Midwest Weekly Highlights - June 24-30, 2009

A Week of Change

Hot, humid weather continued into the last week of June across most of the Midwest, but relief finally arrived as much cooler, drier air spread over the region the last half of the week. Showers and thunderstorms continued to fire in the hot humid air the first half of the period, while lighter showers occurred over the northern Midwest as an upper low slowly spun over the Great Lakes.

Temperatures during the first half of the week were much above average, running 4°F to 6 °F above normal from southern Missouri northeastward through Illinois and into western Michigan. There were numerous record high temperatures and record high minimum temperatures set June 24-26. Following the passage of a strong cold front on June 27 temperatures dropped to 3°F to 6°F below normal across much of the region as cool dry Canadian air spread southward. Temperatures for the week were near normal across northern Iowa and much of Minnesota, and 3°F to 5°F above normal across the southeastern half of the region (Figure 1). High temperatures ranged from the mid 90s in the southern Midwest to the mid and upper 80s north the first half of the week, but by week's end had dropped to the low 60s north and in the low to mid 80s south and far west. After many record high temperatures the first part of this week, a number of stations in the northern Midwest set record low maximum temperatures the last two days of the month.

Most of the Midwest was dry the last week of June. Only northwestern Minnesota, northern lower Michigan, and the eastern Ohio Valley received above to much above normal rainfall (Figure 2). The June 30 U.S. Drought Monitor continued to depict Severe Drought in northwestern Wisconsin and a broader area of Moderate Drought in the western Michigan UP, northern Wisconsin, and eastern Minnesota. There was also some expansion of Moderate Drought into western Minnesota this week (Figure 3).

Severe Weather Thrives in Heat and Humidity

Severe thunderstorms were not as numerous as the previous week, but all nine states in the region reported severe weather this week. Most of the storms were scattered around the region. On June 24 a line of thunderstorms formed in Chicago along a lake breeze moving inland during the early afternoon (Figure 4). These training storms produced more than two inches of rain which caused flash flooding, dime-sized hail, and damaging winds. Midway Airport reported a gust to 63 mph, and the roof of a dugout at a high school baseball field in Burbank (Cook County), two miles south of Midway Airport, was ripped off by high winds. One minor injury was reported.

Two frontal systems produced larger and more organized area of storms. On June 25 a weak cold front (Figure 5) dropping through the Great Lakes triggered severe storms from eastern Indiana through Ohio and from Michigan south to eastern Kentucky. Hail around one inch was common and hail the size of tennis balls was reported. Thunderstorm winds produced tree and power line damage. Winds caused an estimate $1 million in damage to a UPS warehouse in Hebron, OH (Licking County) when a large part of the roof caved in. Intense lightning was responsible for at least five fires in southern Ohio with estimated losses at nearly $1 million. Flash flooding occurred in southern Indiana, southern Ohio, and northern Kentucky. There were no tornadoes reported.

The second organized area of severe weather occurred on June 27 as a much stronger cold front (Figure 6) approached the Midwest. The severe storms were concentrated in eastern Iowa, northwestern Illinois, and central Missouri along the leading edge of the cold front. Again no tornadoes were reported, but thunderstorms winds produced tree and power line damage. The front produced a few severe storms in eastern Kentucky on June 28.

Heat Breaks

The cold front that moved through the Midwest on June 27-28 brought welcome relief from more than a week of heat and humidity. A strong upper level near Lake Superior funneled much cooler and drier air into the central United States (Figure 7). Dew points dropped from the low and mid 70s to the low 50s (Figure 8). On June 29 the only 90 degree readings were found in far western Missouri (Figure 9), with high temperatures only in the mid 50s to mid 60s across northern Minnesota, Wisconsin, and Michigan as clouds associated with the low shrouded the northern Midwest (Figure 10). The cloudiness associated with the low extended further south on June 30 keeping temperatures in the 70s as far south as the Ohio River. Scattered light showers occurred across the Michigan U. P., Wisconsin, and lower Michigan as the upper low moved southeast over Lake Huron. Thunderstorms developed along the southern periphery of the cooler air from extreme southern Missouri eastward into southern Kentucky, with a number of severe storms reported. High temperatures remained in the mid and upper 80s from southwestern Missouri into southern Illinois (Figure 11).

SDH