The winter severity index (WSI) combines spatially explicit temperature and snow-depth data to provide a general indication of winter conditions as they relate to deer populations. The WSI is calculated weekly from weather stations across the state from Nov. 1 to May 31. Although data is collected statewide, winter conditions in Minnesota’s northern forested zone generally have a greater impact on deer populations than elsewhere in the state.

Scores accumulate throughout the winter, with one point for each day with an air temperature of 0 degrees Fahrenheit or lower and an additional point for each day with a snow depth of 15 inches or greater. A total end-of-season WSI value of 50 or lower is considered mild; 51 to 119 is moderate; and 120 or higher is severe. 

Categories of severity compare the current season WSI to annual winter conditions over the past 30 years. Deer mortality has been directly linked to winter severity, particularly the depth of snow, which makes WSI an important tool.

 Image: MN DNR

The 2023-2024 WSI estimates indicate this winter to be one of the most mild winters Minnesota has seen in recent history. This bodes well for Minnesota deer as the usual stressors associated with winter (such as reduced food availability, mobility, and survival) were less of an issue compared to other years.

The WSI scale was updated in 2020 to clearly define winter severity categories. Based on 36 years of data from winter conditions across deer permit areas in the northern forest zone, 25% of winters had values of 50 or less; 50% of winters had values of 51 to 119; and 25% of winters had values of 120 or higher.

Keep in mind though: One mild winter doesn’t guarantee a significant increase in deer numbers. For example, the winter of 2020-2021 was very mild, but was also followed by two consecutive harsh winters which likely impacted population growth and ultimately deer harvest over the following years. 

Overall, WSI is a means to simplify winter conditions but is not predictive of deer populations alone. Many other factors such as quantity and quality of habitat, deer densities, age and sex structure, and predation are all important in deer winter survival and are important factors in deer management.