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 HARD TO FORECAST: SNOW ACCUMULATION

METEOROLOGIST JEFF HABY

Forecasting snowfall amounts can be a challenge. There are several reasons for this. One reason is that it is on the condition that snow falls in the first place. This makes it doubly difficult since not only does the forecaster have to forecast if precipitation will occur but on top of that it needs to be forecasted how much will occur. A second reason is that a small difference in the precipitation amount will have a big difference in the accumulation in inches of snow. For example, a 1/10th of an inch of liquid equivalent can produce 1 inch of snow while 4/10ths of an inch of liquid equivalent can produce 4 inches of snow. For a rain forecast, this difference is not that apparent but with snowfall accumulation it is very apparent. Another reason is due to the variety of liquid equivalent amounts that can occur. 1/10th of an inch of liquid equivalent can produce anywhere from less than a half inch of snow up to several inches of snow depending on the snow to liquid equivalent that occurs. A good typical value is a 10:1 ratio which means 10 inches of snow will occur from each inch of liquid equivalent. Depending on the temperature profile, the snow can be a fluffy 20:1 ratio or a wet snow with a 5:1 ratio. Thus, it is important to forecast the temperature profile in order to determine how fluffy or dense the snow will be. Another difficulty is compaction which means the snow becomes denser each hour the snow is on the ground. This occurs when gravity and melting cause the snow to pack more densely. Thus for example, 6 inches of snow on the ground in the evening can compact to 4 inches by morning. Also, a warm ground temperature can melt snow and cause compaction. If the ground is too warm, then much of the potential accumulation can be lost to melting. The wind can add difficulty to snowfall measurements since the wind will drift and pile the snow to different depths in different places. This can make it difficult to determine a precise number of inches that has fallen. These various reasons make snowfall accumulation forecasts a challenge.