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Who’s Had The Worst Winter? The Updated Winter Severity Index

31 Jan 2017, 2:16 pm

The Midwest Regional Climate Center (MRCC) has been updating its Winter Severity Index to shed some light on “who’s had the worst winter?”.  From the MRCC, “Winter seasons have significant societal impacts across all sectors ranging from direct human health and mortality to commerce, transportation, and education. The question “How severe was this winter?” does not have a simple answer. At the very least, the severity of a winter is related to the intensity and persistence of cold weather, the frequency and amount of snow, and the amount and persistence of snow on the ground. The Accumulated Winter Season Severity Index (AWSSI) was developed to objectively quantify and describe the relative severity of the winter season.”

The scores are specific for each region, because what would be an extreme winter in the South might be mild for the Upper Midwest. Nearly a dozen cities, mainly in the Northwest part of the country, have had an extreme Winter so far based on the AWSSI. Redmond, Oregon has had one of the most extreme Winter’s based on it’s percentile. Bismarck, North Dakota has the highest score on the map so far this year, with a total over 1100.

The MRCC lists the following for Goals of the AWSSI:

  • Objectively index winter conditions
  • Use commonly available data—max/min temperature, snowfall, and snow depth or precipitation
  • Create a historical database of AWSSI for any location with daily temperature, snow, and precipitation data
  • Allow comparisons of season to season severity at one location in the context of the climatology of that location or between locations
  • Use as a baseline to scale subjective impacts such as those to snow removal, commerce, and transportation
  • Apply to multiple users and their needs

The AWSSI is not limited to meteorological winter (December ‐ February) but is intended to capture winter weather from its earliest occurrence to its last. The winter season begins when the first of any one of the following instances occurs:

First measurable snowfall (>= 0.1 inch)
• Maximum temperature at or below 32°F
• December 1

The winter season ends at the last occurrence of any of the following:
• Last measurable snowfall (>= 0.1 inch)
• Last day with 1 inch of snow on the ground
• Last day with a maximum temperature of 32°F or lower
• February 28/29

Daily scores are calculated based on scores assigned to temperature, snowfall, and snow depth thresholds. The daily scores are accumulated through the winter season, allowing a running total of winter severity in the midst of a season as well as a final, cumulative value characterizing the full season. Accumulations of the temperature and snow components of the index are computed separately and then added together for the total index. This allows comparison of the relative contribution of each to the total score.

The AWSSI has been processed for 52 locations across the continental U.S. to provide a variety of locations in different climate regimes for analysis. The AWSSI is calculated for each season from 1950‐1951 to 2012‐2013. The seasonal data is then subject to quality control, and seasons missing data that would contribute 5% or more of the seasons AWSSI are removed . Averages and standard deviations are calculated for running accumulations of daily temperature and snowscores as well as the total AWSSI.

Quintiles of AWSSI scores were determined for each location. Descriptive categories were assigned to each quintile as follows:

AWSSI quintile categories

Some of the limitations:

  • Does not include wind (e.g. wind chill, blowing snow)
  • Does not include mixed precipitation or freezing rain explicitly (a precip‐only version of AWSSI may help address impacts of these events)
  • Thresholds have been set with impacts in mind and are subject to adjustment in the future as analysis continues.

Last Winter, there were no cities in the extreme category, with only 4 western cities listed as severe.

You can read more on the AWSSI at http://mrcc.isws.illinois.edu/research/awssi/indexAwssi.jsp

For WeatherNation: Meteorologist Mace Michaels

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