# Blizzards in the Upper Midwest

Copyright Lawrence Burkett 2015, minor rev. 2018

## Results

*Annual Blizzard Frequency*

Table 1 summaries extremes and central tendencies calculated. In total, ninety-three blizzards impacted the Upper Midwest from 1980-2013. On average, 2-3 blizzards impacted my study each year. However, as indicated by Figure 2 there is a considerable spread in terms of the number of blizzards annually. For example, in 1980, 1982, 1989, and 1990, no blizzards occurred while in 1996 and 2013, nine blizzards occurred. Between 1980 and 2013 there were nine years with one blizzard, and twelve years with four or more blizzards. As a result and due to high variability in blizzard frequency, annual probabilities were not constructed.

Table 1. Total, measures of central tendency, and other relevant values pertaining to blizzard frequency in the Upper Midwest from 1980-2013 are indicated. Source: *Storm Data*, 1980-2013.

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Number of blizzards 93

Mean 2.7

Standard deviation 2.3

Median 2.0

Mode 1

Years with no blizzard 4

Years with 2-3 blizzards 9

Years with >3 blizzards 12

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The long-term trend in annual blizzard frequency in the Upper Midwest forms a positive slope as indicated in Figure 2. However, the low sum of squares value from the ordinary least squares regression performed offers little evidence for a long-term increasing or decreasing trend in blizzard frequency from 1980-2013 (i.e. R^{2 }= 0.09).

Figure 2. Annual blizzard frequency in the Upper Midwest from 1980-2013 is shown with black bars. Annual blizzard frequency averages 2-3, and ranges from no blizzards in 1980, 1982, 1989 and 1990 to nine blizzards in 1996 and 2013. The mean number of blizzards is shown in red, and the median number of blizzards is shown in green. The annual trend line, and ordinary least squares linear regression equation and sum of squares are shown in blue. Source: *Storm Data*, 1980-2013.

*Monthly Blizzard Frequency and Trends*

Figure 3 indicates monthly blizzard frequency in the Upper Midwest. Unlike annual blizzard frequency which varied considerably from year to year and exhibited no clear long-term trend, when blizzards occur, it does appear they tend to happen at certain times of the year. In particular, from Figure 3 notice how all blizzards from 1980-2013 occurred during the months of October through April with a peak in occurrence in January. To make predictions about the time of year blizzards are most common, the second-order polynomial is quite reliable (i.e. R^{2} = 0.83), although the polynomial does not capture the subtle changes in slope from month to month.

Figure 3. Monthly blizzard frequency in the Upper Midwest from 1980-2013 are shown with black bars. The second-order polynomial curve, equation and sum of squares for monthly blizzard frequency are shown in blue. Source: *Storm Data*, 1980-2013.

*Blizzard Duration and Area Impacted*

Table 2 shows extremes and several measures of central tendency for blizzard duration in the Upper Midwest. Blizzard duration in the Upper Midwest ranged from three hours to as long as two days. On average however, most blizzards typically range in duration from approximately 6 to 18 hours.

Table 2. Extremes and several measures of central tendency for blizzard duration in the Upper Midwest from 1980-2013 are shown. Source: *Storm Data*, 1980 2013.

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Shortest duration 3 h

Longest duration 48 h

Mean duration deviation 17.3 h

Median duration 14.0 h

Mode duration 12 h

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Table 3 shows extremes, and two measures of central tendency for area impacted by blizzards in the Upper Midwest. The area impacted by blizzards ranged from one county to the entirety the Upper Midwest; however cases of blizzards having impacted all of the Upper Midwest or one or two counties were quite rare. The average area impacted by a blizzard was on the order of roughly 50,000 km^{2}.

Table 3. Extremes and several measures of central tendency for area impacted by blizzards in the Upper Midwest from 1980-2013 are shown. Source: *Storm Data*, 1980-2013.

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Largest area 180,333 km^{2}

Smallest area 1,489 km^{2}

Mean area 60,145 km^{2}

Standard deviation in area 37,544 km^{2}

Median area 52,018 km^{2}

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*County Blizzard Frequency and Trends*

Figure 3 shows county blizzard frequency from 1980-2013. Blizzard frequency ranged from 46 or more blizzards along and west of line from Kittson County south through Wilkin County to fewer than 10 blizzards along and east of a line from Koochiching County south through Cass County in Minnesota – a difference of about 100 to 200 km from line to line. Walsh County in North Dakota had the highest frequency of with 67 blizzards, while Aitken, Cass, Crow Wing, and Pine counties in Minnesota had the lowest frequency with four blizzards.

Figure 4. County blizzard frequency in the Upper Midwest from 1980-2013 is shown using a six-class Jenks natural breaks classification algorithm. Sources: *ESRI*; *Storm Data*, 1980-2013.