About WPC Winter Weather Forecast Products

PRODUCT DELIVERY SCHEDULE

The WPC Winter Weather Desk (WWD) is staffed two shifts a day
from Sept. 15 through May 15.

The WWD routinely produces 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.

Provide feedback on these products using our "Comments" link. 

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.
  • 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.


PROBABILITY LEGEND

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.

Beginning with the 2013-14 season, all of the probabilistic forecasts are generated automatically based on the WPC Winter Weather Desk forecasts of 24-h accumulations of snow and ice. Therefore, the forecasts depicted here are usually not edited directly by the forecaster. Please see the PWPF description for more information on how the probabilistic forecasts are generated.

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


LOW TRACKS GRAPHIC

These graphics depict 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, low positions from a multi-model ensemble are depicted with symbols. The typical constituency of the ensemble is:
    • 1 NCEP North American Mesoscale (NAM) run
    • 1 NCEP Global Forecast System (GFS) run
    • 1 European Centre for Medium-Range Weather Forecasts (ECMWF) run
    • 1 Canadian Global Model (CMC) run
    • 1 Naval Research Lab (NAVGEM) run
    • 1 United Kingdom Met Office (UKMET) run
    • 21 NCEP Short-Range Ensemble Forecast (SREF) members
    • 1 NCEP SREF ensemble mean
    • 1 NCEP SREF bias-corrected ensemble mean
    • 20 NCEP Global Ensemble Forecast System (GEFS) members
    • 1 NCEP GEFS ensemble mean
    • 1 NCEP GEFS bias-corrected ensemble mean
    • 1 ECMWF ensemble mean
    • 1 mean of the NAM and GFS runs
    • 1 mean of the GFS and ECMWF runs
    In each case, the latest available model run is utilized to extract the low positions. 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.