Midwest Weekly Highlights - September 25-30, 2004

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Fall Begins Finally

During the last six days of September, the Midwest experienced the onset of fall with the passage of a nearly rain-free cold front on the 27th and 28th. While the front did not have much moisture, it reflected a change in the upper level flow that allowed the first Canadian high of the season to move southeastward into the Midwest. Conditions were dry and cool throughout the region, with only isolated patches experiencing normal rain near the Michigan / Indiana / Ohio meeting point (Figure 1). About 90% of the Midwest had less than a quarter of the normal precipitation for the period (Figure 2). Unlike the rest of September, the central area of the Midwest was below normal in temperature for the September 25-30 period by 1-3°F (Figure 3), although the edges of the Midwest came out slightly above normal despite the late period cooling. However, the cooling was still a significant change in the trajectory of the season, bringing the first freezes to the northern Midwest on the 28th, 29th, and 30th (Figure 4). Low temperatures reached the mid-20s in the northern tier of the region, and otherwise ended the growing season in much of Minnesota, Wisconsin, and Michigan.
 

The Month Ends Drier and Cooler Than It Began

The substantial lack of rain in the central Midwest culminated during the last week of September with even the wet northwestern and southeastern edges of the region not receiving significant precipitation. The remnants of Hurricane Jeanne, unlike Frances and Ivan, stayed to the east of the region. The soil moisture model of the Climate Prediction Center (CPC) showed increasing dryness in the northeastern and southwestern areas of the Midwest (Figure 5, CPC). However, the drying area was much larger, and is better revealed in the change in CPC soil moisture estimates since the end of August (Figure 6, CPC). A large swath of the central U.S. from Texas to Michigan had undergone a great shift towards drier soils in just 30 days. The U.S. Drought Monitor released at mid-period reflected these changes in greatly expanded abnormally dry (D0) regions covering Michigan, the southeastern half of Wisconsin, northern Illinois, and parts of southern Missouri (Figure 7, National Drought Mitigation Center). The only reason the entire Midwestern portion of the drying soil swath has not been designated as abnormally dry is simply that much of northern Missouri, Iowa, central Illinois, and Indiana initiated the drying trend from a much wetter starting point than Michigan or eastern Wisconsin.

The tightly packed isotherms (dashed lines of equal temperature in Figure 8, College of DuPage) indicate the position of the cold front on the morning of the 28th, trailed by a strong Canadian high pressure center. After just a brief period of cloudiness, the almost perpetual sunshine of this September returned, ameliorating the air mass temperatures somewhat during the day. However, at night, seasonably cold temperatures were recorded that ended the growing season in much of the northern Midwest (for example, (Figure 4). No records were set, as this was a return to normal climate conditions after the unexpected extension of climatological summer into late September.