Snow versus Rain: Looking beyond the “Magic” Numbers

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Abstract
Compelling evidence suggests that various lower-tropospheric thickness calculations (e.g., 1000–850- and 850–700-mb thickness) should be considered a vital element of wintertime precipitation-type forecasts. The traditional 1000–500-mb thickness often does not adequately resolve lower-tropospheric temperature profiles, which are so critical in the determination of precipitation types. During a three-winter period (1989–92) for Pittsburgh, Pennsylvania, and Albany, New York, rain events were observed with 1000–500-mb thickness values as low as 5220 m and 850-mb temperatures as low as −7°C, both values of which are well below critical-thickness thresholds, traditionally used to delineate frozen versus liquid precipitation events. One particular parameter, the 850–700-mb thickness, was occasionally more valuable as a predictor than the 1000–500-mb thickness. Snow rarely occurred with a 850–700-mb thickness > 1550 m. This study uses surface and radiosonde data to generate an array of layer-thickness statistics, which are subsequently used to develop a standard deviation model for snow events at Pittsburgh and Albany. Precipitation-type forecasts are also strongly influenced by static stability and humidity profiles.

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