionThe 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:
* 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.
*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.
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:
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 article 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
The medium range graphical forecast products include:
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. The workflow consists of three shifts per day. Two meteorologists focus on the contiguous U.S., one during the overnight hours (0000-0900 UTC) and one on the day shift (1130-2030 UTC). They generate a set of North American 3-7 day pressure systems/fronts and 500mb forecasts, 3-7 day sensible weather grids, four 24-hour QPFs covering Days 4-7, winter weather outlooks for Days 4-7, and a discussion. In addition, the overnight forecaster prepares a Hawaiian discussion focusing on numerical forecast guidance and significant weather threats. The third meteorologist, who works from 1500-0000 UTC primarily focuses on Alaskan forecasting. See the Alaska Medium Range section below for more information. All forecasters 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.
Storm SummariesStorm 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:
Tropical Public AdvisoriesThe 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.
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