Experimental Snowfall Probabilities

New Information for Snow Accumulation Forecasts This Winter


The National Weather Service in Mount Holly, NJ has done a retrospective study of snowfall amounts for the past five winters and compared those amounts to what our forecasts said would occur. The study was done for Philadelphia, Allentown, and Mount Pocono in Pennsylvania, for Atlantic City in New Jersey, and for Georgetown in Delaware, and is based on the forecast 24 to 36 hours prior to the onset of snowfall. These five sites were chosen to represent climatologically distinct areas, therefore locations near an individual site would have similar probabilities.

So what are the results? This assessment allows us to issue a new product containing a set of probabilities for expected snowfall accumulation for each storm this winter. The probabilities will be issued as a Public Information Statement when sufficient snowfall accumulation is expected to occur to require the issuance of a winter storm watch over any portion of the Mount Holly service area. The WMO header for this product is NOUS41 KPHI. The AWIPS identifier is PHLPNSPHI. This product can also be viewed on the web by clicking here.

The text below is an example of how this product will appear: 

PUBLIC INFORMATION STATEMENT - EXPERIMENTAL SNOWFALL PROBABILITIES
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ

330 AM EST FRI NOV 01 2002

...THIS IS AN EXPERIMENTAL PRODUCT FOR WINTER 2002-2003...

THE PROBABILITIES GIVEN BELOW ARE BASED ON A COMBINATION OF 
STATISTICAL THEORY, OUR SNOWFALL FORECASTS OVER THE PAST FEW WINTERS, 
AND THE MOST LIKELY SNOWFALL TOTALS FOR THIS POTENTIAL STORM.  THE 
PERIOD OF EXPECTED SNOWFALL IS XXXXXX INTO XXXXX NIGHT.

=====================================================================

                  PROBABILITY (PERCENT) OF REACHING OR EXCEEDING 
THE FOLLOWING STORM-TOTAL SNOWFALL AMOUNTS (INCHES)

====================================================================
STATION       2IN    4IN   6IN   9IN   12IN   18IN   24IN
====================================================================

MPO           >95    >95    90    80     50     15     5
ABE           >95     95    90    65     30     10    <5
PHL            95     90    75    40     20      5    <5
ACY            95     80    50    20     10     <5    <5
GED            85     50    25    10      5     <5    <5
=====================================================================
PROBABILITIES ARE ROUNDED TO THE NEAREST 5 PERCENT.

STATION IDENTIFIERS:

MPO - MOUNT POCONO, PA (MOUNT POCONO AIRPORT)
ABE - ALLENTOWN, PA (LEHIGH VALLEY INTERNATIONAL AIRPORT)
PHL - PHILADELPHIA, PA (PHILADELPHIA INTERNATIONAL AIRPORT)
ACY - POMONA, NJ (ATLANTIC CITY INTERNATIONAL AIRPORT)
GED - GEORGETOWN, DE (SUSSEX COUNTY AIRPORT)

MORE INFORMATION ON THIS EXPERIMENTAL PRODUCT CAN BE FOUND ON THE 
NWS MOUNT HOLLY INTERNET HOMEPAGE AT WWW.ERH.NOAA.GOV/ER/PHI.
	

In this example, the probabilities are based on the mid-range value of the expected snowfall for this storm. A snowfall forecast of 10 to 14 inches has a midrange value of 12 inches for Mount Pocono, 10 inches for Allentown, 8 inches for Philadelphia, 6 inches for Atlantic City, and 4 inches for Georgetown. Other storms would have a table with different values, depending on what is forecast to occur.

Who are some of the potential users of this information? Basically anyone who has to take some action based on the occurrence of certain amounts of snowfall. In the example above, if a school official in Georgetown needs to take action if two or more inches of snow is expected then the official knows there is an 85 percent probability that will occur, a Department of Transportation official in Atlantic City who needs to take a specific action when snowfall is expected to exceed 12 inches knows there is only a 10 percent chance that will occur with this storm, an emergency management in the Poconos planning for a winter sporting event knows there is an 80 percent probability that at least nine inches of snow will fall.

We hope these numbers will provide decision-makers with useful information on how to assess the risk with a particular snowstorm. We welcome comments on how useful and informative you find this product. We plan to update these numbers after the end of the upcoming winter season.

Please send any comments and questions to:

Gary Szatkowski
Meteorologist-In-Charge
National Weather Service Forecast Office
732 Woodlane Road
Mount Holly, NJ 08060

Phone: (609) 261-6600
Email: Gary.Szatkowski@noaa.gov