The average August temperature for the Midwest was 71.1°F, which was 0.1°F above the 1991-2020 normal (Figure 1). Temperatures were generally 1-3°F above normal in the west and 1-3°F below normal in the east (Figure 2). Statewide average temperatures ranged from 1.5°F above normal in Iowa to 1.7°F below normal in Michigan (Figure 1). Temperatures were rather seasonal, or even mild, for much of the month until an extremely warm and humid air mass settled across the region from August 19 to August 25 (details in the Humid Heat Wave section below). Minneapolis, Minnesota tied for their 10th warmest August in 151 years. Conversely, Russellville in western Kentucky had the 6th coldest August in 94 years. The average summer (June-August) temperature for the Midwest was near normal, with slightly above-normal temperatures in the west and slightly below-normal temperatures in the east (Figure 3).
August precipitation totaled 3.81 inches for the Midwest, which was 0.15 inches above normal, or 104 percent of normal (Figure 1). Precipitation was 25-75 percent of normal in the northwestern portion of the region, with precipitation near or above normal in the southern and eastern portions of the region (Figure 4). Statewide precipitation totals ranged from 2.49 inches above normal in Missouri to 1.52 inches below normal in Wisconsin (Figure 1). Rankings indicate Missouri had tied the 6th wettest August since 1895. In southeast Missouri and western Kentucky, multiple rounds of significant heavy rainfall during the first half of August resulted in a wide swath of the region with monthly precipitation totals of 10-18 inches (Figure 5). Marble Hill, Missouri reported 17.19 inches, which included 3 separate days with precipitation exceeding 4 inches. Rainfall moved eastward for the latter half of the month, with high precipitation in the Detroit, Michigan and Cleveland, Ohio areas. Cleveland had the 9th wettest August in 152 years and Detroit had the 9th wettest in 150 years. Conversely, Decorah in northeast Iowa tied the 4th driest August in 122 years with just 0.91 inches for the month. Summer (June-August) precipitation for the Midwest totaled 10.38 inches, which was 1.93 inches below normal (Figure 6). Summer precipitation deficits of 4-10 inches were widespread across the northwest, while precipitation surpluses of 2-8 inches were observed across the south-central and eastern Midwest (Figure 7).
Throughout August, drought conditions improved in the east, with abnormal dryness and drought significantly reduced or eliminated in Illinois, Indiana, Kentucky, Michigan, and Ohio. Conversely, drought expanded and intensified in the northern and western portions of the region. By month’s end, about 41 percent of the Midwest was in drought, according to the U.S. Drought Monitor (USDM), with the most dire conditions in the Upper Mississippi River Basin (Figure 8). Wisconsin reached a Drought Severity Coverage Index (DSCI) of 248 in late August, which tied for the highest DSCI since those records began in 2000. Drought in the northwest intensified rapidly over the last 12 weeks since the start of summer when the Upper Mississippi River Basin was either abnormally dry or drought-free. By late August, the ongoing summer drought resulted in low flows and navigation restrictions on the Mississippi River, reduced corn and soybean yields, and increased wildfire risk.
Humid Heat Wave August 19-25
An unusually strong, long-lasting upper atmospheric heat dome trapped a hot and humid air mass across the lower Midwest from August 19-25. Triple-digit heat index values were paired with excessively warm overnight temperatures, prompting the National Weather Service to issue widespread excessive heat warnings across most of the Midwest. Kansas City, Missouri (Downtown Airport) had a record-setting 5 consecutive days (August 19-23) with the heat index at or above 115°F, and they tied the all-time record heat index of 120°F on August 20. Sioux City, Iowa set a new all-time record on August 21 when the heat index reached 122°F. Heat Index values in excess of 120°F were measured as far north as Minneapolis, Minnesota. St. Louis, Missouri had 3 consecutive days (August 23-25) with record-setting warm overnight temperatures, and the area had air temperatures at or above 80°F for 142 consecutive hours from August 20-26 (4th longest duration going back to 1945). The widespread warmth resulted in school closures, pavement buckling, livestock fatalities, and increased energy demands that resulted in Midwest grid operators seeking usage reductions.
The Twin Cities area in Minnesota had its most damaging thunderstorm event of the year on August 11 when a line of storms produced golf-ball to baseball-sized hail across a multi-county region (Figure 9). Extensive damage was reported to vehicles, rooftops, windows, and siding. On August 24-25, damaging thunderstorms and tornadoes moved across southern Michigan and northern Ohio. Heavy rain paired with 70-80 mph wind gusts created extremely hazardous driving conditions, downed trees, and knocked out power to over 200,000 customers. Unfortunately, 5 fatalities in Michigan resulted from these storms.