Above Normal Temperatures

The May average temperature for the Midwest was 61.5°F, which was 1.8°F above the 1991-2020 normal. Statewide monthly average temperatures ranged from 0.8°F above normal in Minnesota to 2.8°F above normal in Ohio and Michigan (Figure 1). An unusual mid-May heatwave brought hot and humid conditions across the region from May 8-14, with over 1,500 daily high temperature records broken or tied during that week (Figure 2). Columbia, Missouri had six consecutive days with record high temperatures (dating back to 1890). Three heat-related fatalities were reported in Chicago, where a new 3-day minimum temperature record was set (72.3°F). Many locations across the region exceeded 90°F for the first time this season, which is about a month earlier than normal across the central and northern Midwest. Dew point temperatures in the upper 70s and lower 80s pushed heat index values over 100°F in portions of Iowa, Illinois, and Wisconsin during the height of the heatwave. The Midwestern region average minimum temperature was 50.6°F, which was tied with 1964 for the fifteenth warmest on record. Many areas had minimum temperature departures in excess of 2°F above normal (Figure 3). Maximum temperature departures were above-normal in the east and below-normal in Minnesota (Figure 4).

Variable Precipitation

May precipitation was 4.55 inches for the Midwest, which was 0.17 inches above normal, or 104 percent of normal. Precipitation totals were highest in Missouri, Minnesota, Indiana, Kentucky, and Ohio (Figure 5). Statewide precipitation totals ranged from 1.28 inches below normal in Iowa to 1.41 inches above normal in Minnesota (Figure 6). Minnesota had the 16th wettest May and the 6th wettest spring since 1895. Ohio had the 17th wettest May since 1895. Regionwide there were 285 daily high precipitation records broken or tied for the month. Collegeville, Minnesota had its wettest May on record (since 1892). Columbus, Ohio had the 3rd wettest May on record (since 1878).

Flooding

Above-normal winter snowpack, delayed ice-out, and repeated heavy rainstorms in April and May spurred widespread flooding across northern Minnesota. Record high monthly average streamflows (Figure 7) were reported in May along the Rainy River (Minnesota/Canadian border) and the Red River of the North (western boundary of Minnesota). Damage along the Red River of the North was minimal, while the Rainy River basin had significant impacts including damaged homes, National Guard deployments, and numerous closures of roads, trails, and recreational areas. In northeast Minnesota, historic raging waterfalls caused substantial damage to bridges, docks, and roads with many paths and overlooks closed. Flooding across northern Minnesota is expected to continue into June.

Corn Planting

Midwest corn planting started the month behind schedule following a cool and damp April. By May 8, corn planting was 18-49 percent behind the 5-year average across the region. Indiana, Illinois, and Iowa reported just 11 percent, 15 percent, and 14 percent of the total corn crop planted, respectively. However, as conditions rapidly warmed in mid-May, farmers moved quickly to close the gap on delayed planting. By May 29, corn planting was 0-14 percent behind the 5-year average. Only two states, Minnesota and Ohio, had double-digit planting delays at 14 percent and 10 percent behind the 5-year average, respectively. In Iowa, 94 percent of the total corn crop had been planted by month’s end.

Notable Weather Events

A straight-line wind event (derecho) on May 12 traversed the north-central US, affecting Iowa and Minnesota in the Midwest (Figure 8). There were 228 preliminary storm reports across those two states, including 20 tornado reports, that resulted in one fatality and extensive damage to buildings, vehicles, and trees. Osceola County, Iowa recorded an 82-mph wind gust while a 94-mph gust was measured in Madison, Minnesota. Strong winds also produced a dust storm, called a haboob, that traveled across portions of Iowa and Minnesota. On May 25, several locations in Indiana received wind storm reports in excess of 60 mph. Unfortunately, Boone County, Indiana had a fatality as a boom lift was toppled as a result of the winds. A tornado outbreak occurred in Minnesota on May 30 (Figure 9). Both the Twin Cities, Minnesota and the Grand Forks, North Dakota National Weather Service Offices provided summaries for each of the tornadoes. There were 4 EF-2 and 10 EF-1 tornadoes. Fortunately, there were no reported fatalities or injuries from these tornadoes.