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 have been making Quantitative Precipitation
Forecasts since 1960. Quantitative Precipitation Forecasts, or QPFs, depict the amount of liquid
precipitation expected to fall in a defined period of time.
In the case of snow or ice, QPF represents the amount of liquid that will be measured when the precipitation
is melted. Precipitation amounts can vary significantly over short distances, especially when thunderstorms
occur, and for this reason QPFs issued by the WPC are defined as the expected "areal average"
(on a 32 x 32 km grid) in inches.
Methods for producing QPFs are similar to other meteorological forecasts. First, meteorologists
analyze the current state of the atmosphere. Then they use model forecasts of pressure systems, fronts,
jet stream intensity, etc., to form a conceptual model of how the weather will evolve.
The WPC has unique access to the full suite of operational and ensemble model guidance from modeling centers
in the U.S., Canada, and Europe (the foreign models are global models, so they also make predictions over
the U.S.). The WPC also stores output from several consecutive runs of all of these models,
allowing for trend analysis of model QPFs. Forecasters determine which models are showing a reasonable
amount of precipitation in roughly the correct place and time. Those choices serve as the starting
point for QPFs, but forecasters also make manual adjustments based on their experience.
WPC forecasters often engage in discussion with some of the 122 local National Weather Service Forecast
Offices, 12 River Forecast Centers, and other national centers such as the Storm Prediction Center
and National Hurricane Center. In fact, per a 2005 agreement, the WPC provides the rainfall forecast
(known as a rainfall statement) that the National Hurricane Center inserts into each tropical cyclone
advisory it issues. The WPC is also co-located with NOAA's National Environmental Satellite,
Data, and Information Services (NESDIS) Synoptic Analysis Branch (SAB). The SAB provides information
on satellite trends which helps refine short range QPFs. Together, the SAB and Day 1 QPF desk at the
WPC are known as the National Precipitation Prediction Unit (NPPU). This collaborative process
makes WPC forecasts generally more accurae than any individual model, and consecutive
versions of WPC forecasts do not differ as much as consecutive model forecasts sometimes do.
Technical Description of QPFs:
The QPF contours (isohyets) are drawn to encompass areal average amounts of 0.01, 0.25 inch, 0.50 inch,
1 inch, 1.50 inches, and 2.00 inches. Any values greater than 2.00 inches are drawn in one-inch
increments. It is important to note the valid time period when viewing each product.
Specifically, for the Day 1, 2, and 3 forecasts, QPFs are manually created for 6-hour periods and an
accumulated 24-hour total QPF is also issued. For the Days 4/5 and Day 6/7 QPF, forecasters manually
create a 48-hour accumulation of areal average rainfall. Computer programs then take advantage of
model forecasts of the timing of precipitation to break the WPC forecast down into 6-hourly QPFs.
Forecasts issued on the day shift correspond to periods ending at 0000Z (for issuance times
between 1815Z and 2215Z), and those issued on the night shift correspond to periods ending at 1200Z
(for issuance times between 0615Z and 1015Z).
Multi-day Accumulation Products:
Through simple addition of the various 24-hour and 48-hour QPFs, WPC is able to provide multi-day
accumulation forecasts. Days 1-3, Days 1-5, and Days 1-7 accumulations are made available,
and can be powerful planning tools for many applications.
Schedule of QPF Issuance:
||06Z Today (Day 1) - 12Z Tomorrow (Day 2)
||12Z Today (Day 1) - 12Z Tomorrow (Day 2)
||12Z Tomorrow (Day 2) - 12Z Day 3
||12Z Day 3 - 12Z Day 4
|Days 4 and 5
||12Z Day 4 - 12Z Day 6
|Days 6 and 7
||12Z Day 6 - 12Z Day 8
||18Z Today (Day 1) - 00Z Tomorrow Evening (Day 2)
||00Z This Evening (Day 1) - 00Z Tomorrow Evening (Day 2)
||00Z Tomorrow Evening (Day 2) - 00Z the Evening of Day 3
||00Z Day 3 - 00Z Day 4
|Days 4 and 5
||00Z Day 4 - 00Z Day 6
|Days 6 and 7
||00Z Day 6 - 00Z Day 8
* Forecasts made on the night shifts are verified versus observed rainfall. The valid time of 1200z to 1200z for
QPFs issued by the night shift coincides with the 24-hour observation cycle for National Weather Service
Cooperative Observers and many other rainfall networks such as CoCoRaHS.
All Times shown are in Zulu (Z), or Greenwich Mean Time, and correspond to the time in Greenwich, England,
as seen on a 24-hour clock.
