September 22-30, 2007
Midwest Weekly Highlights - September 22-30, 2007
Dog Days of Fall?
The beginning of the fall season was embraced by unseasonably warm temperatures across the entire region. September 23rd was the official start to autumn, and average temperatures ranged from 6°F above normal near Detroit, MI to over 20°F above normal in Fergus Falls, Minnesota (Figure 1). Temperatures cooled midweek with the passage of a strong cold front, but still remained mostly above normal for the entire week. The largest departures from normal (9.5°F +) occurred across western and central Illinois and throughout central and southern Indiana (Figure 2), with coolest temperatures in central Iowa and Minnesota where temperatures ranged from 4.5°F to 7°F above normal.
Precipitation was highly variable across the region throughout the final week of September. Minnesota, Wisconsin and the Upper Peninsula of Michigan ranged from normal to over three times their normal precipitation for the week (Figure 3). Kentucky continues to stay dry; seeing roughly 5% to 25% of their normal precipitation.
Low pressure developed over Colorado on Saturday the 22nd into Sunday the 23rd. Strong surface winds allowed the advection of warm and humid air to infiltrate the region and send highs up into the record territory. The Indianapolis National Weather Service Office noted that their record temperature of 92°F on the 23rd is only the fourth time that a day of 90°F temperatures were observed this late in the year. The last occurrence was on September 27th of 1998 where temperatures reached 93°F. Prior to 1998, the other two greater than 90°F September days occurred in 1953 and 1871. Upper level winds guided the surface low pressure northeast over Minnesota on Monday the 24th and into the Ontario Province by Tuesday the 25th. A trailing cold front provided welcome showers across Wisconsin and much of the Midwest throughout the week (Figure 4). By Friday the 27th, the slow moving cold front (Figure 5) had moved out of the area after dropping welcome, but limited, rain over Kentucky and the Ohio Valley (Figure 6). Rainfall totals in northwest Wisconsin ranged from 3" to 5" where drought has been taking its toll. Scattered storms occurred last week and a few were accompanied by high winds, hail and even a few tornados (Figure 7). During the evening of Sunday September 30, 2007 two tornadoes struck portions of Marion, Jasper, and Poweshiek Counties. The first tornado produced EF0 to EF2 damage along its track and was rated EF2
on the Enhanced Fujita Scale with maximum winds of 125 mph. The track was 22 miles in length
from 4 miles northeast of Pella to 1 mile north of Interstate 80, 2 miles west of Malcom.
Much of the track was three to five tenths of a mile wide, but as wide as seven tenths of a mile
at times.
The Good and Bad of Drought
With autumn comes a decrease in temperature and evapotranspiration rates, so soil moisture levels should be increasing as precipitation occurs now that the growing season is effectively over. Any additional precipitation should accumulate and help to alleviate drought conditions. The continued precipitation surpluses from the rain events this week, coupled with decreasing evaporative demands resulted in a one-category improvement across most of the northern half of Minnesota and extreme northwestern Wisconsin (Figure 8). Areas of Illinois, Indiana and Kentucky had conditions deteriorate with the lack of precipitation and warmer than normal temperatures. Extreme Drought (D3) conditions edged further into southern Indiana and filled in a larger area of Kentucky. Kentucky is seeing the hardest conditions across the Midwest. The entire state is in Severe Drought (D2) or worse while over 87% is classified as Extreme Drought (D3) or worse (Figure 9).
Daily Records September 22-30
Many records were broken over the final week of September. Only significant records are listed in Table 1.
SSS