Photo credit: State Climatology Office
A massive heat wave, affecting everything from the Gulf of Mexico to the Northern Plains and Upper Midwest, moved into Minnesota, breaking daily temperature and dew point records, and producing the second-highest Heat Index values on record across southern Minnesota. This was a strong, humidity-driven heat wave, meaning that while the temperatures were quite high-- reaching the 90s to low 100s F-- the high humidity made it feel much hotter. This was the most humid air mass since 2011 at some locations.
Origins
The hot weather was produced by a "heat dome," resulting from a massive atmospheric high pressure system that had shifted from the southwestern into the central US. Heat domes form as hot air rises, gets trapped by stable air above it, and then expands to cover wide areas, often equivalent to several large US states. Heat domes severely limit precipitation, and temperatures are almost uniformly hot across the entire area under one's influence.
Such was the case with this episode: the Heat Index, which combines the effects of both temperature and humidity into a single metric, was basically the same in southern Minnesota and Iowa as it was in traditionally-steamy areas of southern Texas. Also, the northern edge of the heat formed a "razor's edge," with much cooler conditions a mere 50-75 miles from the sweltering heat.
Timeline
The heat first moved into southern Minnesota during the afternoon of Monday August 21st, as temperatures rose into the low 90s F and dew point temperatures climbed into the upper 70s F, producing Heat Index values of 100-110 F, especially south of the Minnesota River late Monday afternoon. More extreme conditions were unfolding just to the south, in Iowa, and would move into Minnesota the next day.
Indeed, the extreme heat seemed to peak on Tuesday August 22, as temperatures rose into the 90s and low 100s in some areas, and dew points climbed to the upper 70s and low 80s F by the end of the day. The highest temperature recorded by a National Weather Service cooperative observer was 103 F at Canby. The intense moisture made the air mass seem much hotter, however, and Heat Index values across the southern half of Minnesota were generally in the 110s F. The very highest Heat Index values were found near the Twin Cities, with 122 F reported by a Hennepin-West Mesonet sensor in Spring Lake Park, 121 F at Elko/New Market, and 120 F at Minneapolis and Hanover. Airport stations reported Heat Index values of 119 F at Princeton, 118 F at Madison, and 117 F at several locations including Fairmont and Buffalo.
All of these values eclipsed the highest reported on July 19, 2019, placing them second only to the phenomenal humid heat wave of July 19, 2011, when Moorhead, MN recorded a state-record 88 F dew point, and the Twin Cities broke its dew point record with 82 F. Minnesota towns recorded Heat Index values of 115 to 125 F on that day, with Moorhead reporting values over 130 F.
After peaking on Tuesday, the heat continued at only a slightly lower intensity on Wednesday, with temperatures again reaching the mid-90s to lower 100s and dew points generally in the 70s F. This time, the highest temperatures were recorded in southeastern Minnesota, with Winona recording 102 F, and Grand Meadow, La Crescent, and Caledonia all recording 101 F.
Records
The event broke some records at long-term climate observation sites. The Twin Cities set daily high temperature records, reaching 98 F on August 22nd and 23rd; the previous records had been 97 F for both days. The Twin Cities also set daily dew point records (based on hourly observations), with 79 F on the 22nd, and 78 F on the 23rd. With a low temperature of just 79 F on the 23rd, this was also the highest minimum temperature on record for that date at the Twin Cities. Saint Cloud also tied its high temperature record for August 22nd, with 97 F. The Twin Cities area temperature record stretches back 151 years, and St. Cloud's goes back 130 years.
Geographic Limitations
This was not a statewide heat wave. One interesting feature present was the extremely sharp cut-off on the north side of the hottest air. Within a span of 50-75 miles, air temperatures often fell by 25-35 F and Heat Index values dropped by 40 degrees F or more. Parts of northern Minnesota remained in the 60s and 70s F for the entire event.
August 24, 2023