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A Generalized Approach to Precip Type Forecasting. Temperature of the Atmosphere . The key aspects are diagnosing the temperature of the entire atmosphere. Think about warm (above freezing) layers and cold layers (below freezing). What is the surface temperature?
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Temperature of the Atmosphere • The key aspects are diagnosing the temperature of the entire atmosphere. • Think about warm (above freezing) layers and cold layers (below freezing). • What is the surface temperature? • If it is well above freezing, rain is likely. • If it is near or below freezing, frozen (or freezing rain) is likely.
Examples of Freezing Rain • Note the large warm layer from ~900mb-750mb and the shallow cold layer near and at the surface. • Surface temperature is also below freezing.
Freezing Rain Once again, note large warm layer, (~900mb-680mb) shallow near surface cold layer, and subfreezing surface temperature.
Examples of Sleet • Note smaller (less depth) and cooler warm layer between ~830mb-760mb and deeper cold layer near the surface. • Surface temp is below freezing.
Sleet • Once again, note shallow and cooler warm layer and deep cold layer near surface. • Also note the rather cold temperatures at the surface.
Examples of Snow • Note the entire atmosphere is below freezing. • Temperatures are quite cold (<-20C) high in the cloud layer and at the surface. • Note: A cloud layer is located where the temperature dewpointspread is ≤4C.
Snow • Entire atmosphere is below freezing. • Cold high in cloud layer (<-20C) and near surface
Examples of Rain • Note large layer above freezing and surface temp is well above freezing.
General “Rules of Thumb” • Large warm layer, (>3C) shallow near surface cold layer, and surface temp below freezing = freezing rain. • Small and cooler (1-3C) warm layer, deep near surface subfreezing layer, and surface temp <0C = sleet. • Entire atmosphere below freezing = snow • Exceptions! • It can snow when the surface temp is above freezing as long as it is below freezing just above the surface. • It can rain when the entire atmosphere is below freezing (when there is a shallow cloud layer close to the surface) • Large warm layer and surface temp above freezing = rain.
The 540 Thickness Line • Another very general “rule of thumb” • If the 1000-500mb thickness is below 540dm snow is possible. If it is above, rain is more likely. The thick blue line is the 540dm (5400m) line.