Midwest Weekly Highlights - September 11-17, 2004

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Flooding Rains Coming and Going

The week of September 11-17 produced two separate flood events in the Midwest. One event was due to training thunderstorms traveling over the same ground on the 14th and 15th and dumping 8-12 inches of rain in northern Iowa and southern Minnesota. The other event was just beginning at the end of the period, as the remnants of Hurricane Ivan brought heavy rains over regions already drenched by the remains of Hurricane Frances in eastern Kentucky and Ohio. Most of the rain for the week feel in these two areas (Figure 1), leaving much of the Midwest dry from southern Missouri to eastern Michigan. Only a small area from northeastern Missouri into far western Iowa received above normal precipitation in this band (Figure 2). The Midwest was also much warmer than normal for the week, especially in Wisconsin and Michigan, where temperatures were 6 to 9°F above normal. The rest of the Midwest ranged from 2 to 4°F above normal (Figure 3). This unseasonable warmth is greatly appreciated by farmers in the northern Midwest, where crops have still not reached maturity due to cold conditions during August. Record corn and near record soybean production is anticipated for this growing season in the U.S. Corn Belt.

It has indeed been an unusual September in which not one but two former hurricanes impacted the region. Fortunately, Ivan (Figure 4, NWS) moved faster than expected and cleared the region early in the next period. Unfortunately, Ivan’s flood waters still caused tremendous damage and several deaths in the Midwest; this will be discussed in detail in next week’s summary.

Heavy Rains in Iowa and Minnesota

On the 13th of September, weather systems were blocked from entering the Midwest due to a lack of winds above the surface to steer systems through the region (Figure 5, College of DuPage - COD). A surface low pressure stalled over the Dakotas (Figure 6, COD) brought warm and humid air from the south, which then was lifted along a stationary front over the cold air to the north. Thunderstorms broke out late on the 13th near the joining point of Iowa, Minnesota, and South Dakota (Figure 7, NWS), and proceeded from there across northern Iowa and into southeastern Minnesota. A heavier set of storms repeated the process on the 14th (Figure 8, NOAA Radar Operations Center Archive), with the heaviest rain falling on the evening of the 14th and morning of the 15th. Total rain amounts for the period exceeded 10 inches on radars located at Sioux Falls (Figure 9a, NWS), Des Moines (Figure 9b, NWS), and Minneapolis (Figure 9c, NWS), and at several unofficial ground measurement sites. Some of the largest 24-hour rainfall measurements at official sites reported by media and the NWS included:

Originally posted: January 30, 2023 9:51am EST