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The Accumulated Winter Season Severity Index (AWSSI)

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) is being developed to objectively quantify and describe the relative severity of the winter season.

Goals of the AWSSI2013-14 AWSSI:  Detroit Area, MI

  • 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

How the AWSSI Accumulates

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

AWSSI Point ThresholdsDaily 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 snow scores as well as the total AWSSI.2013-14 AWSSI:  Urbana, IL

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



  • 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.
See annotated scoring page sample: See current scoring and charts for sample locations:
See annotated score page Chicago
Current Chicago scoring
Current Detroit scoring
Current Minneapolis scoring

Additional Work

The development of the AWSSI and analysis of results are ongoing. We hope to complete soon a version of the AWSSI that will use temperature and precipitation data to estimate snowfall and snow depth where that data is not available. This will allow us to look at historical winters (late 19th century) at locations for which that data may be available. There will also be work to identify temperature‐dominant and snow/precipitation‐dominant winter regimes, examine trends, teleconnections, other statistics by temperature and snow/precipitation, and eventually develop decision support tools utilizing the AWSSI. Before next winter we hope to have an interactive AWSSI product available for users on the MRCC web site.

Additional Information

Information Sheet (2-page) - pdf

An Accumulated Winter Season Severity IndexBarbara Mayes Boustead, NOAA/NWS, Valley, NE; and S. Hilberg, M. D. Shulski, and K. G. Hubbard. Presented at 93rd Annual Meeting of the American Meteorological Society, January 2013. 

An Accumulated Winter Season Severity Index (AWSSI). Webinar for the NWS Central Region, January 2014. 

For more information, please contact Barb Mayes Boustead (barbara.mayes@noaa.gov) at the National Weather Service or Steve Hilberg (hberg@illinois.edu) at the Midwestern Regional Climate Center.



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