|QPF Overview||Medium/Extended Range|
|Excessive Rain||Alaskan Desk|
|QPF Verification||Model Diagnostics/Biases|
|Mesoscale Precip Discussion||Surface Analysis|
|Short Range||International Desks|
|Storm Summaries||Winter Weather|
|Tropical Public Advisories|
and others interested in the details of WPC operations.
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.). 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 accurae 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. 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 1800Z and 2030Z), and those issued on the night shift correspond to periods ending at 1200Z (for issuance times between 0600Z and 0830Z).
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:
* 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 at 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.
The risk of excessive rainfall is expressed both probabilistically and categorically according to the table below.
Flash floods are rare events at any one specific location, and, therefore, the point 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 the possibility of very organized heavy rainfall and flash flooding affecting numerous locations. In an ideal case, as confidence of the threat increases (usually as lead time decreases) the category may be updated from Slight to Moderate to High. In other cases, risk areas may be introduced quite suddenly and with short lead time, owing to the difficult nature of flash flood forecasting.
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"
WPC produces Excessive Rainfall Outlooks for Days 1, 2, and 3. Outlooks for Day 2 and 3 differ from those issued for Day 1 in the following ways:
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. 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. Forecasts of main stem river flooding are made by the RFCs out to 5 days 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 varies 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 the forecasters' confidence in both the available model output and the manual forecast.
Schedule of QPF Discussion (QPFPFD) Issuance:
* 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 plottd 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.
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 indiate longitudes greater than or equal to 100oW. From the above message, the following table gives the decoded lat/lon pairs:
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.
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)
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)
Model DiagnosticsThe 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.
|0445Z/1645Z||Evaluate NAM and GFS initializations
Compare NAM/GFS and other available model/ensemble guidance
Review model trends and biases as appropriate Discuss model preferences
|0645Z/1845Z||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.
PLEASE NOTE: The Pacific Ocean analyses (East and West Pacific) are prepared by NCEP's Ocean Prediction Center.
Probabilistic Heavy Snow/Icing Forecasts
Short Range Forecasts
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.
International DesksPlease click here to find out more about this WPC function.
Storm summaries are issued for significant large-scale storms which: