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Weather Prediction Center
College Park, MD

WPC Product Information

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The following provides technical information on the WPC for forecasters 
and others interested in the details of WPC operations.


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, NMC's Meteorological Operations Division, Heavy Precipitation Branch (HPB) and the Quantitative Precipitation Branch (QPB)] have been issuing QPFs since 1960. All QPFs incorporate the latest surface and upper air analyses, radar data, satellite data, and model guidance from the NAM, NGM, GFS and RUC displayed on N-AWIPS workstations. Forecasters also work in conjunction with meteorologists in NESDIS's Synoptic Analysis Branch (SAB) to obtain information regarding satellite trends, precipitation and moisture availability estimates. This co-location and collaboration between SAB and WPC is bureaucratically known as the National Precipitation Prediction Unit (NPPU). 

An WPC Senior Branch (lead) Forecaster (SBF) is on shift at all times. He or she is responsible for producing the day 1 24-hour precipitation forecasts, coordination of all WPC products (both internally and with other NWS offices), and center administrative operations after business hours. In addition to normal duties, the SBF participates in the East Coast winter storm and NHC hurricane conference calls regarding heavy precipitation, occasional unscheduled FEMA conference calls and numerous media interviews. 

24 Hour QPFs

Product            Issuance Time
94q (preliminary Day 1)   0600Z & 1800Z
94q (final Day 1)   1000Z & 2200Z
98q (preliminary Day 2)   0600Z & 1800Z
98q (final Day 2)   1000Z & 2200Z
99q (Day 3)   0800Z & 2000Z
95e (Day 1-5 total QPF)   1730Z
QPFPFD (prelim. Day 1)   0700Z  & 1900Z
QPFPFD (prelim. Days 1-3)   0900Z & 2100Z
QPFPFD (final Days 1-3)   1100Z & 2300Z

Isohyets of expected basin average rainfall of 0.01,  0.25 inch, 0.50 inch, 1 inch, and 1.50 inch and greater (in inch increments) are drawn for the 24 hour forecast period ending at 1200Z on both days 1, 2, and 3. 

An electronically generated bulletin, which describes the location of the forecast isohyets using latitude and longitude points, is transmitted at the end of the qpf discussion for the Day 1 through Day 3 finals. 

 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...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/long 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

6 Hourly QPFs
Product Valid Period Issuance Time
92e, 93e, 9ee, & 9fe 12-18Z, 18-00Z, 00-06Z, 06-12Z 1000Z
9ge, 9he, 9ie, & 9je Same as above but for Day 2 1000Z
9ke, 9le, 9oe, & 9ne Same as above but for Day 3 0800Z
9pe & 9qe 12-18Z, 18-00Z (forecasts extend 12 hours
beyond the above Day 3 period)
92e, 93e, 9ee, & 9fe 00-06Z, 06-12Z, 12-18Z, 18-00Z 2200Z
9ge, 9he, 9ie, & 9je Same as above but for Day 2 2200Z
9ke, 9le, 9oe, & 9ne Same as above but for Day 3 2000Z

These forecasts depict isohyets of accumulated precipitation in inches expected in each six hour period. The SBF generates the six-hourly forecast for Day 1. The QPF forecaster, known as the Day 2/3 six-hourly forecaster, generates the Day 2 and Day 3 six hourly products during 2 shifts per day (~9am-5pm/9pm-5am EST), issuing QPFs for eight consecutive six-hourly periods ending on synoptic hours. As of June 22, 2010, WPC began issuing two additional 6-hour QPFs for the Day 3½ period once daily at 1000 UTC (5am EST).

Excessive Rainfall Potential

The Excessive Rainfall graphics provide a forecast of the potential for 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 will delineate the probability forecasts, with areas of threat 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 FFG values for a given risk category.

Three probability categories are defined:

Slight (SLGT) 5-10%
Moderate (MDT) 10-15%
High (HIGH) >15%

If the potential exists for precipitation exceeding guidance values, but the expected probability 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 precipitation is expected to exceed five inches will also be indicated.

If conditions are not favorable or are not expected to become favorable for flash flooding then "Rainfall Not Expected To Exceed Flash Flood Guidance" is appended to the graphic.

