Midwest Weekly Highlights - November 17-23, 2007

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Cold North, Mild South, and Wet in the Middle

A frontal system that oscillated north and south through the central Midwest during the first five days of this week produced a large temperature gradient across the Midwest. Temperatures this week ranged from 3°F to 6°F above normal from southern Missouri through the Ohio Valley to 4°F below normal in northern lower Michigan (Figure 1). The central portion of the region saw the greatest day-to-day variations as cold air moved south, then retreated north, and then moved south again.

Precipitation this week was confined to a relatively narrow band in the central Midwest, but here it was as much as 300 percent of normal (Figure 2). Almost all of the precipitation fell in association with a storm system that developed on November 21 and which finally pushed the frontal system through the entire Midwest. Heavy rain fell from central Illinois east northeastward into northwestern Ohio, with amounts exceeding five inches in the Lake Superior snow belt. Snow fell from central Iowa northeastward into Michigan (Figure 3). There was little change reflected in the drought conditions in the Midwest this week, and Extreme Drought continues to exist in extreme southeastern Kentucky (Figure 4).
 

Storm Impacts Thanksgiving Travel

A wave of low pressure developed along the front extending through the Midwest late on November 20. This low moved northeast along the front on Wednesday, November 21, producing heavy rain along and behind the front. More than an inch of rain fell from central Illinois to northwestern Ohio, with a band of 2.00 to more than 2.75 inches of rain from east-central Illinois through north-central Indiana (Figure 5). Thunderstorms dropped some of the heavy rain, but the only severe weather was dime to nickel size hail in southern Missouri on November 21.

The front ushered in some of the coldest air of the season to date, and snow developed in the cold air behind the front. On the Wednesday before Thanksgiving (November 21) snow advisories were issued from southern Iowa eastward through Michigan, and winter storm watches were issued in the lake effect areas of northern Indiana and in the thumb of Michigan (Figure 6). Two five inches of snow fell in parts of southern Iowa (5.0 inches in Grundy Center (Grundy County), and up to 4.7 inches in northern lower Michigan. The biggest impacts on travel were on the roads. Airport delays on Wednesday were surprisingly minimal, partly due to the timing of the snow. At Chicago's O'Hare International Airport, the accumulating snow did not begin until around 9:00 p.m. on Wednesday, by which time most travelers had left the airport. During the day outbound delays at O'Hare were generally a few minutes, and seldom more than an hour.

There was a wintry chill in the air on Thanksgiving Day in all but the far southeastern Midwest. High temperatures were only in the low 20s in northern Minnesota to the mid to upper 40s in far southern Kentucky (Figure 7), 5°F to 10°F below normal for this date.

SDH