Midwest Weekly Highlights - June 17-23, 2009

Severe Weather Lashes Midwest; Heat Arrives

Severe thunderstorms occurred throughout the region the third week of June as a frontal boundary moved south, then north through the region. The thunderstorms, at times slow-moving, brought heavy rain to many areas. Precipitation was heaviest from southern Iowa through northern Illinois and into southern Michigan, where it was three to four times normal for the week (Figure 1). Precipitation was also normal to above normal in Minnesota and western Wisconsin alleviating some of the dry conditions there. The driest portions of the Midwest were from northwestern Wisconsin into northern lower Michigan, where precipitation ranged from 50 percent to less than 10 percent of normal for the week. Rainfall was also below normal in southwestern Missouri and northern Kentucky. The June 23 U.S. Drought Monitor depicted little change in conditions throughout the Midwest (Figure 2). Abnormally dry conditions exist in Minnesota northeastward into Michigan, and Severe Drought continues in northwestern Wisconsin. A swath of abnormally dry conditions is also depicted in central Ohio.

Hot, humid weather finally arrived for an extended stay in the Midwest as an upper level high pressure ridge became established over the southern U.S. (Figure 3). Temperatures across the Midwest were above normal this week, ranging from near normal in eastern Michigan and Ohio to 8°F above normal across southern Missouri and Illinois and northern Wisconsin (Figure 4).

There were a number of temperature records through the Midwest this week. Most of the records were record high minimum temperatures, although there were a few record high temperatures, most of them on June 23.

An Active Week of Severe Weather

The frontal system marking the boundary of very warm and humid air to the south, and cooler, drier air to the north provided the focus for thunderstorm development through the week. Severe thunderstorms occurred in all nine states, but the storms were most numerous and heavy from southern Iowa east-southeast through Illinois into western Kentucky, and from southern Wisconsin to southern Michigan (Figure 5). The distribution of the severe weather reflected the northern extension of the upper level ridge and the mean position of the front during the week (Figure 6). June 17 was the most active severe weather day in the country this year with 479 reports, with about two-thirds of these in the Midwest region. Hail damaged 150,000 acres of crops in Chickasaw and Fayette counties in northeast Iowa on the night of June 17 with about 10 percent of these acres being a total loss.

Thunderstorms continued on June 18 and 19 as the frontal system sagged south through the region. On June 19 thunderstorms were concentrated from eastern Iowa through Illinois into Indiana and southeastern Michigan. An 87 mph thunderstorm wind gust was reported in Garrison, IA (Benton County) during morning thunderstorms, and a 75 mph thunderstorm wind gust was measured near Oswego, IL (Kendall County) during thunderstorms that struck during the afternoon. Straight-line wind damage occurred across the affected region. 112,000 ComEd customers were without power in northern Illinois Friday night in the wake of the storms. High winds knocked 12 empty rail cars off the tracks in Greene County, IN. Tornadoes touched down in Illinois, Indiana, Wisconsin, and Michigan. Most of the tornadoes were weak and caused only minor damage. However, two EF2 tornadoes were confirmed in Michigan, one in Allegan County and a second in Kalamazoo County. More information on these storms in Michigan can be found in this report from the NWS Grand Rapids, MI office . There were numerous reports of large hail from the storms that developed during the afternoon and evening, with hail the size of grapefruit (4 inches) reported near Talma, IN (Fulton County).

Flash flooding was also common as thunderstorms produced heavy rain, sometimes at a rate of five inches per hour or more. Flash flooding closed a section of Interstate 80 in Iowa (Iowa County) for 30 minutes. Numerous roads were flooded in the Chicago Metropolitan area from the heavy rain. The Des Plaines River reached flood stage June 20 and remained above flood stage at the end of this week. Two to four inches of rain in southern Michigan on the evening and night of June 19 (Figure 7) caused the Grand River (Figure 8) to rise rapidly early Saturday June 20. Flood waters inundated the Ionia County fairgrounds where a country music festival was being held. 1,000 vehicles were stranded in floodwaters in the parking lot. At one point water was rising on the Grand River at the rate of one foot every two hours, five times faster than with typical flooding. Rainfall amounts upstream of Ionia were in excess of three inches (Figure 9). Four to 4.50 inches of rain were measured in Holland, MI (Ottawa County) from the storms (Figure 10).

Storms continued on June 21 and June 22, although coverage was much less extensive. Severe thunderstorms were mostly confined to an area from southern Minnesota to northern Missouri on June 21. There were 23 tornado reports in Iowa (16) and southern Minnesota (7) from late afternoon through early evening that day. Most touchdowns were brief with only minor damage reported. One tornado in Grundy County, IA was reported to have extensively damaged at least one home and a barn at a hog operation.

Severe storms were again concentrated in Iowa on June 23, with severe weather reported in 41 of Iowa's 99 counties. There were a number of reports of winds in excess of 70 mph. Most damage was downed trees and power lines. The complex of storms moved southeast into western Illinois downing trees and power lines there as well. There was one tornado touchdown reported near Edgerton, WI (Rock County).

SDH