The Day 1 graphics and associated discussion are issued four times per day, at 03, 06, 15, and 18 UTC. The valid times of the products varies as noted in the table below. The 03 and 15 UTC issuances are valid for 21-hour periods, while the 06 and 18 UTC issuances are valid for 30 hours. In addition, if significant changes to the outlook are necessary, the forecaster has the option to issue products at 00, 12, and 21 UTC.

The Day 1 graphics and associated discussion are issued four times per day, at 03, 06, 15, and 18 UTC. The valid times of the products varies as noted in the table below. The 03 and 15 UTC issuances are valid for 21-hour periods, while the 06 and 18 UTC issuances are valid for 30 hours. In addition, if significant changes to the outlook are necessary, the forecaster has the option to issue products at 00, 12, and 21 UTC.

On June 5, 2007, WPC began producing experimental Day 2 and Day 3 excessive rainfall forecasts. 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.
  • No five-inch QPF areas will be indicated.
  • There is no written discussion accompanying these products, but if an area is forecast, a text 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. There is no Flash Flood guidance issued for Days 2 and 3. Flash Flood guidance will change (increase or decrease) in response to future precipitation amounts. Increased variability in model guidance and the inability of the models to resolve mesoscale features result in greater uncertainty forecasting excessive rainfall in the day 2 and 3 period. 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 a threat region is depicted.

Note: The definition of exceeding flash flood guidance is broad. Flash flood guidance values depict the amount of rainfall necessary in a specific period of time to cause flash flooding over a given area. The River Forecast Centers typically issue guidance values for 1-, 3-, and 6-hour periods, and in some cases, 12- and 24-hour periods. WPC forecasters make a determination of the threat that precipitation will exceed any of the 1-hour, 3-hour, or 6-hour FFG values associated with the specific time interval in the valid time period of the forecast. This product is not intended as a specific forecast of flash flooding, but rather as a probabilistic indicator of rainfall amounts exceeding flash flood guidance over an area.

Issuance and Valid Times
Product Issuance Time Valid Time
94e 03 UTC 03 UTC - 00 UTC (21 hours)
06 UTC 06 UTC - 12 UTC (30 hours)
15 UTC 15 UTC - 00 UTC (21 hours)
18 UTC 18 UTC - 00 UTC (30 hours)
Optional 94e 00 UTC 00 UTC - 00 UTC (24 hours)
12 UTC 12 UTC - 12 UTC (24 hours)
12 UTC 12 UTC - 12 UTC (24 hours)
21 UTC 21 UTC - 00 UTC (27 hours)
98e 0630 UTC 12 UTC Day 2 - 12 UTC Day 3
1830 UTC 00 UTC Day 2 - 00 UTC Day 3
99e 0830 UTC 12 UTC Day 3 - 12 UTC Day 4
2030 UTC 00 UTC Day 3 - 00 UTC Day 4
QPFERD Discussion Required
03, 06, 15, 18 UTC
Same as associated 94e graphics listed above
00, 12, 21 UTC

Example 1:   A Day 1 (94e) issued at 1500 UTC June 5, 2007 will be valid from 1500 UTC June 6 - 0000 UTC June 7, 2007.

Example 2:   A Day 2 (98e) issued at 0630 UTC June 5, 2007 will be valid from 1200 UTC June 6 - 1200 UTC June 7, 2007.

Example 3:   A Day 3 (99e) issued at 2030 UTC June 5, 2007 will be valid from 0000 UTC June 8 - 0000 UTC June 9, 2007.

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 about the verification of WPC precipitation forecasts, read the article by Olson, Junker and Korty in Weather and Forecasting.Volume 10, 1995, pgs. 498-511. 

Medium Range (3-7 days)

Product Issuance Time
Day 3-7 Fronts/Pressure Graphics - CONUS 0330 UTC and 1530Z
Day 3-7 Fronts/Pressure Graphics - Northern Hemisphere 1900 UTC
Discussion 0700 UTC and 1600 UTC
Day 3-7 Min/Max/PoPs 0530 UTC and 1430 UTC

The medium range graphical forecast products include:

  • Surface pressure patterns, circulation centers, and fronts for days 3-7 into the future
  • Daily maximum and minimum temperatures and anomalies for days 3-7
  • Daily precipitation probabilities for days 3-7
  • Total 5-day precipitation for days 1 through 5

The surface pressure and fronts graphics are generated three times per day, while the Min/Max/PoP graphics are issued twice per day. The 0900 UTC and 1400 UTC sets of graphics are preliminary and unofficial, intended for inter-office coordination purposes only. The surface pressure patterns and fronts on the preliminary forecasts are only drawn for the continental U.S. The final set of graphics is issued by 1900 UTC, with the surface pressure patterns and fronts encompassing much of the Northern Hemisphere.

