Extreme May Cold
Temperatures more typical of early April compared to early May blanketed the region during the first ten days of May
(Figure 1). Temperatures were 3-6°F below normal across most of the region, with some areas in southeastern Ohio and the U.P. of Michigan as much as 9°F below normal. Average temperatures during the period were fairly similar to the long-term average temperature for April 1-10 (Figure 2). Minimum temperatures averaged below freezing in the Upper Great Lakes
(Figure 3). More than 530 daily low maximum and minimum temperature records were broken across the Midwest during the period (Figure 4).
May 8-10 Late Freeze and Snowfall
A polar vortex system invaded the central and eastern U.S. on May 8-10, causing widespread freezes and uncommon late-season snowfall in the Midwest. The most widespread freezes occurred on the morning of May 9
(Figure 5). Most of the Upper Midwest and the Ohio River Valley had temperatures in the upper 20s, with lower 20s in northern Minnesota and Michigan. Freezing and near-freezing temperatures also were common on the morning of May 10 (Figure 6). A total of more than 400 daily low temperature records were tied or broken combined on May 8
(Figure 7), May 9 (Figure 8) and May 10
(Figure 9). Scattered snowfall was reported in Minnesota, northern Wisconsin and Michigan as well, with lake-effect snowfall in the U.P. of Michigan (Figure 10). The only state where freezes weren’t widespread in the Midwest was Missouri, where only scattered station in the eastern third of the state reached 32°F
(Figure 11). In the Ohio River Valley, freezes this late in the season were a very rare occurance (Figure 12). Freezes at 28°F and below stayed mostly north of the Ohio River Valley
(Figure 13), where more than 150 growing degree days had been accumulated in 2020. However, delayed freeze damage was expected to be reported.
Precipitation was very limited across the Midwest during the week (Figure 14). Amounts of less than half an inch were reported across the Upper Midwest, with 1-2 inches falling in southern Missouri and the Ohio River Valley. Only a few areas in eastern Kentucky, southern Missouri and extreme western Minnesota had more than the normal amount for the period
(Figure 15). Most of Wisconsin, eastern Minnesota and western Lower Michigan meanwhile had less than a quarter the normal amount. However, the Midwest remained drought free, with only minimal areas of abnormal dryness in Minnesota and Michigan
A dry first ten days of May allowed farmers to complete a large amount of field work according to USDA NASS Crop Progress Reports for the week ending May 10. Corn planting in Iowa and Minnesota were nearly complete, as 91 percent and 89 percent respectively of each state had been planted, compared to a five-year average of just 66 percent and 57 percent. Only Ohio and Missouri were behind their five-year average for corn planting at 3 percent and 11 percent behind respectively. Iowa soybeans were also at 71 percent planted, which was 47 percent ahead of the five-year average. Soybean planting in the Midwest was ahead in every state except Missouri.