August 18-24. 2009
Midwest Weekly Highlights - August 18-24, 2009
A Wet and Wild Week
Many areas in the central Midwest received a month's worth of rain this week as showers and thunderstorms were almost a daily occurrence. In addition, severe weather returned in earnest after a period of relative quiet. Every state except Kentucky recorded severe storms, and tornadoes dropped from the skies in seven states early in the week as a strong low pressure system spun over the upper Midwest.
Rainfall this week was two to three times normal across Missouri, much of Illinois, the northern three-quarters of Indiana (Figure 1). Rain was also at least twice normal in eastern Minnesota and across the Michigan U. P., areas that have been very dry for much of the summer. Rainfall total in some locations in northwestern and southwestern Missouri exceeded 5.00 inches this week, and many locations across the Midwest received from 3.00 to more than 4.00 inches of rain (Figure 2). Northern Minnesota and southeastern lower Michigan were the driest areas this week, receiving less than half the normal expected rainfall.
The heavy rain in the upper Midwest came after the August 18 U.S. Drought Monitor (Figure 3) was released and was not reflected in that depiction.
Summer warmth continued into the early part of this week as the strong low over the upper Midwest produced strong southerly winds. A few record high minimum temperatures were set in the eastern half of the region August 18-20. By the middle of the period, however, colder air was again plunging south through the region dropping temperatures to well below normal levels. Low temperatures plunged into the 30s across northern Wisconsin and the Michigan U. P. the morning of August 23, and record low maximum and minimum temperatures occurred in all nine states from August 20-24. A strong upper level low spinning over the Great Lakes produced cloudy skies and some showers over parts of the region while maintaining the flow of cool air. Temperatures this week ranged from 4 °F above normal in extreme northeastern Ohio to 8°F below normal in western Missouri (Figure 4).
Severe Storms and Heavy Rain
A cold front moving south through the Midwest was responsible for the the stormy start to the week. On the morning of August 18 the cold front extended from western Michigan southwest into central Missouri (Figure 5). A few severe thunderstorms occurred on August 18, including one which produced an EF1 tornado near Watseka, IL (Iroquois County). This cold front stalled in the southern Midwest on August 19 as another push of cold air entered the Northern Plains (Figure 6). As low pressure intensified in the Plains, the frontal boundary began to move north as a warm front. The warm humid air overrunning the cooler air to the north, combined with favorable wind shear led to a large outbreak of severe storms on August 19 from Minnesota south to Missouri and eastward to western Ohio. There were 30 reports of tornadoes during the day
During the early afternoon four tornadoes were confirmed to have touched down in eastern Minnesota and western Wisconsin, including one in Minneapolis, MN (Hennepin County). Three of the tornadoes were classified as EF0 and one as EF1. An EF0 tornado touched down in south Minneapolis and then moved into downtown, eventually lifting near the Convention Center. Most damage was to trees and power lines, with some structural damage to homes and businesses.
A complex of thunderstorms that developed in Missouri late on August 18 and moved east overnight entered Illinois and intensified on August 19. Tornado watches were issued for an area from eastern Missouri into central Indiana. Tornado warnings and severe thunderstorms warnings were issued for areas ahead of the line of severe storms (Figure 7). The first tornadoes dropped down over Morgan, Scott, and Sangamon County in Illinois. In Williamsville, IL (Sangamon County) 25 structures were severely damaged or destroyed on the northwest side of town by an EF3 tornado. This tornado continued into Logan County, where it caused damages estimated at more than $4.8 million. In Loami (Sangamon County) ten homes were destroyed and another 12 were badly damaged by an EF2 tornado. Nineteen people were injured by the storms in central Illinois. In all, six tornadoes were confirmed in central and east-central Illinois during the afternoon of August 19.
Storms moved into northern Illinois during the late afternoon and produced two tornadoes within 15 minutes of each other near Elburn, IL (Kane County). The tornadoes were rated EF1 and EF0, respectively, and resulted in tree damage and some structural damage to homes.
The line of storms continued to march steadily eastward during the evening hours. Around 7:30 p.m. CDT a tornado touched down near Chesterton, IN (Porter County). This tornado, rated EF2, damaged trees, homes, and power lines along a 2.5 mile path through the town. Emergency officials reported 211 structures damaged, including eight destroyed and 54 with major damage. At least 13,000 customers were without power. Only one minor injury was reported. The storms weakened as they moved into Ohio but still packed a powerful punch. In the Dayton, OH area (Montgomery County) 12,000 customers lost power as high winds downed trees and power lines. A man suffered minor injuries while on the phone when lightning struck the building he was in.
Storms continued to fire overnight on August 19 into the morning of August 20th along the front in southwestern Missouri. The thunderstorms produced a broad swath of rain three inches or more. A number of observers in the Community Collaborative Rain, Hail, and Snow network (CoCoRaHS) reported five to more than six inches of rain in southwestern Jasper County, MO (Figure 8). The observer near Oakland Park, MO (Jasper County) reported 6.54 inches for the 24-hour period.
A Taste of Autumn
The second cold front pushed through the Midwest on August 20 still accompanied by showers and thunderstorms. Severe weather was confined to the eastern two-thirds of Ohio, with a brief tornado touchdown reported in Ross County. Much cooler air spilled into the region behind the front on August 21, with temperatures in the 60s across the northern half of the Midwest (Figure 9). A strong upper level low spun over the lower Great Lakes (Figure 10), and the cloudiness associated with the low (Figure 11) helped keep high temperatures on August 22 in the 60s and 70s across the region (Figure 12). The weather remained fair through the rest of the period, although by late on August 24 showers and thunderstorms began to develop over Minnesota as another cold front entered the Northern Plains.
SDH