In addition to the graphical forecasts, the forecasters prepare three daily written discussions. The preliminary narrative highlights medium-range model differences and provides initial model preferences, and is available by 0900 UTC. This discussion is updated by 1400 UTC to include any possible changes to MOS (Model Output Statistics) and incorporate global model ensemble guidance. The final discussion incorporates the latest model guidance and updates forecast reasoning/model preferences. In addition, it highlights any significant weather expected to impact the continental U.S. and Alaska during the day 3-7 time frame. Forecasters also provide a separate discussion describing model differences and preferences across Hawaii.

One meteorologist works during the overnight hours, while two prepare the forecasts during the day shift (1030-1930 UTC). The overnight forecaster generates the initial set of preliminary 3-7 day pressure systems/fronts and discussion due at 0900 UTC. During the day shift, one of the meteorologists updates the preliminary forecasts and, late in the shift, issues a final discussion and a set of 3-7 day pressure systems/fronts. The other forecaster prepares the 5-day precipitation, 3-7 day temperature and PoP progs, and Hawaiian discussion. They routinely use output from the GFS, ECMWF, and UKMET medium range models and also consider the Canadian, the Navy's NOGAPS model, the MRFX, a semi-operationally experimental version of the GFS, and ensembles.

During hurricane season, at 1200 noon ET time on a daily basis since June 1, 1997, the medium range pressure forecaster 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. 

Model Diagnostics

The 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 four 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.

Issuance Time Content
0315Z/1515Z Evaluate NAM initialization
Compare the NAM to latest runs of other models/ensembles
0445Z/1645Z Evaluate GFS initialization
Compare GFS to NAM and other guidance
Review NAM/GFS trends and biases as appropriate Discuss model preferences
0530Z/1730Z Compare UKMET/Canadian global models to current guidance
Update model preferences as needed
0645Z/1845Z Evaluate latest ECMWF run
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. In addition, a coded surface bulletin is created that describes in text format the locations of highs and lows, fronts, and troughs.

Analysis Time Issuance Time Web Posting
00Z 0115Z 0145Z
03Z 0415Z 0445Z
06Z 0715Z 0745Z
09Z 1015Z 1045Z
12Z 1315Z 1345Z
15Z 1615Z 1645Z
18Z 1915Z 1945Z
21Z 2215Z 2245Z

PLEASE NOTE:  The Pacific Ocean analyses (East and West Pacific) are now being done by NCEP's Ocean Prediction Center and Tropical Prediction Center.

Probabilistic Heavy Snow/Icing Forecasts


The WPC Winter Weather Desk (WWD) is staffed two shifts a day from September 15 through May 15.  The WWD forecaster routinely releases updated forecasts twice daily at 0900Z and 2100Z (4am/pm EST or 5am/pm EDT respectively). Forecasts may be updated if warranted by rapidly changing situations.


These graphics indicate the probability (potential) for a location to receive specific thresholds of accumulated snow or ice. 

  • Snowfall - closed lines represent the probability (slight, moderate, and high) that enclosed areas will receive equal to or greater than a specific threshold accumulation (4", 8" or 12") of snowfall in a 24 hour period.
  • Freezing Rain - depicts the probability in the same manner and time period as snowfall, but with an accumulation threshold of .25" (one quarter of an inch) of  freezing rain.
  • Note the 4" threshold on the Snowfall Probability Graphic is drawn only for elevations less than 7,500 feet.  Elevationcriteria is not imparted for the 8" and 12" thresholds. 

    CAUTION ! The probability contours may appear to inordinately expand, contract or "jump" geographically after a scheduled update.  This is partially due to the frequency which the products are updated.  The "new" 24 hour period covers the last 12 hours of the previous issuance AND the next 12 hours. Additionally two model cycles have passed since the last scheduled issuance.

