Originally posted by (Matt) MVH

 I thought I would run through the general list of meteorological 'ingredients' that are needed to produce snowfall. I stress that the list below is a guide, a rule of thumb, the more of the criteria that are met the overall greater chance you have of seeing snowfall. Snow forecasting is tricky and minor local variations can make all the difference between rain or snow.

Upper Air Ingredients:

A precipitation maker, low pressure, trough, vorticity feature. Any cyclonic event that can produce precipitation, the more potent the cyclonic feature the better.

500mb Temps: At or below -25ºC (below -30ºC preferable)
850mb Temps: At or below -5ºC (below -7ºC/-8ºC preferable)

1000-850mb thickness: At or below 130dm
1000-500mb thickness: At or below 525dm (below 520dam preferable)

850mb theta-w (wet bulb potential temp): At or below 2ºC (below 0ºC peferable).

Wet Bulb Zero (WBZ): Height figures will vary, but gives a general/good idea as to what altitudes (200m, 300m etc) snow will fall down to. Calculated in conjunction with evaporational cooling.

3000ft or over: Always rain, snow highly unlikely.
2000ft-3000ft: Mostly rain, snow unlikely, low risk of sleet.
1000ft-2000ft: Snow more likely than rain/sleet.
below 1000ft: Mostly snow, light precipitation and low lying ground may still see sleet/rain.

Surface Conditions:

Surface Temperature: At or below 1.5ºC (below 0ºC preferable).
Surface Dew Point: **Highly variable**, At or below 0ºC.


As a general guide if 4 of the above criteria have been met then in general there would be a 50% risk of precipitation being snow. If 6 criteria had been met the risk would be 75% and obviously if all 8 have been satisfied than the general risk of snow would pretty much be 100%.