July 18-24, 2008
Midwest Weekly Highlights - July 18-24, 2008
More Heavy Rain
Heavy thunderstorms rumbled through the central Midwest this week, dumping four to more than six inches of rain in central Iowa on many of the same areas that suffered from severe flooding in early June (Figure 1). Most of the rain came between July 18-20 as a frontal system became stationary across Iowa and the southern Great Lakes and raised new threats of river flooding. On July 24 heavy rain also fell across northern and northeastern Missouri, causing extensive flash flooding. Precipitation this week was focused along this frontal system which eventually pushed south through the region by July 23. Rainfall ranged from less than 25 percent of normal in the far southern and northern portions of the Midwest, to more than 500 percent of normal in the central Midwest (Figure 2).
Temperatures reflected the average position of the frontal boundary this week. The northern half of the Midwest was cooler than normal, with temperatures departures ranging from near 0°F to -4°F in the northwestern half of the region, and from 0°F to +4°F in the southeastern half (Figure 3) Daily high temperatures reached the mid 90s from southern Missouri into Kentucky the first four days of this week, and the heat was compounded by dew points in the 70s (Figure 4).
Deluge in Iowa
Heavy rain was dumped on a large portion of central Iowa again at the start of this week as a frontal system slowly sagged into the Midwest. Beginning late on July 17 and through July 19 thunderstorms poured another four to 6 inches of rain on many of the same areas that received the heavy rain during early June (Figure 5). The rain caused another rise on area rivers, but by the end of the week river levels were on a steady decline as high pressure kept additional rainfall at bay (Figure 6).
Severe Weather Across the Region
After a one-day respite from severe weather, severe storms returned to the upper Midwest on July 19. The storms were concentrated across Iowa in in east-central Minnesota. There was one tornado reported in Shelburne County, MN, one tornado in eastern Wisconsin, and three tornadoes across central Iowa. The twisters were brief mostly produced only minor damage.
Severe storms continued along the cold front as it moved south the next two days. On July 20 most of the reports of severe weather were from damaging winds, with numerous gusts in excess of 60 mph. A gust of 79 mph was measured in Switzerland County in southeastern Indiana during the evening of July 20. Severe wind gust from 75 to 80 mph were reported across Iowa during the early morning hours of July 21. The slow-moving front kept storms active across mostly Illinois, Indiana, and Ohio on July 21. At least two tornadoes were reported in Missouri and one in Illinois. Tennis ball size hail (2.50 inches) was also reported with the Illinois storm near Chrisman in Edgar County. A wind gust of 84 mph was clocked in Princeton, IL (Bureau County), and a gust to 81 mph was measured at Roanoke, IL (Woodford County).
By the morning of July 22 the cold front was pushing into the southern portions of the Midwest. While cooler, drier air cut off storms north of the front, showers and thunderstorms continued to develop in the warm and very humid air south of the boundary. Most of the storms occurred in northeastern Ohio and in southwestern Missouri where a possible tornado was reported. A few severe storms occurred during the morning of July 23 in southwestern Missouri and into the early afternoon in northern Ohio, but most of the region enjoyed pleasant summer weather as the center of high pressure moved across the northern Great Lakes.
More Heavy Rain and Flash Flooding
The high moved to the east by July 24, and showers and thunderstorms developed ahead of the warm air returning from the west. Although the storms were not severe, two to three inches of rain fell across portions of west-central Iowa, prompting the issuance of flash flood warnings for a number of counties (Figure 7). During the afternoon of July 24 a persistent area of showers and thunderstorms set up from north-central Missouri to near St. Louis. A mesoscale convective system developed over southern Iowa and northern Missouri during the evening of July 24 (Figure 8), and flash flooding was extensive across a number of counties in northern Missouri. Heavy rain continued through the night into the morning of July 25. Impacts of this event will be detailed in the July 25-31 Climate Watch.
SDH