August 1-7, 2008
Midwest Weekly Highlights - August 1-7, 2008
Warm, Humid, and More Heavy Rain
An upper level ridge of high pressure over the western U.S. expanded to the east the first week of August, bring muggy conditions to much of the region (Figure 1 - 500 mb map). The hot weather over the Plains the last two weeks began to creep into the Midwest this week, but only the western portions of the region experienced the hot weather for any length of time. Temperatures this week ranged from near to slightly below normal along the northern periphery of the Midwest to 6°F to 7 °F above normal across much of Missouri (Figure 2).
A significant portion of the Midwest was dry this week, but severe thunderstorms brought heavy rain to an area from northeastern Illinois through Indiana and into southwestern Ohio (Figure 3). Rainfall was 150 to more than 400 percent of normal in this band, with the heaviest rain in northwestern Indiana (Figure 4). The persistent drier than normal weather in Minnesota has resulted in Moderate Drought being depicted on the August 5 Drought Monitor in a narrow band from Minneapolis-St. Paul westward (Figure 5).
Hot Weather Makes a Push
A cold front dropped south through most of the Midwest on August 1-2 providing a couple of days of seasonably warm summer weather. The exception to this was an area generally south of a line from Kansas City, MO to Louisville, KY where high temperatures remained in the low to mid 90s south of the stalled front. This boundary began to return northward on August 3, and by August 4 hot, humid weather had spread as far north as central Iowa and east through the southern two-thirds of Illinois (Figure 6). Temperatures in the 90s combined with dew point temperatures in the mid to upper 70s (Figure 7) produced heat indices from 105 to 115 across most of Missouri, southern Iowa, and Illinois on August 4.
Severe Weather Hammers Illinois, Indiana
A strong jet stream aloft and hot humid air near the surface provided the ingredients for potent storms along the warm front advancing across the northern Midwest. In the late afternoon and evening of August 4 severe thunderstorms erupted beginning in Iowa and then raced through northern Illinois, northern Indiana, and western Ohio. At 5:00 p.m. CDT a cluster of strong thunderstorms was moving from eastern Iowa into northwestern Illinois (Figure 8). These storms, moving to the east at up to 60 miles per hour, with wind damage reports across northern Illinois. By 8:00 p.m. CDT the line of thunderstorms, towering to almost 70,000 feet, had developed a classic "bow echo" configuration as they moved through the Chicago metropolitan area (Figure 9). The line of storms then swept out of Illinois and across southern Lake Michigan and through northeastern Indiana, entering western Ohio by midnight CDT (Figure 10). In northern Illinois the most significant damage in the Chicago area and suburbs. Commonwealth Edison reported that 427,000 customers were affected by the storm, with 238,000 still without power on the morning of August 5, with 102,000 of those in Chicago. At least three tornadoes were confirmed in northeastern Illinois, and a wind gust of 94 mph was recorded by an automated station located at the Harrison Crib in Lake Michigan about 4 miles NNE of downtown Chicago. A sellout crowd of 40,867 at Wrigley Field for the Chicago Cubs-Houston Astros game was directed to seek shelter in the stadium concourse areas when tornado sirens went off at 8:09 pm CDT. The game was later called in the eighth inning due to intense lightning and more rain. Passengers at Chicago's O'Hare Airport were evacuated to the lower level of the terminals when the storms hit, and 350 flights were canceled.
A 23 year-old man was killed in La Porte County, IN when a tree toppled by high winds fell on his car. A confirmed EF2 tornado touched down in Griffith, IN (Lake County) about 8:40 pm CDT. The storm produced at least a five-mile path of damage. The National Guard was called out to prevent looting to stores damaged by the tornado. An estimated 80 to 100 homes had received some type of structural damage. A second Indiana tornado (EF0) was confirmed in Boswell (Benton County).
In addition to the high winds and tornadoes, much of northwestern Indiana was deluged by heavy rain (Figure 11). Rainfall amounts ranging from 3 to more than 6 inches were reported by CoCoRaHS observers across central Lake and Porter and southwestern La Porte counties (Figure 12).
Cooler, Drier
The cold front moved into the northern Gulf States by the morning of August 7 and cooler, drier air spilled across the Midwest in its wake. High temperatures in the 70s and 80s along with dew points in the 50s and 60s brought a comfortable end to this week.
SDH