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

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

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. The AWSSI data is gathered every hour throughout the day.

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

See annotated scoring page sample

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.

Additional Information

Information Sheet (2-page) - pdf

The Accumulated Winter Season Severity Index (AWSSI)
Barbara E. Mayes Boustead, Steven D. Hilberg, Martha D. Shulski, and Kenneth G. Hubbard. Journal of Applied Meteorology and Climatology, Vol. 54, No. 8, August 2015: 1693-1712.
Abstract | Full Text | PDF (2545 KB)

An Accumulated Winter Season Severity Index. Barbara 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, February 2017.

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