The mission of the WPC is to provide forecast,
guidance, and analysis products and services to support the daily public
forecasting activities of the NWS and its customers, and to provide
tailored support to other government agencies in emergency and special
situations.
A brief history of the WPC.
View a slide presentation describing the WPC in more detail, including:
- Vision and mission
- Partners and customers
- Products and services
- Value added by WPC forecasters
- WPC plans
The primary functions of the WPC:
Quantitative Precipitation and Winter Weather Forecasts
The QPF desk prepares and issues forecasts of accumulating (quantitative) precipitation, heavy rain,
heavy snow, and highlights areas with the potential for flash flooding.
The basic QPF products are primarily directed to the NWS forecast offices and are available
on the Internet for public use. The heavy snow forecast products, in association with the short-range
public forecast products (described below), serve as a coordinating mechanism for the NWS winter storm watch
and warning program. Through a continuous watch for excessive rainfall, heavy snow, and winter storms,
this desk ensures that the highest quality forecast products are constantly available.
The QPF desk is co-located with the National Environmental Satellite Data and Information Service (NESDIS),
and together they comprise the National Precipitation Prediction Unit (NPPU). NESDIS meteorologists
prepare estimates of rainfall and current trends based on satellite data, and this information is used by the QPF
short term forecaster as part of the input for individual 6-hourly forecasts that cover the next 12 hours.
With access to WSR-88D/Doppler radar data, satellite estimates, and NCEP model forecast data
as well as current weather observations and WPC analyses, the forecaster has the latest data
for use in preparation of short-range precipitation forecasts. Meteorological reasoning discussions are
regularly written and issued with the forecast packages to explain and support the forecast.
Winter Weather Forecasts
The Weather Prediction Center (WPC) Winter Weather Desk issues heavy snow and icing
operational products. These products provide support to the NWS winter weather
watch/warning/outlook program. These forecasts are for the contiguous United States
(CONUS) and are routinely issued from September 15 to May 15. Graphical forecasts
are issued twice daily at 0900 UTC and 2100 UTC (4AM/PM EST respectively), although
updates may be warranted by rapidly changing conditions.
The Winter Weather Desk issues probabilistic heavy snow and icing guidance products
for Days One, Two, and Three. The forecasts represent the probability that freezing rain
or combined snow/sleet accumulations will meet specific criteria in a 24-hour period.
These products are issued in probabilistic form to better represent the forecast uncertainty
associated with a particular event. Specific (deterministic) accumulations for a
particular location in the United States can be obtained from the National Weather Service home page.
The probabilistic graphics combined with the deterministic forecasts provide a user two elements;
the most likely amount expected from an event and a sense of "what's possible" in terms of accumulations.
The Winter Weather Desk produces a heavy snow and icing discussion that provides the
meteorological reasoning for the 24-hour probabilistic heavy snow and icing guidance graphics
for Days One, Two, and Three. This text message is used by internal and external clients
including NWS field offices, Department of Homeland Security, FEMA, the White House, Department of Commerce,
FAA, and the general meteorological community (private sector and the media).
Short Term Forecasts (days 1-2)
The short range forecasters are responsible for preparing forecasts for the time period
of 12 to 48 hours. These products are issued twice daily using guidance from the
NWS's global forecast system model (GFS) and North American Mesoscale model (NAM), as well as
models from the European Centre for Medium Range Weather Forecasting (ECMWF), the United Kingdom's
Meteorology Office (UKMET), the Canadian model, and ensembles. Coordination with the
surface analysis, model diagnostics, quantitative precipitation, winter weather, and tropical forecast desks
is also performed during the short range forecast process. The short range forecast
products include surface pressure patterns, circulation centers and fronts for days 1 and 2, and a
depiction of the types and extent of precipitation that are forecast at the valid time of the map.
In addition, meteorological reasoning discussions are written on each shift and issued with the forecast
packages that highlight forecast reasoning and significant weather across the Continental United States.
Medium-Range (days 3-7) Public Forecasts
The medium range forecasters are responsible for preparing forecasts for days 3 through 7.
These forecasts are issued twice per day (preliminary and final issuances) using guidance from the NWS
medium range forecast model (GFS) as well as models from the European Centre for Medium Range Weather
Forecasting (ECMWF), the United Kingdom's Meteorology Office (UKMET), Canadian model, the Navy NAVGEM model, and ensembles.
The medium range forecast products include: 1) surface pressure patterns, circulation centers and fronts
for days 3-7; 2) daily maximum and minimum temperatures and anomalies for days 3-7; 3) Probability
of Precipitation in 12 hour increments for days 3-7; and 4) total 5-day precipitation for days 1 through 5.