Excessive Rainfall Outlooks
The Excessive Rainfall Outlooks provide a forecast of the risk of 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 delineates the
probability forecasts, with risk areas 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 flash flood guidance values for a given risk category. It is important to note that the probabilities assigned
to each risk category are small numbers because the risk of flash flooding at any single location is relatively low,
even when heavy rain is expected to fall somewhere in the vicinity. When forecasters outline risk areas, however,
they are expecting greater organization of excessive rainfall than would be observed under average conditions.
As confidence of excessive rainfall increases the category respectively evolves from Slight to Moderate to High.
||Probability of Rainfall
Exceeding Flash Flood
Guidance at a Point
If the potential exists for precipitation exceeding guidance values, but the expected probability for a point location
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 maximum precipitation amounts may exceed five inches will also be
indicated by a red contour and hatching.
If conditions are not expected to become favorable for flash flooding then "Rainfall Not Expected
To Exceed Flash Flood Guidance" is appended to the center of the graphic.
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:
- Only two probability categories are defined - SLGT and MDT.
- Written discussion is provided in the routine Day 2/3 portion of the QPF discussion (QPFERD) rather than in a separate
product. Furthermore, a text-based 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, respectively. There is no Flash Flood guidance issued for Days 2 and 3.
Forecasters must make an educated guess as to how the guidance will change (increase or decrease) in
response to future precipitation amounts, for instance in the Day 1 period. Increased variability in model
guidance and the inability of the models to resolve mesoscale features results in greater uncertainty in forecasting
excessive rainfall for the day 2 and 3 periods. 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 depicting a threat area.
Note: Flash Flood Guidance values incorporate soil type, land coverage, and a host of other factors in an attempt to
describe the rain rate necessary to yield significant surface runoff and flash flooding over a given area.
The River Forecast Centers issue guidance values for 1-, 3-, and 6-hour periods. Flash Flooding is
considered to be caused by rainfall occurring in 6 or fewer hours, whereas longer duration rainfall represents
areal flooding or inundation. The WPC excessive rainfall products focus specifically on flash flooding.
Schedule of Excessive Rainfall Outlook Issuance:
Day-1 Excessive Rainfall Outlooks (graphic and associated discussion) are issued four times per day: 03, 06, 15, and 18 UTC.
Day-2 and Day-3 versions are issued twice daily. The valid times varies as noted in the table below.
For Day-1 the 03 Z and 15 Z issuances are valid for 21-hour periods, while the 06 Z and 18 Z issuances are valid for 30 hours.
Unscheduled, event-driven updates may be issued as needed.
|| 03Z - 24Z (21 hours)
|| 06Z - 12Z (30 hours)
|| 15Z - 12Z (21 hours)
|| 18Z - 00Z (30 hours)
|| 12Z Day 2 - 12Z Day 3
|| 00Z Day 2 - 00Z Day 3
|| 12Z Day 3 - 12Z Day 4
|| 00Z Day 3 - 00Z Day 4
Forecast Discussions/Text Products:
National Weather Service forecast discussions have been used for decades to summarize our assessment of the upcoming
weather pattern and associated impacts. Discussions are also used to convey the forecasters'
confidence in both the available model output and the manual forecast.
Schedule of QPF Discussion (QPFPFD) Issuance:
|Day 1 Excessive Rainfall Discussion
||0230Z, 0700Z, and unscheduled updates
|Day 1 Excessive Rainfall Discussion
||1500Z, 1900Z, and unscheduled updates
* Days 4/5 and 6/7 QPFs are produced by the WPC medium range desk which issues a medium range forecast discussion (PMDEPD).
Text-based coding of the 24-hour QPF contours
Included at the end of the QPF Discussion for Days 1-3 is a bulletin that describes the shape of the forecast isohyets using
latitude and longitude pairs. The order in which the pairs appear is important as this gives the contour or isohyet
a clockwise direction as the points are plottd to create the contour. The forecast value associated with a given isohyet
is valid in the enclosed area to the right of the direction in which the contour is drawn. The 0.50" contour described
in the example below would represent an areal average of 0.50" forecast over parts of
northwestern Colorado and southern Wyoming.
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 the values are 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 indiate longitudes
greater than or equal to 100oW. From the above message, the following table gives the decoded lat/lon pairs:
Tropical Rainfall Statement
Per an agreement between the WPC and the NHC dated March 13, 2005, the WPC is responsible for providing NHC with the
wording for rainfall statements in all public advisories for tropical cyclones. The WPC provides rainfall
statements for active tropical systems that threaten land masses in the Atlantic and eastern Pacific basins.
The Tropical Rainfall Statement describes specific islands or continental regions expected to be impacted by heavy rain.
The statements include a range of expected areal average rainfall as well as the maximum amounts expected at isolated locations.
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 on
the verification of WPC precipitation forecasts, read the
by Olson, Junker and Korty in Weather and Forecasting.
Volume 10, 1995, pgs. 498-511.
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
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 1500 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 NAVGEM 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.