May 25-31, 2010
Midwest Weekly Highlights - May 25-31, 2010
Very Warm and Humid
The weather the last week of May was more typical of the last week of June across much of the Midwest. Temperatures were well above normal across the entire region. A nearly stationary ridge of high pressure and light southerly surface winds transported more humid air northward as well, fueling the development of showers and thunderstorms.
Average temperatures this week ranged from 4°F above normal along the southern and western periphery of the region to 10°F to 12°F above normal through southern lower Michigan (Figure 1). Maximum temperatures on most days reached the mid 80s over the southern one-half to two-thirds of the region. High temperatures on May 29 and May 30 reached 90°F as far north as central Minnesota.The heat and humidity was punctuated at times by weak cold fronts that brought somewhat cooler and drier weather to the region for a day or so. There were 284 daily record maximum temperatures set or tied during the week, with about 200 of those recorded on May 25-26.
Scattered Storms, Some Heavy
A significant portion of the Midwest received little rain the last week of May (Figure 2), but those areas that did get rain often saw it come down in torrents. Rain missed much of the drying areas of northern Wisconsin and the Michigan Upper Peninsula, and this week the area of Extreme Drought on the U.S. Drought Monitor expanded in the south-central U. P. and into northeastern Wisconsin (Figure 3).
Thunderstorms developing under the upper level ridge on May 25 remained nearly stationary during their lifetimes due to very weak winds aloft, resulting in heavy rainfall amounts in those locations under the storms. On the east side of Springfield, IL (Sangamon County), a CoCoRaHS observer reported 4.43 inches of rain in the span of 90 minutes during the afternoon. Flooding was widespread throughout the city. More than 200 vehicles were stranded, and numerous basements were flooded. Exit ramps from Interstate 55 in Springfield were flooded for an hour or more. The thunderstorm caused scattered power outages, and golf ball-sized hail was reported on the west side of the city.
An intense low pressure system over North Dakota on May 25 also produced heavy rain across northwestern Minnesota as it interacted with warm, humid air (Figure 4). The highest measured amount in Minnesota was 4.74 inches near Highlanding (Pennington County), although radar estimates suggested amounts of six to seven inches may have fallen. Area rivers rose to and over flood stage. Overland flooding was also reported with a number of secondary roads washed out.
St. Louis experienced urban flooding on May 26 as the result of heavy thunderstorms. Numerous streets in north St. Louis were under water and dozens of homes had flooded basements.
Scattered thunderstorms popped up almost every day somewhere in the Midwest. On May 29-30 extreme northwest Minnesota was hit again by heavy rain (Figure 5). Three to four inches of rain were measured in Hallock (Kittson County), causing rapid river rises and overland flooding. More than six inches of rain fell in adjacent portions of North Dakota. Thunderstorms were more numerous in the Midwest on May 31 as a weak cold front pushed eastward. The heaviest rain with these storms fell across northern Indiana and northern Ohio.
Severe storms occurred in all nine states this week and consisted of mostly hail and high wind reports. Two brief tornadoes were observed in Illinois on May 25 and May 26.
Flooding Conditions Persist
Minor to moderate flooding was occurring at the beginning of the week on the Mississippi River from the Clarksville Dam north of St. Louis to Cape Girardeau, MO (Figure 6). On the Illinois River minor to moderate flooding was occurring from Havana south to the confluence with the Mississippi River (Figure 7). By the end of the month flooding had subsided on the Mississippi River, but continued at minor to moderate levels on the Illinois.
-SDH-