July 11-17, 2008
Midwest Weekly Highlights - July 11-17, 2008
Thunderstorms and Heavy Rain Shift North
As an upper level high pressure ridge established itself over the south-central U.S. the jet stream was pushed north across the Midwest this week (Figure 1), bringing damaging thunderstorms and heavy rain to Minnesota, Wisconsin, and Michigan. Rainfall was 200 to 300 percent of normal from central Illinois northward through much of Wisconsin and into the western Michigan U.P. (Figure 2). Rainfall amounts for the week were more than five inches in portions of south-central Wisconsin and north-central Illinois (Figure 3). East and west of this band, rainfall dropped off to less than 50 percent of normal for the week.
Temperatures this week were once again below normal across most of the region, ranging from 4°F below normal in western Missouri and southeastern Kentucky to only 1°F to 2°F above normal in extreme northeastern Ohio (Figure 4). Stretches of hot and humid weather were short as cold fronts periodically moved through the Midwest and brought cooler and drier air. At mid-month average temperatures for July ranged from 3°F in the southwestern portions of the region to just above normal in the northeast.
Warm, Humid, and Stormy Start
Strong southerly winds were set up across the region on July 11 as a strong cold front low pressure system spun up in the Northern Plains. Thunderstorms moved into Minnesota during the morning as a cold front associated with the low approached western part of the state. Severe weather was reported across central Minnesota during the morning and afternoon, with at least 5 tornadoes in western Minnesota. There was widespread wind damage as winds gusted in excess of 80 mph. A thunderstorm wind gust of 84 mph was measured in Waite Park, MN (Stearns County), and a gust to 75 mph was measured by an automated RAWS station in Little Fork, MN (Koochiching County). Hail 2.75 inches in diameter was observed near Starbuck, MN (Otter Tail County). There were also scattered severe storms in Wisconsin, Iowa, and Illinois.
Scattered severe weather occurred along the front on July 12 from southern Missouri eastward, with one brief tornado touchdown in Putnam County, IN.
Quiet Midweek, then More Storms in the Northland
The Midwest enjoyed two quiet, cooler, and less humid days following the frontal passage. Another cold front moving south out of Canada triggered a few severe storms, mostly hail producers, in northern Minnesota. The storms also caused extensive wind damage near Isabella in Lake County. What was initially thought to be a tornado damage was determined by NWS storm damage surveys to caused by a microburst. Forty acres of trees were significantly damaged or blown down. The cold front did not much make progress over the next several days, stalling out across southern Minnesota, Wisconsin, and central Michigan on July 16 (Figure 5). An upper level wave moving eastward (Figure 6) combined with the cold front to generate heavy and severe thunderstorms from eastern Minnesota across south-central Wisconsin. Most of the Severe weather occurred across central Wisconsin into central Michigan, but up to 4.00 inches of rain caused flash flooding and mudslides in eastern Minnesota, especially Houston and Winona Counties. In Wisconsin, a tornado was reported by the public in Sheboygan County, and flash flooding was widespread across Waupaca, Outagamie, Calumet, and Manitowoc counties as 3.00 to 4.00 inches of rain fell in a short period of time. The roof of a plywood factory in Marion, WI (Waupaca County) was reported to have collapsed due to the excessive rainfall.
Storms continued to fire in Minnesota, Iowa, and Wisconsin along the front as it moved back northward on July 17. Many of the storms were slow moving or training (developing and repeatedly moving over the same area) producing rainfall rates of three inches per hour or more across central Iowa during the afternoon and evening. The rain resulted in some flash flooding and closed roads. One observer northeast of Ames (Story County) reported 4.73 inches of rain, and 24-hour totals in excess of three inches were common in eastern Dallas County, Polk County, and Story County (Figure 7). Scattered severe storms were reported in Iowa, Minnesota, and northern Wisconsin. Meanwhile, the surface ridge of high pressure centered over the central Appalachians and extending back to the Southern Plains kept the rest of the region warm and dry.
SDH