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About the Probabililty of Freezing Rain Forecasts
The images depicted on the Day 1-3 page represent forecast probabilities that freezing rain or drizzle will fall within the 3-hour period ending at the valid time of the product. There is no conditional dependency on accumulation of precipitation above a threshold greater than zero.
The probability of freezing rain is based on the precipitation type diagnosed using output from several medium-to-high resolution models along with output from the Short Range Ensemble Forecast (SREF) system.  The individual models include the Global Forecast System (GFS) model, the North American Mesoscale (NAM) model, and the global model at the European Centre for Medium Range Weather Forecasts (ECMWF). The relative frequency of occurrence of freezing rain in each 3-hour period is computed initially from the combined output of the GFS, NAM, and ECMWF; then, the frequency of occurrence is determined among the members of the SREF. The displayed probability at each point on the domain is a weighted average of 25% of the GFS-NAM-ECMWF frequency and 75% of the SREF frequency. The probabilities are displayed in percent.

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Page last modified: Tuesday, 08-Dec-2009 13:41:53 UTC