Midwest Weekly Highlights - May 18-24, 2005

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The Wet Get Wetter, the Dry Get Drier in the Midwest

During the week of May 18-24, storm systems tended to travel over the northern and eastern edges of the region, bringing substantial rains to these areas. Unfortunately, the core of abnormally dry conditions in the central and southwestern Midwest received less than one inch of rain (Figure 1). Precipitation totals well below 75% of normal enhanced dryness in Missouri, while in Illinois, the rain was spotty, bringing relief to narrow bands but not to widespread areas. Precipitation totals for some of these area-averaged grids reached 75% to 100% on the precipitation departure map (Figure 2), but many locations in Illinois still missed out on even moderate rain. The dryness was related to a cool northerly flow dominating the weather most of the week, with temperatures 3 to 6°F below normal in the eastern Midwest (Figure 3). Toward the end of the week, some warmth leaked into the western Midwest around a large western high pressure ridge, bringing about temperatures there for the week that were slightly above normal. The cool temperatures helped stem evaporative losses somewhat, but the air was still dry, and conditions deteriorated to a moderate drought state across much of central Illinois in this week's U.S. Drought Monitor map (Figure 4, National Drought Mitigation Center).

Severe Weather on the 19th and 20th

During the week, the jet stream was often flowing through the northern part of the Midwest. However, on the 19th, a strong upper air disturbance dipped down into the region in association with a surface cold front and brought a widespread outbreak of severe weather to central and southeastern portions of the Midwest. More than 200 severe hail and wind events were reported, including several tornadoes (Figure 5, Storm Prediction Center). The outbreak of large thunderstorms is quite evident in a late afternoon visible satellite image (Figure 6, College of DuPage). One set of storms formed near the low center in northern Illinois, and more storms formed ahead of the system in Indiana, southern Illinois and Kentucky. By early evening, the Illinois storms had joined a squall line except for one supercell on the Indiana/Illinois border, while widespread thunderstorm activity in and near Kentucky was coalescing into a strong squall line (Figure 7, University Corporation for Atmospheric Research). Interestingly, streaks of moderate precipitation totals crisscrossed eastern Illinois (Figure 8, NWS), but heavier overall totals fell in Kentucky (Figure 9, NWS). The week ended cooler and drier, with substantial rates of water loss at the surface as measured in evaporation pans.