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. It used 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 on the ground to estimate the snow hazard. Days when both conditions occurred are scored as 2. These are added together from 1 December through 30 April 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 initially 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. Since 1999–2000, four stations were discontinued (Iron River, Cumberland, Medford, and Pound) and Copper Falls State Park, Alvin, and Giles were added as new stations in 2000, 2009, and 2010, respectively. Daily snow depths and minimum temperatures were recorded at these stations from 1 December through 30 April on a standardized form, and this information was sent to the Northern Wildlife Research Group at the end of each month. Survey instructions request that the presence of crusts be recorded. To date, information on snow crust has not been incorporated into the index, but this information may affect our interpretation of the index.
Starting in the winter of 2014-2015, we created monthly WSI maps, to better show the spatial variation in WSI. This required augmenting traditional weather station data with data from the National Oceanic and Atmospheric Administration’s National Operational Hydrologic Remote Sensing Center (NOHRSC). We used spatial interpolation (kriging) to create WSI maps.
Using the metric
Winters are considered “mild” if the calculated WSI is less than 50, “moderate” if it is between 50 and 80, “severe” if it is 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 (Wisconsin Department of Natural Resources 2001:5.11). Historically, severe to very severe winter conditions were commonly reported across the northern forest region from the early 1960s through the late 1980s, whereas 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.

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Future needs
Contact Dan Storm 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

Current Winter Severity reports are available here: Winter Severity Reports

Winter Severity Index
For questions on this deer metric data contact:
Dan Storm 715-365-4712 DanielJ.Storm@wisconsin.gov
Kevin Wallenfang 608-261-7589 Kevin.Wallenfang@wisconsin.gov