1. Forecasting Winter Precipitation

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

3. 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

4. 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

5. All Snow (below freezing throughout column

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

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

8. Possible Sleet (Deep freezing and melting level)

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

10. 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

11. 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!!!

12. Where Will the Heaviest Snow Fall?

13. 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 mb thickness contours
K Index values of (instability measure)
Several others