Wisconsin’s location in the Upper Midwest, bordered by Lake Michigan and Lake Superior, subjects the state to a wide array of natural hazards year-round. The continental climate variability results in four distinct seasons, each presenting different atmospheric and hydrological risks. This combination creates conditions for everything from violent summer thunderstorms to paralyzing winter storms. The state’s vulnerability is primarily driven by rapidly changing weather systems that frequently cross the region.
High-Impact Summer and Spring Weather Events
The transition from spring into summer is often marked by the most violent atmospheric hazards, with the peak season for severe weather occurring in June. Wisconsin averages approximately 23 tornadoes annually, though the vast majority are on the lower end of the Enhanced Fujita (EF) scale. Less than three percent of all recorded tornadoes reach EF3 intensity or greater, which is the threshold for significant damage.
Tornado activity is most frequent in the southwestern and southern portions of the state, particularly near the Mississippi River. Beyond tornadoes, severe thunderstorms are common and produce powerful straight-line winds, sometimes organizing into extensive, fast-moving systems known as derechos. The 1998 derecho, for example, produced measured wind gusts that exceeded 100 miles per hour in southern Wisconsin, causing widespread damage.
Large hail is another significant hazard during warmer months; the state’s record hailstone measured 5.7 inches in diameter. Hail two inches or greater in diameter is responsible for the majority of property damage. Strong updrafts within severe thunderstorm cells suspend these hailstones, allowing them to grow until they fall, damaging crops and property.
Flooding and Great Lakes Coastal Hazards
Wisconsin’s extensive network of rivers and its location on two Great Lakes make it highly susceptible to hazards involving excessive water. Riverine flooding frequently affects major water bodies, including the Mississippi River and the Wisconsin River, following rapid snowmelt or heavy rainfall. The statewide flooding event of 1993, for instance, saw every major river overflow its banks.
Flash flooding is a localized threat, particularly in urban areas and along smaller tributaries, where rainfall rates can quickly overwhelm drainage systems. Historic events have seen rainfall totals exceed ten inches, leading to record crests on many rivers and streams. This rapid accumulation of water can wash out roads, damage infrastructure, and severely impact communities.
The Great Lakes coastline presents hazards, including coastal erosion and fluctuating water levels. High lake levels combined with powerful storms accelerate the erosion of the state’s high bluffs, which are composed of unstable glacial till and clay deposits. Storm-driven wave action can also cause seiches, which are large standing waves resulting from atmospheric pressure gradients and wind pushing water across the lake surface, causing rapid changes in water level along the shoreline.
Winter Storms and Extreme Cold Events
The cold season in Wisconsin is dominated by hazards relating to frozen precipitation and dangerously low temperatures. Blizzards are defined by a combination of heavy snow or blowing snow, sustained winds of 35 miles per hour or more, and visibility reduced to one-quarter mile or less. These conditions create whiteouts that can paralyze transportation and isolate communities.
Lake-effect snow is a localized hazard that significantly impacts communities along the Lake Superior and Lake Michigan coastlines, such as Racine and Kenosha. This phenomenon occurs when cold air masses move across the relatively warmer, unfrozen lake water, picking up moisture and dropping it as intense, narrow bands of snow. An approximate temperature difference of 13° Celsius between the water surface and the air at 5,000 feet is required for this process to begin.
Extreme cold poses a serious public health risk, with Wisconsin averaging about 50 cold-related deaths each year. The danger is quantified by the wind chill index, which measures the rate of heat loss from exposed skin. Wind Chill Advisories are issued when the wind chill reaches -20° Fahrenheit, as frostbite can occur on exposed skin within 30 minutes. Extreme Cold Warnings are issued when wind chills drop to -30°F or -35°F, signaling life-threatening conditions.