Wisconsin Department of Natural Resources
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Winter Severity Index
   View statewide historical information regarding annual Winter Severity Index (WSI). For additional Information….
Prior to 1975, Wisconsin did not have a formal procedure for measuring winter severity and predicting its impact on deer herds. Our winter severity index (WSI) was developed after testing several procedures for quantifying winter conditions. The WSI uses the number of days with a minimum temperature of <=0°F as a measure of winter air-chill, and the number of days with >=18 inches of snow depth to estimate the snow hazard. Days when both conditions occur are scored as 2. Daily scores are added from December 1st through April 30th to obtain the WSI.
Collection and analysis methods
United States Department of Commerce (USDC) weather data were initially used to measure winter severity because they were easily obtained and allowed us to compare WSI for previous winters with historical deer data (i.e. results of dead deer surveys, Summer Deer Observations, and buck harvests). The WSI was calculated for each of 12 USDC stations and then averaged to obtain the northern forest WSI for each winter back to 1959-60. Beginning in the winter of 1986–87, weather data were collected at 35 DNR stations across the north. These stations recorded daily snow depths and minimum temperatures from December 1st through April 30th and sent their data to the Northern Wildlife Research Group at the end of each month. Starting in the winter of 2014-2015, we began creating monthly WSI maps to better show the spatial variation in WSI. WSI maps are created in ArcMap using the kriging spatial interpolation tool. Mapping requires augmenting DNR station data with data from the National Oceanic and Atmospheric Administration’s National Operational Hydrologic Remote Sensing Center.
Using the metric
Winters are considered “mild” if the WSI is less than 50, “moderate” if between 50 and 80, “severe” if between 80 and 100, and “very severe” if the WSI exceeds 100. These designations are based on observed associations between WSI and winter mortality, fawn production, and buck harvest during the following year. Severe and very severe winter conditions were common across the northern forest region from the early 1960s through the late 1980s, whereas mild and moderate winter conditions have prevailed across the region since the early 1990s.
Limitations and precautions
Local habitat and winter conditions can and do vary from regional data. Deer in better habitats tend to be less effected by extremes of winter weather.
Future needs
Contact Dan Storm at Daniel.Storm@wisconsin.gov for more information.
Additional background materials related to this metric
Winter Severity Reports are available for viewing on the Wisconsin DNR website: Wildlife Survey Reports

Winter Severity Index
For questions on this deer metric data contact:
Dan Storm 715-401-2715 DanielJ.Storm@wisconsin.gov
Jeff Pritzl 920-366-3450 Jeffrey.Pritzl@wisconsin.gov