The First Heat Wave of Summer 2025

The first major heat wave of the summer is finally fading from Wisconsin, after 80- and 90-degree temperatures and tropical-feeling humidity June 19 through 23. 

A majority of the central and eastern regions of the United States were feeling the heat right along with us, thanks to a heat dome in the atmosphere. This phenomenon occurs when the jet stream shifts northward and a high-pressure center becomes trapped in one spot (like a traffic jam on the highway!), and creates persistently hot and often humid conditions in a region for days at a time.

Despite sweltering temperatures, only one high temperature record was set at one of Wisconsin’s first-order stations. Green Bay reached  94 degrees on June 22, tying the record for that date set in 1910.

It was the record warm low temperatures that were tumbling left and right across the state! Nine low temperature records across the state were either tied or broken between June 21 and June 23, 2025— another event displaying the changing characteristics of Wisconsin’s heat waves.

La Crosse’s 80-degree low temperature on June 22 tied the record for the location’s warmest low temperature in June. The previous occurrence was on June 29, 1931. 

That same day, Green Bay bottomed out at 79 degrees, tying its record for all-time warmest low temperature, which occurred on July 4, 1897, and on July 30, 1916. Wausau also tied its all-time warmest low temperature record on June 22 with a low of 78 degrees, which previously occurred on July 4, 1999.

In Eau Claire, the mercury climbed above 80 degrees at noon on June 21 and didn’t fall below 80 degrees until 9 p.m. on June 23. At 80 hours, this set a new city record for the longest amount of time spent at or above 80 degrees. Eau Claire’s previous record was 42 hours, set in June 1991.

Those temperatures, combined with dew points in the 70s and 80s, pushed heat indices, or feel-like temperatures, into the triple-digits in many locations.

The National Weather Service doesn’t keep track of heat index records, but we know that the hottest heat index ever reported in the state was 124 degrees in July 1999 at the Kenosha Regional Airport. For this heat wave, the hottest heat index reported by the National Weather Service was 111 degrees in the village of Winter (Sawyer County) on June 22.

This is a product of the Wisconsin State Climatology Office. For questions and comments, please contact us by email (stclim@aos.wisc.edu) or phone (608-263-2374).