Blizzards in the Upper Midwest
Copyright Lawrence Burkett 2015, minor rev. 2018
In total, 93 blizzards impacted the Upper Midwest from 1980-2013. On the average, 2-3 blizzards were reported each year in the Upper Midwest. However, blizzard frequency from year to year varied considerably with 2-3 blizzards having occurred in only 26% or about one-quarter of the years studied, while 35% of years studied had four or more blizzards. Moreover, in four out of 34 years in my study, there were no blizzards, and in two years there were as many as nine blizzards; hence, notations of average or annual probabilities seems rather inappropriate. In addition, no linear trend was evident in blizzard frequency from 1980-2013. However, when blizzards did occur, they tended to occur most often from December-February when 66% of blizzards occurred, in a blizzard season that last from October-April. Typical blizzard duration in the Upper Midwest ranged from about 6 to 18 hours. However, some blizzards last as long as 48 hours while others were as short in duration as three hours; the minimum threshold for a blizzard in terms of time. Due to differences in reporting efficacy however, identifying specific hourly start and end times for all blizzards was not possible. Nevertheless, it is clear that most blizzards generally last on the order of several hours. The average area impacted by blizzards in the Upper Midwest was on the order of several tens of thousands of kilometers which is roughly one-quarter to one-half the size of the Upper Midwest. The largest area impacted by a blizzard covered the entire Upper Midwest while the smallest covered one county.
Differences in blizzard frequency from county to county in the Upper Midwest can primarily be explained by forest cover as evidenced in Table 4. Provided that blizzards in Upper Midwest occur most often during the passage of what Black (1971) referred to as Alberta lows during the blizzard favorable months of December-February, and to a lesser extent Colorado lows in other months, suggests that forests impeded the flow of air circulating through intense cyclones with falling or blowing snow as they tracked through the Upper Midwest, which agrees with authors in Finnigan and Brunet (1995) in Wind and Trees in that forests absorb air momentum, lowering wind speed. Supposing there had been a spatial reporting bias in Storm Data, it would be expected that winter storms would have correlated more strongly with percent forest cover, which was not the case. Also, it appears that the frequencies of winter storms are not related to the frequencies of blizzards; meaning that high or low winter storm frequency is not an indicator of blizzard frequency despite their regard as severe snowstorms. This is probably because blizzards, unlike winter storms, are defined on the basis of a wind speed near the surface which is more sensitive to obstacles such as forest cover which tend to slow air that would have otherwise been circulating rapidly through cyclones; hence, blizzards are much more frequent a hazard to flat or gently rolling landscapes such as cropland and prairie than in areas with substantial forest cover during the passage of intense cyclones associated with falling or blowing snow. Whether or not blizzard frequency is sensitive in part to other obstacles on the mesoscale such as hills, for example, might explain why I was unable to more strongly correlate blizzard frequency to forest cover since part of the Upper Midwest is located within a broad, flat river basin while other parts are located in hills. And indeed, on even finer scales, it is reasonable to consider the possibility that blizzards may be locally altered in terms of their severity by buildings, and other wind-reducing features such as shelter belts, fences, overpasses, and utility poles, for example, unless winds are high enough to remove such obstacles.
In summary, my findings stress the need for further refinement to blizzard climatology in the United States, and perhaps other parts of the world where blizzard or blizzard-like storms are known to occur. Aside from offering improvements to our understanding of blizzard frequencies annually and monthly, and providing new rough estimates of duration and area impacted, my study was successful in establishing a strong relationship between blizzards and forest cover; hence, my methods could be applied to other parts of the United States in particular wherever intense cyclones that are associated with falling or blowing snow occur in order to develop a blizzard susceptibility index.