Midwest Overview - April 24-30, 2003

The last week of April continued the trend towards wetter conditions established the previous week. The total precipitation for the period of April 24-30 is quite substantial, yet does not reflect the tremendous overnight deluges on the 30th; these storm events will be discussed below, but the overnight precipitation will be reflected in the totals for May. The amount of precipitation for the week generally exceeded 1.5 inches of rain in southern Iowa, Illinois, and Indiana, and in most of Missouri (Figure 1). The northern Midwest either missed out on heavy precipitation or had precipitation after most measurements were reported for the morning of the 30th. Therefore, these areas had less than 25% of normal totals for the period, while the southern and southwestern portions of the region received more than 200% of normal (Figure 2). With several system passing through the region, temperatures oscillated above and below normal as air masses changed, resulting in near normal temperatures for the week in the central Midwest, slightly warmer conditions on the southern and western edges of the region, and cooler than normal conditions in the northeastern Midwest (Figure 3). In fact, lake ice was still a problem in some harbors along Lake Superior, and Marquette, MI, tied a low temperature record of 19°F on the 29th. The April 29th U.S. Drought Monitor (Figure 4, National Drought Mitigation Center) does not reflect late-period precipitation, some of which reached areas of intensified drought in the central Midwest in northern Illinois, southern Wisconsin, and southern Michigan. Significant improvements in drought status in these areas of the Midwest should be expected in the first May Drought Monitor map.

The period began with a low pressure center traveling through the southern tier of the Midwest. By the morning of the 25th, outflow from a substantial area of heavy rain (Figure 5, NWS) was triggering strong thunderstorms that resulted in a weak F0 tornado in southern Illinois and a more damaging F1 tornado in western Kentucky. However, most of the severe weather with this low was associated with a strong cold front sweeping across the southern U.S. (Figure 6, Storm Prediction Center). At the same time, the drought areas of the central and northern Midwest reported increasingly dry grasslands and brush. When the warm air returned from the south later in the week, several major and many minor brush fires broke out in northern Illinois, Michigan, and Ohio. The most spectacular blaze involved most of the Mentor Marsh along the shore of Lake Erie northeast of Cleveland. Firefighters from 35 fire departments were required to keep flames reaching 40 feet in height confined to natural areas and away from homes and buildings. The smoke could be seen for 20 miles, and the whole event was covered on live TV from traffic helicopters. Michigan reported a total of 195 wildfires through April 24, with 3,171 acres burned and 13 structures destroyed. Additionally, a number of dangerous fires were extinguished in grasslands near houses in the Detroit suburbs during the week. Finally, the largest fire involved 700 acres of scrub and grasses at Illinois Beach State Park north of Chicago.

The drought was also noted as having several other benefits and costs this week. Farmers in some of the drier areas of Illinois and Ohio were able to plant much earlier than the previous year, when a wet April and May delayed planting. Some of the wild fires occurred in locations that would normally require controlled burns for ecological health. It was also reported this week that several municipal and county governments in snow drought plagued Wisconsin now have thousands of tons of unused road salt in storage for next year. Milwaukee alone has 20,000 extra tons worth about $600,000. But the costs outweigh the benefits at this time. The Army Corp of Engineers was working to dredge more than 50,000 tons of sediment from the confluence of the Osage and Missouri Rivers so that overdue shipments of fertilizers for farmers could be shipped to central Missouri despite low water levels. The Lake Huron-Michigan system is 22 inches below normal for the time of year, and is now within 7.5 inches of the all time low set in April 1964. Lake St. Clair, the heavily used lake between Huron and Erie, is now 9 inches lower than last spring. Lake Erie is 9 inches below normal presently. All of these water levels are greatly affecting leisure boating and marinas, sports fishing for cold water species, and transportation by lake freighters.

After several near misses, a major precipitation event finally reached the arc of severe drought from northwestern Missouri through southeastern Iowa, into northern Illinois, northern Indiana, and southern Michigan. A strong low pressure center in the Great Plains brought a stationary front northward in the form of a warm front, introducing warm, humid, and unstable air to the drought axis. On the 29th, a strong upper level disturbance triggered the formation of a mesoscale convective system over western Iowa that moved across the state, leaving 2 to 4 inches of rain across a wide swath (Figure 7, NWS). Some remnants of these storms brought welcome moisture across the central Midwest. More importantly, these remnants created atmospheric boundaries that helped to destabilize the lower atmosphere in advance of another upper level disturbance arriving ahead of a cold front on the 30th. During the late afternoon, storms started to break out in eastern Iowa and northern Illinois (Figure 8, NWS). By 7:30 PM CDT, a broad outbreak of severe thunderstorms was in progress (Figure 9, NWS). A particular supercell crossing the Iowa/Illinois border at this time spawned a long lived series of tornadoes along its path through western Illinois. This path shows up very clearly on storm reports for the day (Figure 10, SPC). A tornado that formed south of the Davenport, IA, NWS Nexrad can be seen clearly in the base reflectivity and storm relative velocity of the radar (Figure 11, redrawn from source images at the College of DuPage). The reflectivity map on the left shows an exaggerated notch at the location of the tornado, with precipitation echos hooking around the south side (blue circle in the left panel of (Figure 11). In the storm relative velocity map, a very strong counter-clockwise rotating mesocyclone is evident where air is rushing away from the radar (red) and towards the radar (green) in the same location as the base reflectivity signal (blue circle in the right panel of (Figure 11).

At the time of this report a detailed survey of the tornado paths by National Weather Service personnel was still taking place. Fortunately, most of the tornado paths crossed rural farmland. The most damage was reported initially in Mercer County, IL, were one tornado occurred near the town of Viola and another near the town of Aledo. Outbuildings at farms were destroyed, and power poles were snapped along the path near Viola. Even after the original supercell passed the area, a small bow echo of severe thunderstorms followed the same path from west to east (Figure 12, NWS). This training effect of storm after storm brought a tremendous amount of rain to the area. Radar estimates in the core of the path indicate up to 8 inches of rain may have fallen (Figure 13, NWS), although some of this may be exaggerated by the radar reflections of hail. According to ground-based observation, totals between 3-5 inches were observed over the rainy period. Flash flooding was evident in places, and the Edwards River near New Boston, IL, rose from 12 feet to 22 feet (5 feet over flood stage) in a matter of a few hours overnight (Figure 14, USGS). Further east in the Chicago area, a daily precipitation record of 2.76 inches of rain was set at O'Hare International Airport, and 33,500 customers of Commonwealth Edison lost power due to strong winds and lightning strikes across northern Illinois. A home in the Burr Ridge suburb of Chicago was heavily damage by a lightning-sparked fire, and street flooding was common. Storms continued into the new month; these will be discussed in the next Climate watch, along with the drought relief brought to the Midwest by the heavy rains.