Ninety years ago this September, Minnesota — along with other Midwest states — experienced the most intense September heat wave in history.
The heat wave in Minnesota peaked between Sept. 7 and Sept. 13, 1931, when most counties saw daily temperatures that ranged from 15 to 20 degrees F above normal. Many temperature records were set that still stand today. The weather pattern for September was set up by a long, hot, sunny and dry summer that brought the first extreme drought of the Dust Bowl era.
At the beginning of the 1931 growing season, much of the state was already showing signs of severe drought. By July and August, this had evolved to extreme drought for some areas of the state. Portions of southern and western Minnesota had received less than half of normal rainfall for the summer months of June, July and August. As a result, the landscape was dry going into the month of September.
Heat settled in
Hot weather began to flirt with the state over Sept. 4-5 as afternoon temperatures climbed into the upper 80s and lower 90s. But the real heat settled over the state beginning Sept. 7, when observers in 16 western and southern counties reported afternoon temperatures from 95 to 104 degrees.
From Sept. 8 to Sept. 11, the heat wave peak values occurred. The climate observer at Beardsley reported 111 degrees. It was 108 degrees at Wheaton and Milan. The temperature reached 106 degrees at Morris, New Ulm and Canby, with 105 degrees reported at Fergus Falls, Montevideo and Redwood Falls.
Overall, 51 Minnesota climate stations reported daytime temperatures of 100 degrees or greater during this period. Many overnight low temperatures remained in the low to mid-70s, making for uncomfortable sleeping.
The Minnesota State Fair reported the hottest temperature ever measured on the fairgrounds with a reading of 104 degrees on Sept. 10.
On Sept, 13, a cold front crossed the state, bringing some rainfall and a drop in temperatures. A few northern Minnesota counties received over 1 inch of rain (the most for the entire month), but most of the state reported less than half an inch. Following the cold front, overnight temperatures dropped into the 30s in many areas.
For the remainder of the month, temperatures were generally warmer than normal, with several more days in the 80s, and rainfall was scarce. Many parts of the state saw less than 1 inch of rainfall for the entire month. It was the first of many episodes of heat and drought during the 1930s.
September 1931 was the warmest in history up until September 2015, which surpassed it.
However, the heat and drought of 1931 left its mark on depressed crop yields and low flows in Minnesota watersheds. Overall, 1931 brought the hottest year in history to the state as well, until it was surpassed in 1987, which has since been beaten by the year 2012.
The 111-degree reading at Beardsley in Big Stone County remains the hottest temperature ever measure in the state during September.
In just the previous year, 1930, a minimum temperature of 10 degrees had been measured on Sept. 30 at Big Falls in Koochiching County. Thus, in the course of one year, a temperature range of 101 degrees was reported for the month September in the state.
Only in Minnesota!
Seeley is Extension professor of meteorology and climatology, and the author of “Minnesota Weather Almanac.”