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Snowfall variance in the northeastern United States: Looking at what defines a "normal" winter

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Thursday, 27 January 2011
Snowfall variance in the northeastern United States: Looking at what defines a "normal" winter
Paul, O.G. Heppner, Global Science & Technology, Inc., Greeenbelt, MD

The public pays significant attention to seasonal snowfall, particularly when the media and broadcast outlets make the “winter outlook.” Seasonal snowfall is often framed relative to the seasonal average. If the average snowfall in an area is 20”, the public often perceives that a winter is unusual if the snowfall deviates from that value.

In reality, significant differences are the norm. At Philadelphia, for example, the standard of deviation is 13” on an average of 20.” In this case, it is statistically ‘normal' for winter snowfall to be within the 7 to 33 inch range at Philadelphia. A winter snowfall of 10” may be lower than average, but it is not an unusual occurrence. Media and broadcast outlets that make a “winter outlook” should consider framing it in the context of standard of deviations, rather than seasonal averages.

30-year snowfall totals across the northeastern United States are examined, with standard of deviations computed at each location.

A measure of how large the standard of deviation is relative to the mean is given by the coefficient of variation (Cv), which shows the normalized dispersion of a probability distribution. The greater the Cv value, the greater the dispersion or variance of the sample (flatter bell curve). The 30-year snowfall distributions in the Northeastern United States were examined for their Cv values. This study presents those Cv values and shows which stations experienced large deviations relative to their statistical means.

Although interior Northeast USA stations have large standard deviation values, they show less variance (smaller Cv) when compared to the coastal plain stations. The interior stations of New York and Vermont had the smallest Cv values. All of the coastal plain stations (Washington through Boston) had the largest Cv values. It is interesting to note that even though Boston is much further north than Washington, Boston had the same Cv value The greatest Cv values fall along and to the right of the “Fall Line”, which demarcates the Appalachian mountains and the coastal plain.

The largest Cv value (0.78) was at Baltimore, which indicates a very large spread in the members and very flat bell curve. Baltimore can have winters with only a few inches of snow and that is considered ‘normal' statistically, just as it is if Baltimore has a 30-inch winter snowfall. Relative few winters in Baltimore cluster near the climatological average for seasonal snowfall.

Because interior Northeast USA stations have small Cv values, the bell curve is much steeper. There is less variation relative to the mean in the interior. The consistency in the smaller Cv values for the interior Northeast USA stations may indicate that ocean proximity is very influential on snowfall variation. For example, Boston has far greater Cv than Albany, yet both are at similar latitude, similar elevation, and only 150 miles apart. Moreover, Boston has the same mean snowfall (40”) as Pittsburgh, but the Cv is much larger at Boston, even though this station is several degrees latitude farther north. In lay terms to the general public, these data suggest that even though storm tracks greatly influence snowfall during any given winter, the amount of snowfall at the coastal plain stations shows much greater variability, as measured by Cv, than interior stations. At all stations, winter snowfall that is within the standard of deviation is ‘normal' statistically.