Midwest Weekly Highlights - September 10-16, 2011

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Cool Autumn Weather

The second week of September started with summer-like weather across the region, but a strong cold front brought sharply colder weather to the region, and much of the northern Midwest experienced the first freezing temperatures since the spring. Temperatures this week were above normal through September 13th, but then were much below normal the remainder of the week. For the week as a whole temperatures ranged from near normal in Minnesota, Wisconsin, and eastern Ohio to 3°F to 5°F below normal in Illinois, Iowa, and Missouri (Figure 1)
 

Finally, Some Rain

Most of the Midwest was dry this week with the exception of the southern Midwest and eastern Ohio (Figure 2). There were a few scattered severe storms during the week (Figure 3) An upper level wave moving through Midwest on September 14-15 produced a wide shield of rain across the southern Midwest. The heaviest rain fell in eastern Missouri and southwestern Illinois, where upwards of two inches of rain accumulated (Figure 4). This rain only affected the far southwestern portions of the drought across the central Midwest. Severe drought persisted from Iowa across central Illinois and southwestern Indiana on the September 13 Drought Monitor (Figure 5).
 

Wildfire in Northeastern Minnesota

The Pagami Creek Wildfire began with a lightning strike in the Boundary Waters Canoe area of northern Minnesota on August 18th. Dry weather across Minnesota has made the fire difficult to control and it has continued to burn. On September 13th smoke from the fire was transported across Wisconsin and into Illinois. Afternoon visibilities that day in Chicago dropped as low as 3 miles especially in lakeside counties. The development of a lake breeze forced air to sink offshore, drawing dense smoke above the ground down to lake level where it was able to move onshore with the breeze. Satellite photos clearly showed the plume of smoke from the fire extending southeastward into Wisconsin and northern Illinois (Figure 6). Smoke actually was detected as far south as central Illinois and Indiana.
 

Freezing Temperatures

Following the passage of a cold front on September 13 and 14, a sprawling high pressure system settled in over the Midwest (Figure 7). Clear skies and the cold, dry air resulted in record lows and record low maximum temperatures across the Midwest on September 14-16. International Falls (Koochiching County) dropped to 19°F on September 15th, breaking the old record of 24°F set in 1964. The freezing temperatures extended as far south as southern Iowa and northern Illinois (Figure 8). The Mason City, IA (Cerro Gordo County) airport minimum of 26°F this year was the lowest reading anywhere in Iowa for so early in the fall since 1955. At Cedar Rapids, IA (Linn County) and Jefferson, IA (Greene County) this event tied with 2007 recording the earliest 32°F reading on record. A hard freeze (less than 28°F) occurred on both September 14th and 15th across northern Minnesota (Figure 9), bringing an end to the growing season there.
 

-SDH-
The Iowa State Climate Office also contributed to this report.