I was asked a while back how we predict snowfall… and I totally forgot to answer it. I’ll try and address it here. The answer is simple yet complex.
We get the liquid total, then apply “ratios” of expected snow-to-liquid to get a final amount. In our area, that ratio is usually around 10::1, so you just take the liquid pcp and x10… to get snowfall in inches. If it’s colder, you use a higher ratio… like 12::1 to 14::1. Some of our fluffiest snows we’ve gotten near 30::1, but these super-fluffy events are by nature usually small and not laden with moisture. To get a lot of moisture, you need to tap the Gulf and/or Atlantic, and those air masses are warmer and will consequently have lower ratios (a more normal 10::1, maybe 12::1) but more pcp. We rarely will get a foot of powdery snow. It’s happened, but not often. Sometimes, it’s the opposite… where our ratio drops to like 8::1 or 6::1, if it’s relatively mild yet still cold enough for snow.
Interesting side note: I looked up our liquid pcp estimate up here for Feb 9th, and it’s right around 0.40″… and I measured 4.0″ of snow, so the climo 10::1 held. This was likely more a function of the cold air aloft, cos we were in the upper 20s and lowers 30s for much of it.
Speaking of aloft…. It gets more complicated around here, where warm air aloft will lower your ratio by altering the snow structure where it’s created. It can also flat out kill the snow entirely by causing it to fall as sleet, freezing rain, or rain. I’ve seen cases where it was bitter cold at the surface but warmer aloft, and we had a dry but dense snow. In 2003, we had temperatures in the teens at the surface… but readings aloft were in the upper 20s and lower 30s, so despite the bitter cold at the ground, we ended up with a dry snow that was quite dense… and I believe our ratio was right around 10::1 despite the arctic air mass at the surface.
And, from my co-worker Brad Rippey…
The good folks at St. Louis University have studied thousands of frozen-precipitation events over the past several decades and have put together some cool graphics. I’ll only bore you with one of them. J
The mean snow-to-liquid ratio in the D.C. area is 10.5 to 1, but as Eric said we might occasionally see 30:1 or higher when the snow is very powdery.
A lot of our low-ratio events tend to be mixed snow and sleet – we call it a “slurpee.” If I recall, a pure sleet event tends to average around 3:1.
For future reference, here’s the list of all available WFO snow-liquid graphics via St. Louis University (and CIPS):
Snow to Liquid Ratios
It’s really interesting to compare some of the coastal and inland locations.