Blizzards in the Upper Midwest
Copyright Lawrence Burkett 2015, minor rev. 2019
Mining Storm Data Publications
To determine blizzard frequency in the Upper Midwest, I used reports of blizzards in monthly Storm Data publications that are compiled by the United States National Weather Service using information from a variety of sources which include: 1) weather reporting stations; 2) county, state and federal emergency management officials; 3) local law enforcement officials; 4) storm spotters; 5) official damage surveys; 6) newspapers; 7) the insurance industry; and, 8) the general public. In each monthly Storm Data publication, I searched for the keyword “blizzard” in the “Character of Storm” column and recorded the date of the storm, and the counties reported to have had blizzard conditions; identical to Schwartz and Schmidlin (2002).
Using “Character of Storm” in Storm Data, I identified and assigned a 1 for blizzard occurrence or a 0 for non-occurrence by counties impacted using an Excel 2010 spreadsheet according to date. In cases where specific counties were not listed, a consistent logical method was used to assign counties to area impacted. In my spreadsheet, blizzards spanning several days were regarded as one continuous blizzard event, while two or more blizzard reports separated by more than three hours were regarded as multiple blizzard events by assigning a two or greater for those dates, however rare. Using the time column in Storm Data, the start and end times of blizzards were recorded in my spreadsheet to the nearest hour. In cases where specific hourly start and end times were not listed, times were estimated from text provided in the description of the event.
Annual Blizzard Frequency and Trends
To determine annual blizzard frequency, I organized blizzards in terms of the year they occurred using years that begin on August 1st and end on July 31st such that the midpoint of each year occurs during the winter in the Earth's northern hemisphere; hence, the period from August 1st, 1980-July 31st, 1981 would be regarded simply as 1980, for example. An important note to consider is that all blizzard reports included multiple counties, and as such, a ‘blizzard’ in this case refers to all counties impacted by that particular blizzard.
To describe blizzard frequency, I created a table that included calculations of the total number of blizzards in the Upper Midwest as well as several measures of central tendency such as the mean, standard deviation, median, and mode, and extremes. To determine long-term trends in blizzard frequency, I created a histogram from data organized in my spreadsheet and performed an ordinary least square linear regression to determine whether there was a linear increase or decrease in blizzard frequency. By regarding sum of squares (R2) values greater than 0.50 as meaningful, I determined whether blizzard frequency was very reliably changing; that is, to the extent that the ordinary least square linear regression is a reasonably good fit to the data.
Monthly Blizzard Frequency and Trends
Similar to how I organized annual blizzard frequency, I organized blizzards in terms of the month they occurred. Again, an important note to consider is that all blizzard reports included multiple counties, and as such, a “blizzard” in this case refers to all counties impacted by that particular blizzard.
To analyze annual blizzard frequency, I created a histogram in my spreadsheet and performed a regression by fitting the monthly blizzard frequency data to a second-order polynomial to determine trends in frequency since my data roughly exhibited the shape of a higher order polynomial with extreme values. By regarding R2 values greater than 0.50 as meaningful, I was able to determine whether or not blizzard frequency exhibited a pattern.
Blizzard Duration and Area Impacted
To crudely determine blizzard duration, I calculated the difference between the start and end times for each blizzard in my spreadsheet. In cases when blizzards on the same date had different hourly start and end times, I calculated the average start and end times for that date. To summarize my results, I created a table that included extremes and several measures of central tendency to allow for differences in reporting efficacy in Storm Data.
To obtain the area impacted by each blizzard in the Upper Midwest, a GIS shapefile of county boundaries that I obtained via ESRI was used to replace the 1 list in my spreadsheet with county area. Area impacted by blizzards was summarized with a table in my spreadsheet that includes extremes, and several measures of central tendency.
County Blizzard Frequency
To determine county blizzard frequency in the Upper Midwest area, I summed all reports of blizzards by county in my spreadsheet and loaded the sums into ArcMap 10.2. Using a GIS shapefile of county boundaries that I obtained via ESRI, I projected my map layer as a North America Lambert Conformal Conic and classified blizzard frequency using a six-class Jenks scheme.