These graphics indicate the potential (probability) for a location to receive specific thresholds of accumulated snow or ice. 

• Snowfall - closed lines represent forecaster confidence (low, moderate, and high) that enclosed areas will receive equal to or greater than a specific threshold (4", 8" or 12") of snowfall in a 24 hour period.

• Freezing Rain - depicts the confidence 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.  Elevation criteria is not imparted for the 8" and 12" thresholds.

Specific (deterministic) accumulations for a particular location in the United States can be obtained from the National Weather Service home page. 

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 confidence levels used can be considered comparable to the probability of occurrence of the forecasted parameter as follows:

• LOW - 20% to 30% chance of occurrence within the outlined area.
• MODERATE (MDT) - 30% to 70% chance of occurrence within the outlined area.
• HIGH - 70% chance or greater of occurrence within the outlined area.

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 Discussion (HSD) that accompanies these graphical products.

• This graphic depicts the forecast location and central pressure of significant surface lows impacting the 48 contiguous United States in 12 hour increments out to 72 hours into the future.

• The low position and track forecast by the meteorologist at NCEP WPC is depicted in black. 

• There are no less than 20 different computer model forecasts of low position for a given time period available to the WPC forecaster - these are depicted with symbols.

• 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.   This is very similar to how the NCEP Tropical Prediction Center's National Hurricane Center conveys the uncertainty with tropical systems.

• Generally the spread of symbols will increase with increasing forecast hour.