The National Weather Service (NWS) defines blizzards as areas of considerable falling and/or blowing snow with sustained wind speeds of 35 miles per hour or greater for an extended period of time (3 hours or greater) in which visibility is frequently reduced to less than a quarter of a mile. Using this operational definition, blizzard occurrence by county was collected from Storm Data for the 1959/60 through 2013/14 winter seasons (n = 55). The conterminous U.S. recorded 714 blizzards, averaging 13 blizzards per season for the entire record; however, the overall trend in blizzard activity has increased. In comparison with the 1960-1994 period, mean annual blizzard frequency in the past two decades has more than doubled (mean = 19) with more blizzards occurring outside the traditional October to March season. Blizzards have occurred somewhere in the contiguous U.S. in all months except August and September. In addition to the positive blizzard trend, blizzard frequency has an apparent cyclical pattern with a peak every 11-14 years with a secondary peak every 4 years. However, a spectral analysis showed the peaks to be statistically insignificant.
The apparent blizzard periodicity mimics the approximately 11-year sunspot cycle and incited investigation into the potential linkage. A Pearson's correlation analysis between seasonal sunspot totals and blizzard frequency displayed a weak, albeit statistically significant inverse relationship (r = -0.30, p = 0.03). The correlation coefficient and associated significance level increases (r = -0.40, p = 0.01) when the 1960s are removed from the dataset, a time period with less certainty in the blizzard counts. Blizzard counts then usually increase (decrease) during periods of low (high) sunspot activity.
Blizzard activity is strongly concentrated in the northern Great Plains, particularly in the Dakotas and western Minnesota (also known as the “blizzard zone”). Nearly all 119 counties in North and South Dakota and 39 counties in western Minnesota average at least 1 blizzard or more per year (i.e., at least 55 blizzards occurred during the 55-year study period). Outside the blizzard zone and its periphery, blizzard totals are generally between 1-11 blizzards per county with higher frequencies generally in the northern and high altitude locations with a few exceptions. Only six states in the conterminous U.S. (Alabama, Florida, Louisiana, Mississippi, South Carolina, and Tennessee) do not have any reported blizzards for their counties in the Storm Data publication database.
The spatial coverage of blizzards displays a strong interannual variability. The average area affected by an individual blizzard over the study period is 83,474 km2 (or 32, 229 mi2), about the size of South Carolina. Blizzards sizes range from a localized Appalachian storm in North Carolina of 1179 km2 on March 15, 2013 to the massive Midwestern blizzard of 1,054,779 km2 that occurred January 24-27, 1978. By decade, blizzard activity is most widespread in the 1970s and 1990s; the 2000s are also trending toward a wide geographic distribution. The other decades show blizzards generally confined to the blizzard zone of the northern Great Plains. Overall, the average blizzard area impacted decreases over time, most likely due to improved technology and reporting methods.