The following provides technical information
on the WPC for forecasters
and others interested in the details of
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
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
|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
6 Hourly QPFs
|92e, 93e, 9ee, & 9fe
12-18Z, 18-00Z, 00-06Z, 06-12Z
|9ge, 9he, 9ie, & 9je
Same as above but for Day 2
|9ke, 9le, 9oe, & 9ne
Same as above but for Day 3
||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
|9ge, 9he, 9ie, & 9je
Same as above but for Day 2
|9ke, 9le, 9oe, & 9ne
Same as above but for Day 3
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. 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).
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 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:
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.
On June 5, 2007, WPC began producing Day 2 and Day 3 excessive rainfall forecasts. These forecasts were declared operational on February 11, 2008. Aside from the valid periods, there are several differences between these products and the Day 1 forecasts.
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
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.
- 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.
||03 UTC - 00 UTC (21 hours)
||06 UTC - 12 UTC (30 hours)
||15 UTC - 00 UTC (21 hours)
||18 UTC - 00 UTC (30 hours)
||00 UTC - 00 UTC (24 hours)
||12 UTC - 12 UTC (24 hours)
||21 UTC - 00 UTC (27 hours)
||12 UTC Day 2 - 12 UTC Day 3
||00 UTC Day 2 - 00 UTC Day 3
||12 UTC Day 3 - 12 UTC Day 4
||00 UTC Day 3 - 00 UTC Day 4
03, 06, 15, 18 UTC
|Same as associated graphics listed above
00, 12, 21 UTC
Mesoscale Precipitation Discussions
On April 9, 2013, WPC began providing short term guidance during
heavy rain events leading to a threat of flash flooding to the National Weather Service (NWS) Weather Forecast Offices (WFOs),
River Forecast Centers (RFCs), the media, emergency managers and interested partners. Guidance is given in the form of Mesoscale
Precipitation Discussions (MPDs), that are ideally issued 1-6 hours ahead of time, averaging an area equal to roughly half the
size of the state of Kansas. Each MPD consists of a graphic indicating the area of concern and any pertinent meteorological
features as well as a brief text discussion focused on the mesoscale features supporting the anticipated heavy rainfall.
The potential for flash flooding within the area of concern will be highlighted by one of three headlines:
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
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:
- Surface pressure patterns, circulation centers, fronts, and 500mb heights
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
- Two 48-hour QPFs encompassing Days 4-5 and Days 6-7
- 5-Day Total QPF for Days 1-5 and a 7-Day Total QPF for Days 1-7
The surface pressure and fronts graphics are generated three times per day; the 0330 UTC and 1530 UTC issuances display
features over the continental U.S. (CONUS) only, while the 1900 UTC issuance includes fronts and surface pressure fields for much of the Northern Hemisphere.
The 500mb forecasts and Min/Max/PoP graphics are primarily focused on the CONUS and issued twice per day.
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 NOGAPS 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)
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
|Day 4-8 Fronts and Pressure Graphics
|Day 4-8 500 hPa Height Graphics
|Alaska Medium Range Discussion
|Day 4-8 Max/Min Temps and Probability of Precipitation 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.
- 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 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.
||Evaluate NAM and GFS initializations
Compare NAM/GFS and other available model/ensemble guidance
Review model trends and biases as appropriate
Discuss model preferences
||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.
The Pacific Ocean analyses (East and West Pacific)
are prepared by NCEP's Ocean
Probabilistic Heavy Snow/Icing Forecasts
PRODUCT DELIVERY SCHEDULE
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.
|PROBABILITY GRAPHICS FOR SNOW AND FREEZING RAIN
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:
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
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.
To gain further insight into this forecast, please read the Heavy
Snow Discussion (HSD) that accompanies these graphical products.
LOW TRACKS GRAPHIC
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.
Note - winter weather is
not always associated with significant surface lows.
- 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.
Short Range Forecasts
|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)
||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.
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:
If two or more separate storm systems are occurring simultaneously, a storm summary is issued for each individually.
- 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.
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.