Product Information

QPF Overview Medium/Extended Range
24-h QPF Model Diagnostics/Biases
6-h QPF Surface Analysis
Excessive Rain International Desks
QPF Verification Winter Weather
Short Range Selected Cities/Travelers
Storm Summaries Tropical Public Advisories

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. WPC also works 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). 

A 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
Forecast Discussion (QPFPFD)  
Preliminary Day 1
0700Z  & 1900Z
Preliminary Days 1-3
0900Z & 2100Z
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 three consecutive (Days 1-3) 24 hour forecast periods ending at 0000Z (for issuance times between 1815Z and 2215Z) and 1200Z (for issuance times between 0615Z and 1015Z).

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 final products. 

 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
Preliminary     Final
91e 06-12Z N/A 0600Z
92e, 93e, 9ee, & 9fe 12-18Z, 18-00Z, 00-06Z, 06-12Z 0600Z 1000Z
9ge, 9he, 9ie, & 9je Same as above but for Day 2 0600Z 1000Z
9ke, 9le, 9oe, & 9ne Same as above but for Day 3 N/A 0800Z
91e 18-00Z N/A 1800Z
92e, 93e, 9ee, & 9fe 00-06Z, 06-12Z, 12-18Z, 18-00Z 1800Z 2200Z
9ge, 9he, 9ie, & 9je Same as above but for Day 2 1800Z 2200Z
9ke, 9le, 9oe, & 9ne Same as above but for Day 3 N/A 2000Z

These forecasts depict isohyets of accumulated precipitation of 0.01, 0.25, 0.50, 1 inch and greater (in 1 inch increments) expected in each six hour period. The SBF generates the six-hourly forecasts for Day 1, including the 00-06 hour update (91e). Another meteorologist prepares 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.

48-Hour Day 4-5 and 5-Day Total QPFs

The Day 4-5 Quantitative Precipitation Forecast (QPF) products are prepared by our medium-range forecasters twice per day, once at 7:00 AM EDT/EST (11Z/12Z) and again around 1:00 PM EDT/EST (17Z/18Z).

Both forecasts are valid from the beginning of Day 4 through the end of Day 5, with the later issuance offset by 12 hours. For example, a forecast prepared at 7:00 AM EDT/EST September 2, 2004 would be valid for the 48 hour period from 12Z September 5, 2004 through 12Z September 7, 2004. The forecast prepared at 1:00 PM EDT/EST September 2, 2004 would be valid from 00Z September 6, 2004 through 00Z September 8, 2004.

The 5-Day Quantitative Precipitation Forecast (QPF) products are created by adding WPC's 6-hour QPFs for Days 1-3 (a total of 12 6-hour periods) to a 48-hour forecast for Days 4-5 prepared by our medium-range forecasters. This forecast is issued twice per day, once at 7:00 AM EDT/EST (11Z/12Z) and again at 7:00 PM EST/EDT (23Z/00Z).

Both forecasts are valid from the beginning of Day 1 through the end of Day 5, with the later issuance offset by 12 hours. For example, a forecast prepared at 7:00 AM EDT/EST September 2, 2004 would be valid for the 120 hour period from 12Z September 2, 2004 through 12Z September 7, 2004. The forecast prepared at 7:00 PM EST/EDT September 2, 2004 would be valid from 00Z September 3, 2004 through 00Z September 8, 2004.

Excessive Rainfall Potential

The Excessive Rainfall graphic provides a forecast of the potential for flash flooding across the continental United States. As of October 5, 2004, the graphic displays 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 graphic is 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 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.

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.

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)
21 UTC 21 UTC - 00 UTC (27 hours)
QPFERD Discussion Required
03, 06, 15, 18 UTC
Same as associated graphics listed above
00, 12, 21 UTC

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/Extended (3-7 days)

Product Issuance Time
Preliminary Day 3-7 Fronts/Pressure Graphics 1315 UTC
Preliminary Day 3-7 Min/Max/PoPs 1400 UTC
Preliminary Discussion 1400 UTC
All Final Graphics 1900/1800 UTC (1400 EST/EDT)
Final Discussion 1930Z/1830 UTC (1430 EST/EDT)

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
  • A 48-hour QPF encompassing days 4 through 5
  • Total 5-day precipitation for days 1 through 5

All of these forecasts are generated twice per day.   A preliminary, unofficial, set of graphics is issued by 1400 UTC and are intended for inter-office coordination purposes only. The surface pressure patterns and fronts on the preliminary forecasts are currently only drawn for the continental U.S. The final set of graphics is issued by 2:00 pm ET, with the surface pressure patterns and fronts encompassing much of the Northern Hemisphere.