    Specific (deterministic) accumulations for a particular location in the United States can be obtained via the National Weather Service home page.  Note, at this site you will have to click the GRAPHICAL FORECAST tab prior to clicking a location on the map.

    The probabilistic graphics combined with the deterministic forecasts provide a user both the most likely amount expected from an event and the potential the event will produce accumulations in excess of specific thresholds.


The probability thresholds used are defined as follows:

  • SLGT - 10% to 40% chance of occurrence within the outlined area.
  • MODERATE (MDT) - 40% to 70% chance of occurrence within the outlined area.
  • HIGH - 70% chance or greater of occurrence within the outlined area.
At times the forecasters may use only one or two isolines for the forecast. This simply implies slight or slight to moderate probability for the forecasted parameter. 

To gain further insight into this forecast, please read the Heavy Snow Discussion (HSD) that accompanies these graphical products.


These graphics depicts the forecast location of significant surface lows impacting the 48 contiguous United States in 12 hour increments out to 72 hours into the future.  It is provided in two formats,  non-technical and technical

  • The non-technical version depicts the low position and track forecast by the NCEP WPC meteorologist in white.  Each low position is accompanied by a lead time (Eastern Time).  The circle around each low represents a 75% probability the observed low will be located within the circle. Note: The probability is derived using previous season's verification data. For reference, existing surface lows are depicted with a red marker without yellow circles. 
  • The technical version depicts the low position and track forecast by the NCEP WPC meteorologist in black.  Each low is accompanied by a forecast central pressure. Additionally there are no less than 35 different computer model forecasts of low position for a given lead time available to the WPC forecaster - these are depicted with symbols.  Both the central pressure and computer model forecasts are color coded according to lead time (Universal Time).  Together, the WPC forecast position of the low and computer generated position provide a user both the preferred position and track of the low and a sense of the uncertainty with the forecast.
  • For reference, existing surface lows are depicted with a red marker.  Surface lows can also be found on the WPC Surface Analysis. One can see the current location surface lows and the forecast path of both existing surface lows and surface lows expected to develop within three days time.
Note - winter weather is not always associated with significant surface lows.

Short Range Forecasts

Product Issuance Time
6 and 12 hour forecasts 0200Z (Night Shift)
1400Z (Day Shift)
18 and 24 hour forecasts 0430Z (Night Shift)
1630Z (Day Shift)
30, 36, and 48 hour forecasts 0730Z (Night Shift)
1930Z (Day Shift)
60 hour forecast 0800Z (Night Shift)
2000Z (Day Shift)
Discussion 0900Z (Night Shift)
2100Z (Day Shift)

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.

Alaska Medium Range (Days 4-8)

Product Time
Day 4-8 Fronts and Pressure Graphics 1800 UTC
Day 4-8 500 hPa Height Graphics 1800 UTC
Alaska Medium Range Discussion 1900 UTC
Day 4-8 Max/Min Temps and Probability of Precipitation Grids 2200 UTC

To accommodate a request for support from the NOAA National Weather Service (NWS) Alaska Region, the NCEP/Weather Prediction Center's (WPC) Alaska Medium Range Desk is issuing the following products:
  • 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
Surface graphics depict surface pressure patterns (from which surface winds can be inferred), high and low pressure circulation centers and fronts for days 4-8.  The 500 hPa height graphics display the general flow pattern forecast for days 4-8, and the gridded guidance depicts various important meteorological variables for the forecast period.

An WPC meteorologist interprets available deterministic and ensemble model guidance and collaborates with the WPC contiguous U.S. (CONUS) medium range forecasters.  The Alaska forecaster then uses the available model guidance and meteorological reasoning to depict the most likely scenario for days 4-8.   That meteorologist then composes a forecast discussion outlining deterministic and ensemble model differences, preferences and trends.   In addition, within the discussion the forecaster communicates confidence level, forecast uncertainty and any significant weather expected in the forecast period.

International Desks

Please click here to find out more about this WPC function.

Winter Storm Summaries

Storm summaries provide both a summary of 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:

  • 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.
If two or more separate storm systems are occurring simultaneously, a storm summary is issued for each individually.

Tropical Public Advisories

The 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.

Questions? Comments? Click here to send mail to the WPC.

Last Modified June 22, 2010