Early-Season Heat Waves

Heat extremes often punctuate Minnesota's summers, and are most common from the end of June through the end of August. July is by far the state's hottest month, boasting virtually all of our statewide and station-specific heat records, and historically accounts for 40% of all 90-degree F days.

Heat waves that occur during or before the first half of June are less common, and often of a different nature than many (though not all) mid-summer heat waves. A typical July or early-August heat episode may be oppressively humid, whereas late-spring episodes tend to be somewhat "dry."

The worst heat wave on record in Minnesota occurring any time through the first half of June was from June 3 to June 11, 2021. This event demolished daily and early-season temperature records in northern Minnesota initially, and dealt record or near-record warmth to southern and central Minnesota for nine unrelenting days, when the temperature never fell below 70 F in the Twin Cites, and when many locations soared into the 90s or even 100s each afternoon.

Following are some statistics from the Twin Cities historical record, for heat extremes occurring on or before June 15th. Keep in mind, a heat wave is not only defined by the daily high temperatures, but also by warm nights. Some of late spring and early summer's hottest days have had tolerably mild nights, thanks to the dry air in place. Any time the high tops 90 F and the low also tops 70 F, however, the body will struggle to cool itself, day or night.

Many of the above tables focus on "streaks" of consecutive days at or above certain temperature thresholds. However, these sorts of measures do not capture everything. One notable early-season heat wave not listed above occurred June 6 to June 14, 1976, when 8 out of 9 days produced a high of at least 90 degrees F in the Twin Cities. The average high during this period was 91.8 F, and the average daily mean temperature was 79. 1 F, meaning in many ways this event rivaled others listed above. Furthermore, it is doubtful that anyone at the time mistook the 88 F reading on June 10th for the heat "breaking!"

Last modified June 23, 2021