August 15-21, 2018
Midwest Weekly Highlights - August 15-21, 2018
Cool Days, Warm Nights
A sharp divide in daytime and nighttime temperatures was observed across most of the Midwest during the week. Minimum temperatures were warmer than normal across the entire region except northwestern Minnesota
(Figure 1). Areas in Lower Michigan had minimum temperatures 5-8°F above normal. Meanwhile, maximum temperatures in Missouri, Iowa, and the Ohio River Valley were below normal (Figure 2). Average temperatures were largely near normal in these areas, while areas around the Great Lakes were warmer than normal
(Figure 3). Increased cloud cover likely had a large effect on temperatures as a major storm system moved through the region.
Heavy Rainfall
A strong system moved slowly through the Midwest during the week, dumping inches of rain on the southern two-thirds of the Midwest (Figure 4). Areas in western Iowa, southern Wisconsin, southern Missouri, southern Illinois and Indiana had more than three inches. Isolated heavier amounts were also reported in southwestern Iowa, south-central Wisconsin and the Missouri Bootheel, where more than six inches fell. Some of the heaviest amounts included 10.43 inches in Logan, IA (Harrison County), 10.30 inches in Oakland, IA (Pottawattamie County) 13.02 inches in Middleton, WI (Dane County), 11.14 inches at the Charmany Farm, WI (Dane County) 10.56 inches in Watertown, WI (Jefferson County) and 9.49 inches in Hornersville, MO (Dunklin County). These extreme amounts were more than double the normal amount of the entire month of August. While rain occurred throughout the week, most of this precipitation fell through the mornings of August 20
(Figure 5) and August 21 (Figure 6). Gauges along the Missouri River in western Iowa reached moderate flood stage from rain
(Figure 7), while dangerous flash flooding occurred in Wisconsin. Meanwhile, areas in the northern Great Lakes were much drier and recieved less than a quarter of the normal amount of rainfall for the week (Figure 8).
South-Central Wisconsin Flash Flooding
Exceptionally heavy rain in south-central Wisconsin on August 20-21 led to flash flooding in several counties
(Figure 9). The county most affected was Dane County, where 13.02 inches fell in Middleton and 11.14 inches fell at the Charmany Farm, southwest of Madison. Other unofficial reports had totals of more than 14 inches near Cross Plains. Storms formed and reformed over the same areas from 4pm-10pm in the afternoon. In less than 12 hours, a stream gauge near Black Earth went from around 2.5 feet to a new record of 9.73 feet (Figure 10). Roads, bridges and bike paths were closed due to flooding in the western half of the county. Water rescues were being conducted in Mazomanie as evacuations were ordered. A man was killed in the flooding after being pulled under his vehicle. A state of emergency was declared for Dane County on August 21. High water levels on the connected river and lake system around the Madison area were forecast to cause problems into the following week.
Rain Brings Little Drought Relief
Despite moderate amounts of rainfall in Missouri and Iowa, only modest improvements to drought were made in the August 21 U.S. Drought Monitor (Figure 11). Exceptional Drought remained in nearly five percent of Missouri, while nearly three-fourths of the state was considered in moderate or worse drought (Figure 12). Most of these improvements were in southern Missouri, where 2-4 inches of rain fell. However, many areas in southwestern Missouri still had deficits of 4-6 inches since April 1 (Figure 13).
Meanwhile, drought continued to worsen in northern Lower Michigan, as Severe Drought was introduced into parts of ten counties (Figure 14). Shrubs and leaves were beginning to dry in these areas. More than a quarter of the state was in drought. Elsewhere, moderate drought expanded in northwestern Minnesota, while abnormally dry conditions extended across northeastern parts of the state. Overall, 18 percent of the Midwest was in drought, with an estimated seven million people living in drought affected areas.
-BJP-