Midwest Weekly Highlights - May 8-17, 2005

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Active Period in the Midwest

Wet weather finally reached the Midwest during the period May 8-17, as two slow moving low pressure centers brought heavy rains and some severe weather to much of the region. Almost the entire region received at least 0.5 inches of rain, while much of Iowa received well over 3 inches and experienced some flash flooding (Figure 1). The rain did not reach normal levels everywhere, though, with Illinois especially continuing to fall behind (Figure 2). Temperatures during the week oscillated with the frontal boundary passages of the low pressure systems, but the northern Midwest was generally 4-8°F below normal, and the southern tier was near or slightly above normal (Figure 3). The modest temperatures and substantial rains did take a considerable amount of abnormal dryness out of the region in the U.S. Drought Monitor by the 17th (Figure 4, National Drought Mitigation Center - NDMC), compared to the situation at the beginning of the period (Figure 4a, NDMC). Most of the severe weather action occurred in the first 6 days of the period, but a return of severe weather and rain occurred on the evening of the last day as a new system entered the region on the 17th.

Low Pressure Center Crawls Past

During the period May 8-11, a low pressure center took 3 days to cross the Midwest on a trajectory from South Dakota (Figure 5a, College of DuPage - COD), to Minnesota (Figure 5b, COD), and exiting through Lake Superior (Figure 5c, COD). The worst storms with this system occurred on the evening of the 8th (Figure 6, University Corporation for Atmospheric Research - UCAR), with a substantial amount of severe hail in northwestern Iowa, and a tornado with little damage south of Minneapolis (Figure 7, Storm Prediction Center - SPC). Several more bands of showers whipped through the Midwest, spiraling around the low, bringing substantial relief of dryness to the upper Midwest.

Second Low Moves Even Slower

By the 11th, the upper air flow was even more sluggish, with a large cutoff low situated over the Rockies (Figure 8, COD). The surface low pressure center never really reached the Midwest, spinning out on the Great Plains on May 11 (Figure 9a, COD), and May 12 (Figure 9b, COD) before ejecting towards the north on May 13 (Figure 9c, COD). A stationary front projected eastward across the Midwest, providing a center for daily convective activity. The most pronounced outbreak of storms occurred on the 11th and 12th, with large clusters of storms across the Midwest (Figure 10a and Figure 10b, UCAR). A significant band of severe weather was noted on both days (Figure 11a and Figure 11b, SPC). The largest impact was the heavy precipitation, however, leading to flash flooding and some small river flooding. The storminess finally ended on the 14th, leading to the end of the period being cool but dry.