Predicting Snow Accumulation 101

Tuesday, January 11, 2011

John Ghiorse, GoLocalProv Meteorologist

When a winter storm forms and snow is predicted, the first question always asked is … “how much?” Determining that forecast can and often is a very complicated process but here’s the abridged version. The first thing I do is to try to pin down how much liquid precipitation (or melted snowfall) the storm is expected to produce across our area. That usually ends up being a range depending on the expected track, duration and intensity of the storm. That leads to the first potential pitfall. Let’s say I expect 1.00” to 1.25” of liquid, but if the storm eases 50 miles or so off my track or is not as intense as I expect it may only produce, say, 0.50” to 0.75” of liquid. Or it could go the other way … closer and more intense, the storm could produce 2.00" of liquid or more! And, of course, I have to take into account variations due to terrain, proximity to the Bay and coast and so forth.

Next, I try to calculate the expected temperature profile within the storm. That allows me to compute my liquid to snow ratio. Let’s say I expect that each inch of liquid will produce 10” of snow. So, in the above example storm I would predict 10” to 12.5” of snow. That leads to the next potential problem. If the air column turns out to be colder than I expect the ratio may end up being 20 or 30 to 1, thus we could end up with over 2 feet of fluffy snow! A warmer storm profile may give us a 5 to 1 ratio or a slushy 5” to 6” on the ground.

Other factors also come into play. The forecast can be messed up by snow mixing with or changing to sleet and rain for a time. Then there’s the location issue. Rhode Island’s hills are not very high, but most often in a given storm more snow falls there than along the coast. So, 12” in Foster may dwindle to 4” or 5” in Newport even with the same amount of liquid. This higher snow amount in Northwest Rhode Island often can be partially due to slightly greater lift from the terrain and less maritime warming effect from the ocean.

So, I give it my best shot and cross my fingers, hoping the myriad of ifs, ands and buts work out close to being what I predicted!
 

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