September 15-21, 2007
Midwest Weekly Highlights - September 15-21, 2007
Big Contrast
The passage of a cold front late last week into the weekend of September 15th brought cold conditions across much of the upper Midwest (Figure 1). Below freezing temperatures dipped as far south as central Illinois, but the coldest reading occurred in Embarrass, MN where it fell to a chilly 18°F on September 15th. The cold temperatures did not last long as warmer temperatures surged into the region on Sunday the 16th and into the early part of the week (Figure 2). The temperatures shifted dramatically across Iowa as there was a difference of over 55°F from their coldest lows to their warmest highs over the last week (Figure 3). Temperatures across the region primarily remained above normal (Figure 4). Michigan, Illinois and Missouri all had temperature ranges from normal to 5°F above normal. Ohio and Kentucky remained the coolest where average temperatures ranged from near normal to 3°F below normal.
Rainfall failed to fall over the central and eastern areas of the Midwest (Figure 5). Michigan, Ohio, Indiana, Kentucky, Illinois and the southern half of Missouri only saw 0% to barely 20% of the rainfall that should typically fall in the third week of September. Minnesota has seen the majority with anywhere from 100% to over 700% of the normal. West and cental Iowa saw near to slightly above normal precipitation (Figure 6).
An Early Freeze
Iowa State Climatologist Harry Hillaker noted that the freeze that occurred on the 15th was unusual for Iowa. Preliminarily, 56% of the National Weather Service official reporting locations recorded a temperature of 32°F or below (Figure 7). Spencer Airport was the coldest at 28°F; they later went on to reach 92°F for a high just two days later. The 56% freeze rate has not been met or beaten since September 12/13, 1902 where 63% of the state was at or below freezing. As a matter of fact, there have not been any freeze events that have even come close to the 2007 event other than the 1902 case.
Thethe next closest record years in Iowa were:
- Sep. 15, 1916: 39%
- Sep. 14, 1923: 30%
- Sep. 11-12, 1955: 28%
- Sep. 3, 1974: 17% (probably one of Iowa’s most damaging fall freezes)
- Aug. 20, 1950: 4%
Scattered Storms
The warmer temperatures included shower and thunderstorm activity across the Upper Midwest. The jet stream, and high pressure to the east, helped to steer storm systems continually across the area. The clash of cool air from Canada, and warm moist air from the Plains resulted in repeated rounds of stormy weather. A cold front attempted to sag south on Tuesday through Wednesday. Severe storms occurred over Iowa in association with these storms and very high winds were recorded across the state. The highest reading was just north of Vinton, IA (Benton County) where winds remained steady at 46 mph, while gusting up to 79 mph (Figure 8). Vinton is located northwest of Cedar Rapids.
A strong warm front for late September pushed northward across southern Minnesota on the 20th. Warm, moist air surged northward south of this feature. Isolated severe thunderstorms tracked across western Minnesota during the morning. The front grew stronger during the afternoon and focused severe thunderstorms across central Minnesota during the afternoon into the evening. Severe weather reports included several large hail and damaging wind readings (Figure 9). Drenching rains covered a large area of Minnesota from the 18th on. Some of the hardest drought effected areas received a welcome 3"+ from September 18th through the 21st (Figure 10).
Drought Update
The lack of rainfall across the Midwest only helped to expand drought conditions across the majority of the area (Figure 11) when compared to last week (Figure 12). More than 3" of rain fell across drought ravaged areas of Minnesota (Figure 13). One would think that the rain would help to improve the drought conditions, but unfortunately they did not help much. Many parts of Minnesota are more than 5" below normal since June 5th of this year, however this may change with the late week rains. Northwest Wisconsin is seeing a similarly dry fate. The recent rains for this week (Figure 14) started edge off of the prolonged dry period. According to the U.S. Drought Monitor, northwest Wisconsin is 3" to 6" below normal over the last 60 days, and 6" to 12" below for the last 120 days. This shortage coupled with near or record low level stream flows helped to justify an increase in drought conditions. Conditions slightly intensified across central Missouri, central Illinois and along the lower Ohio Valley due the very limited rainfall. Sections of central Kentucky saw improvement from the rains last week as some areas were downgraded from D3 to D2. When looking at the Midwest as a whole, the drought has slightly decreased.
A few drought statistics:
- Iowa is completely drought free
- 76% of Minnesota is experiencing some form of drought, slightly more than last week
- 53% of Wisconsin is experiencing drought, while over 25% is considered severe
- Michigan's upper peninsula is entirely affected by Moderate Drought
- Drought affected areas slightly grew in Missouri
- Severe drought conditions cover all of Kentucky
- Over 70% of Kentucky is experiencing extreme drought
Daily Records September 15-21
Daily records for the third week of September can be viewed in Table 1.
SSS