http://dnr.wi.gov/topic/wildlifehabitat/documents/wsiest2.pdf
This map depicts the Winter Severity Index (WSI) across Wisconsin. WSI is calculated by adding the number of days with a snow depth of at least 18 inches to the number of days when the minimum temperatures were 0◦F or below. WSI points accumulate through the winter. WSI of 50 or less is considered mild, 50 to 79 is moderate, 80 to 99 is severe, and over 100 is very severe. Most WSI readings are taken by WDNR biolo- gists across northern Wisconsin, but some readings are from National Weather Service stations. The stations used are displayed on the map.A statistical procedure uses that data to estimate WSI in the areas in between stations. Questions can be directed to
DanielJ.Storm@wisconsin.gov.
SOURCE: WI DNR - dnr.wi.gov