In addition to the graphical forecasts, the forecasters prepare two daily written discussions highlighting forecast reasoning and significant weather expected to impact the continental U.S. and Alaska during the day 3-7 time frame. One is available by 1400 UTC and accompanies the preliminary graphics, while the final narrative incorporates the latest model guidance and is issued at 2:30 pm ET. Forecasters also provide a separate discussion describing model differences and preferences across Hawaii.

Two meteorologists work this shift once per day. One produces the 3-7 day pressure systems/fronts and written discussions for the continental U.S. and AK, while the other prepares the 48-hour and 5-day QPFs, 3-7 day temperature and probability of precipitation forecasts, 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 NOGAPS model 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 consists of three sections:

1)  An evaluation of the analyses of the primary models (NAM, GFS)
2)  A review of model trends and biases.
3)  A description of model differences and preferences through 84 hours.

This discussion is issued twice during each of the day and night shifts, and may include evaluation of short range ensemble forecasts (SREFs) and other global models.

The following table shows the deadline and content for each issuance.

Issuance Time Content
0400Z/1600Z Evaluate NAM initialization and trends from previous runs
0530Z/1730Z Evaluate GFS initialization
Review GFS trends from previous runs
Discuss model differences and 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.  During the Atlantic hurricane season (June 1 - Nov 30), he or she coordinates with the National Hurricane Center (NHC) whenever a tropical cyclone is located west of 60 degrees West longitude in the Atlantic basin, Caribbean Sea, or Gulf of Mexico.

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.

Analysis Time Issuance Time Web Posting
00Z 0127Z 0147Z
03Z 0430Z 0450Z
06Z 0733Z 0753Z
09Z 1028Z 1048Z
12Z 1329Z 1349Z
15Z 1635Z 1655Z
18Z 1938Z 1958Z
21Z 2230Z 2250Z

PLEASE NOTE:  The Pacific Ocean analyses (East and West Pacific) are prepared by NCEP's Ocean 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 (1-2 days)

Product Issuance Time
12-24 hr progs 0430Z (Night Shift)
1630Z (Day Shift)
36-48 hr progs 0730Z (Night Shift)
1930Z (Day Shift)
Discussion 0715Z (Night Shift)
1915Z (Day Shift)

The "Basic Weather" forecaster is responsible for preparing the 12, 18, 24, 30, 36 and 48 hour surface forecasts. These are issued twice a day based on output from the 00Z and 12Z model runs. Emphasis is placed on the subjective modification of numerical guidance, especially with regard to timing and placement of coded fronts, high and low pressure systems, and instantaneous precipitation (coverage and type). The primary aim is to accurately depict the evolution of major weather systems that will affect the continental U.S. during the next two (2) days. 

* Please note that the transmission space limits the NFDPMDSPD narrative to 40 lines, thus this message is necessarily short and sometimes may not discuss significant weather conditions over the entire continental U.S. in detail and may not be able get into as much technical detail as users may like. 

International Desks

Please click here to find out more about this WPC function

Selected Cities/Traveler's Forecast

The Selected Cities Forecast is a highly visible product that is issued twice daily at 0100 and 1300 UTC.   It includes a tabular array of the previous day's maximum and minimum temperatures along with temperature and weather forecast for the next two days.  The Selected Cities Forecast covers 158 cities in the United States, Puerto Rico, and the Virgin Islands.  Cities are listed in alphabetical order and the product is divided into four sections to enhance readability.  Listed below are the range of cities in each section.
  • Part 1 -- Abilene, TX through Columbus, OH
  • Part 2 -- Concord, NH through Lexington, KY
  • Part 3 -- Lincoln, NE through St. Thomas, V.I.
  • Part 4 -- Salem, OR through Yuma, AZ
Part 4 includes the observed maximum and minimum temperatures from the continental U.S.   Forecast data is obtained from Coded Cities Forecasts generated at local NWS Weather Forecast Offices. 

The Traveler's Forecast is an abbreviated version of the Selected Cities Forecast.  The Traveler's Forecast, like Selected Cities, is transmitted twice daily, one hour before the Selected Cities.  It includes temperature and weather forecasts for the next two days for 30 cities in the United States.  The product consists of three sections; cities are listed in alphabetical order in each.   Forecast data is obtained from Coded Cities Forecasts generated at local NWS Weather Forecast Offices.

Storm Summaries

Storm 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:

  • 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.