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? – 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 dewpoint spread 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 mb thickness is below 540dm snow is possible. If it is above, rain is more likely. The thick blue line is the 540dm (5400m) line.

What is the precip type?