Forecasting Winter Precipitation

Outline Precipitation Type Snow Amounts Rain, Snow, Sleet, Freezing Rain Snow Amounts Heavy snow band Snow’s liquid equivalent

Precipitation Types Rain Snow Sleet Freezing Rain Passage through any warm layer thicker than ~600 ft. will melt (no return to crystal form) Sleet Rain freezes before contact with surface Freezing Rain Rain freezes upon contact with surface

Method For Evaluating Precipitation Type Determine if there will be any precipitation Model surface forecasts MOS QPF If yes, then perform vertical analysis of forecast temperature Skew-T / Stuve diagram

All Snow (below freezing throughout column

All Rain (melting layer at least 1200 ft. all the way to the ground)

Freezing Rain (freezing layer near surface not thick enough (need ~ 800 ft.)

Possible Sleet (Deep freezing and melting level)

Rain/Snow Mix If there is a warm layer near the surface between 600-1200 ft. thick, some snow crystals will melt while others may not Results in a rain/snow mixture

Another factor to consider Beware of dry air near the surface Precipitation will begin to fall from above and sublimate/evaporate before hitting the ground Moistens lower atmosphere Lowers temperature through evaporative cooling Can drastically alter the temperature profile, and thus precipitation type

Forecasting Snow Amounts Check model QPF Average conversion is 10:1, e.g. every 1” or liquid water equates to 10” of snow Must also factor in “wetness” of snow Snow that accumulates at temperatures near freezing will have a lower ratio (~5-9:1) Snow at very cold temperatures may have ratios as high as 30:1 Lake effect snow – up to 100:1!!!

Where Will the Heaviest Snow Fall?

Rules of Thumb 120-150 miles left of the surface low track Along the track of the 700 mb and/or 500 mb low Just north of the 164 height line at 200 mb Between the -2°C and -5°C 850 mb temperature Between the 534 and 540 1000-500 mb thickness contours K Index values of 10-20 (instability measure) Several others.......