In addition, a narrative is issued for each set of forecasts highlighting forecast reasoning and significant
weather over the Continental United States and Alaska. Separate forecasts,
similar to the 5-day mean products, are prepared for Hawaii.
Alaska Medium-Range (Days 4-8) Public Forecasts
The Alaska medium range forecasters review the latest deterministic and ensemble model guidance
(e.g., GFS, Global Ensemble Forecast System, or GEFS; Canadian GEM Global; Canadian GEM Ensembles;
ECMWF; ECMWF Ensembles; Navy NAVGEM; UKMET; and the North American Ensemble Forecast System, or NAEFS) to
compose the most likely forecast for Alaska and surrounding areas valid on days 4-8.
The Alaska Medium Range product suite consists of:
- 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 surface fronts and pressures graphics and 500mb hPa heights are valid at 1200 UTC on forecast days 4-8.
Each of the grids are available downscaled to 5-km horizontal resolution.
Numerical Model Diagnostics and Interpretation
The purpose of the WPC Model Diagnostic Discussion is to provide objective information and subjective
interpretation concerning the current runs of the NCEP short range numerical models. The WPC model
diagnostic meteorologist prepares the Model Diagnostic Discussion twice per day corresponding to the 0000 UTC and
1200 UTC model runs. This narrative consists of three sections: 1) An evaluation of the initialization of
the NAM and GFS. The meteorologist compares the model analyses with available observed data
(radiosondes, aircraft, surface observations, satellite imagery, etc.), and notes features that do not appear
to be properly represented and have an adverse impact on the models' forecasts after the analysis time.
2) A review of model trends and biases. This section describes how the models are trending in their evolution and
progression of important features when compared to previous runs. This section also includes
any significant persistent errors that recent model forecasts have been making with respect to a particular feature or
over a general region. 3) A description of model differences and preferences.
The meteorologist reviews how the suite of models from the latest forecast cycle differ from each other
in their forecasts of significant features. Where differences exist, he or she takes into account other available models
from previous cycles (ECMWF, UKMET, Canadian, NAVGEM, ensembles, etc.) and the information in 1) and 2) above
to formulate a preferred model or model blend.
Surface Analysis
The WPC Surface Analysis is part of the NWS Unified Surface Analysis and a collaborative effort with
the Ocean Prediction Center (OPC) and the National Hurricane Center (NHC). The WPC focuses on the synoptic
and mesoscale features over North America, north of 31N.
The Surface Analysis itself is a manual analysis of surface fronts and pressure over North America and adjacent
oceans at three hour intervals. The Analysis utilizes a variety of weather data in addition to observations
of surface weather conditions, such as upper air observations, global satellite imagery, Doppler radar,
and model mass fields to ensure that the product is meteorologically consistent and of the highest quality.
Tropical Cyclone Forecasts
The WPC is the official back-up center to the National Hurricane Center (NHC). In this capacity, the
WPC is responsible for issuing all tropical cyclone products, including discussions, graphics and watches and warnings
that would normally be issued by the NHC for any tropical system in the Atlantic Ocean or eastern
Pacific Ocean basins, if NHC is unable to so for any reason.
During the tropical weather season which runs from May 15-November 30, the WPC has several other routine duties
pertaining to tropical weather forecasting. The first duty is to provide track forecast guidance
to the NHC whenever there is a tropical cyclone in the Atlantic Ocean basin west of 60W longitude.
As required, this guidance is provided to the NHC four times daily for use in the tropical cyclone package
issued by the NHC at 0300 UTC, 0900 UTC, 1500 UTC and 2100 UTC. The WPC also participates in the
Hurricane Hotline call with the NHC and other forecast offices and government agencies for tropical
cyclones in the Atlantic Ocean basin west of 60W longitude.
Another duty of the WPC tropical desk is to provide the rainfall statement for tropical cyclones that are
expected to make landfall. This statement is included in the Public Advisory issued by the NHC,
and is a forecast of expected rainfall amounts that will occur with the tropical cyclone.
Finally, the WPC tropical desk also has the responsibility for issuing Public Advisories whenever a
tropical cyclone has made landfall in the U.S. or adjacent parts of Mexico, has weakened below
tropical storm status, and the NHC is no longer issuing advisories on the system but the system is
still capable of producing flooding type rains. This WPC Public Advisory will continue to be issued
until the flooding rainfall threat is over. The advisory will contain information on how much rainfall
has occurred with a particular tropical system, and will also include forecast information on
the remnants of the system.
International Desks
The International desks have a variety of responsibilities, principally the training of foreign
visitors in the use of Numerical Weather Prediction products. The International desk routinely
hosts visitors from Central and South America and the Caribbean. Visiting meteorologists train,
and also generate forecasts for their own national centers, and assist WPC forecasters
with QPF related to tropical cyclones in Central America and the Caribbean.