|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|
|Winter Storm Summaries||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, 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
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.
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
6 Hourly QPFs
These forecasts depict isohyets of accumulated precipitation of 0.01, 0.25, 0.50, and 1 inch expected in each six hour period. The SBF generates the six-hourly forecast for Day 1. Another QPF 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.
Excessive Rainfall Potential
This product outlooks the potential for flash
flooding based on current flash flood guidance issued by River Forecast Centers(RFCs). The graphic
and discussion are issued 4 times a day, at 0215Z, 0615Z, 1430Z, and 1815Z.
The forecast issued at 0615Z is valid for the upcoming 24-hour period ending at 12Z. This
forecast is updated at the 1415Z issuance. The 1815Z forecast is valid for the upcoming 00Z-00Z
24-hour period. This forecast is then updated at 0215Z.
Three categories are used to indicate flash flood threat:
An Exceeding area outlines a geographical area to the right of a solid arrow drawn using station identifiers as anchor points where conditions are favorable for rainfall to exceed flash flood guidance values.
An Approaching area is generally used in winter-time or for synoptic-scale situations where flash flood guidance values are relatively high but accumulated precipitation amounts from a larger scale system may lead to run off problems. It outlines an area with a dashed arrow and labeled "APCHG".
An Isolated area is mainly intended for
use in the Western U.S. and during the warm season where events tend to of a smaller scale and more isolated.
It is drawn with a dashed-dotted line
and labeled with "ISOLD".
Other labels on the Excessive Rainfall Graphic:
"SEE FOUS 30/CCCQPFERD" over the area affected or with an arrow pointing to the general area of concern indicates that there is not sufficient cause or confidence to draw an area, but that the QPFERD discussion talks about the situation.
If the forecasters feel that the potential exists for 5 inches or more within the remainder of the outlook period it is depicted on the graphic with a hatched area.
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.
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)
The medium range graphical forecast products include:
With the exception of the total 5-day precipitation, 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 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 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.
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.
Effective January 18, 2005, this discussion is issued four times during each of the day and night shifts and describes the model forecasts through Day 3 (84 hours). Previous to this date, it was issued once per day/night shift and evaluated model forecasts only through Day 2 (60 hours). The discussion may also include evaluation of the latest short range ensemble forecasts (SREF).
The following table shows the deadline and content for each issuance.
|0230Z/1430Z||Evaluate NAM initialization|
|0400Z/1600Z||Review NAM trends from previous runs|
|0445Z/1645Z||Evaluate GFS initialization|
|0530Z/1730Z||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.
PLEASE NOTE: The Pacific Ocean analyses (East and West Pacific) are prepared by NCEP's Ocean Prediction Center.
Probabilistic Heavy Snow/Icing Forecasts
From mid-September through mid-May the WPC routinely issues heavy snow/icing outlooks and accompanying discussions for heavy snow out to 54 hours. They cover two separate 12 hour forecast periods, 06 to 18 hours (93S) and 18 to 30 hours (94S) and a 24 hr period, 30-54 hours (98S). The WPC has decided to modify its current heavy snow guidance product in an effort to make it more useful to those making decisions based on this forecast. This is described in the Public Information Statement. The confidence levels used can be considered comparable to the probability of occurrence of the forecasted parameter. For example:
At times the forecasters may use only one or two isolines for the forecast. This simply implies low or low to moderate forecaster confidence in the occurrence of the forecasted parameter. One should expect the confidence to lower as the forecast is extended out in time. To gain further insight into this forecast, please read the Heavy Snow/Icing Discussion (HSD) that accompanies this graphical product.
Short Range Forecasts (1-2 days)
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 DesksPlease click here to find out more about this WPC function.
The Selected Cities Summary 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 Summary covers 158 cities in the United States, Puerto
Rico, and the Virgin Islands. It is composed of four parts with the last
part including the observed maximum and minimum temperatures from around
the nation. Forecast data is obtained from NWS Coded Cities Forecasts (CCFs).
The Travelers Forecast Bulletin is an abbreviated version of the Selected Cities Bulletin. The Travelers Forecast, like the Selected Cities is transmitted twice daily, one hour before the Selected Cities, for thirty (30) cities in the U.S. The Travelers Forecast consists of three parts which contain the weather and high and low temperature forecasts extracted from CCFs.
Storm summaries are issued for significant large-scale winter storms which: