What factors make snow totals so difficult to predict?

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Dear Tom,

Very often when we receive less snow than forecast, and the explanation is that dry air moved in and kept the totals down. What factors make snow totals so difficult to predict?

Bob McCraken, Milford, Michigan

Dear Bob,

Accurate snowfall predictions are one of the biggest challenges facing meteorologists in cold weather climates. Even though computer models are improving, correct snowfall forecasts remain a major challenge for both man and machine. Often, a prediction of the correct type of precipitation is as difficult to forecast as the amount of precipitation that will fall. The computer determines how much liquid will fall. Should snow be the favored precipitation type, the computer converts this forecast amount into snowfall totals. This conversion is based on many factors, including the atmospheric thermal and moisture structure, lift, and temperature change patterns. Usually, the best snowfall forecasts result when an experienced forecaster adjusts the computer guidance. Deciding where a narrow band of heavy snow will fall when a storm is more than 1,000 miles away is a daunting task, and since the heaviest snow tends to fall just north of the rain-snow line, a small error can mean the difference between a cold rain, a glazing ice storm or major snow. Even when a snow area is accurately depicted, it’s easy to under, or overestimate the total snowfall if the storm unexpectedly changes its speed, intensity, or moisture content.