Midwest Weekly Highlights - September 8-14, 2006

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The second week of September was surprisingly active, and temperatures were once again on the cool side for most.  The average daily temperature departure map for the Midwest shows that much of the Midwest was below average for this time of year, with the exception being across extreme north central Minnesota (Figure 1).  Here, daily temperature departures were very close to normal, to even 1°F above normal.  This was not the norm, however, for the remainder of the Midwest.   The coolest spot this week was western Wisconsin and the Upper Peninsula of Michigan, where temperature departures were 5-6°F below normal and 6-8°F below normal respectively.  Elsewhere, an axis of cooler than normal conditions was located from central Ohio westward into central Illinois, where departures were 1-2°F below normal.  Surrounding these locales, departures were generally 3-5°F below normal for this time of the year.

Precipitation for the week was fairly widespread and heavy, though we saw less thunderstorm activity and more stratiform rains typical of our transition into the fall season.  The heaviest precipitation this week fell across central and northeastern Iowa, southern Wisconsin and northern Illinois, and southern Indiana eastward into southern Ohio (Figure 2).  These locations saw precipitation totals that were 200 to 500% of normal for this time of year, with a large area greater than 300% of normal.  Meanwhile, northern Wisconsin, northern Minnesota, and extreme southern Missouri saw precipitation totals that were only 0 to 25% of normal, exacerbating drought concerns across these areas, especially across northern Minnesota, where drought remains extreme (Figure 3).  The dry conditions that prevailed across the upper Midwest this week, when combined with strong gusty winds, temperatures in the lower 80's, and low relative humidities, prompted the issuance of several Red Flag Warnings, meaning conditions were ripe for the rapid growth and advancement of fire.

 
First Frost/Freeze of the Season

A strong cold front sunk across the Midwest from north to south on September 8, generating strong to severe thunderstorms across Iowa, Minnesota, Michigan, and Wisconsin (Figure 4).  Large hail and a few reports of damaging winds dotted the region.  Behind the front, anomalously cold weather settled in, resulting in new record cold minimum temperatures.  Temperatures dipped into the upper 20's and lower 30's across northern Minnesota and northern Wisconsin, abruptly ending the growing season.  A few new record cold minimum temperatures were set across this part of the Midwest, namely International Falls on the 9th which recorded a low temperature of 25°F (previous record was 33°F in 1956), and again on the 10th when the temperature dropped to 29°F (previous record was 31°F set in 1955).  

    
Damp...Dreary...and Cool

A slow-moving upper level trough and associated surface low pressure system took its time traversing the Midwest from September 9-12.  The system produced widespread precipitation and cloudiness across the Midwest during the period (Figure 5).  This persistent system even resulted in a few Flash Flood Warnings across Iowa and Missouri.  Rainfall totals were in excess of 3.25" in Waterloo, IA on September 10, and generally greater than one inch across the remainder of eastern Iowa during this time. Other notable locations across northeast Iowa include Fayette, IA with 5.40", St. Lucas, IA with 3.95", and Oelwein, IA with 3.54".   In addition, soaking rains fell across much of Wisconsin, with Rockville, WI reporting the highest storm total of 3.38" in western Wisconsin.  The weekly precipitation map shows generally widespread generous rains across much of Wisconsin, Indiana and Iowa (Figure 6).

Maximum temperatures during this part of the week struggled to get out of the 60's over much of the Midwest due to persistent rainy periods and extensive cloud cover (Figure 7).  This resulted in temperature departures that were 6-9°F below normal across Wisconsin, to 1-2°F below normal across Ohio, Indiana, Illinois, and Kentucky (Figure 8).  In fact, on September 9-11, a number of locations across Minnesota and Wisconsin reported record cold maximum temperatures:

Date	Location	New Record	Old Record and Year
September 9	Rochester, MN	55°F	60°F, 1999
September 10	Rochester, MN	57°F	58°F, 1962
September 11	Antigo, WI	52°F	53°F, 1918
	Merrill, WI	52°F	56°F, 1924
	Wausau, WI	53°F	54°F, 1918
	Marshfield, WI	54°F (tie)	54°F, 1918
	Wisc. Rapids, WI	54°F	57°F, 1918
	Appleton, WI	55°F	59°F, 1924
	Stevens Point, WI	55°F	56°F, 1924
	Green Bay, WI	56°F	57°F, 1886

 
More Flash Flooding

The final few days of the second week of September were very active, despite the cooler than average temperatures.  A slow moving area of low pressure, and attendant cold front, the same responsible for the heavy rain in Iowa, trekked across Illinois and Indiana on September 12-13 (Figure 9).  The result was widespread showers and thunderstorms over a good portion of the Midwest, some of which were very efficient rain producers (Figure 10). Many locations received in excess of 300% of normal precipitation for the three-day period spanning September 10-13 (Figure 11).  Unfortunately, the heavy rain resulted in a number of flooding situations, especially in Evansville, IN, and in Highland, IN (just south of Gary, IN).  In Evansville, IN, flooding was most intense on the southeast side of town, where 10-12 feet of water stood in roughly 10 homes, in addition to numerous intersections being totally water-covered.  The South Side of Chicago, IL also saw significant precipitation, with Morris, IL reporting over 4.80" of rain in just 24-hours.  The axis of heavy rain extended northeast into the southern suburbs of Chicago, where low-lying viaducts were completely under water, and where motorists who drove into flooded roads had to be rescued.

Further north, in extreme northwestern Indiana, severe flash flooding resulted in a state of emergency for Highland, IN.  Here, the Little Calumet river and all ditches were 3 to 5 feet above flood stage, and this water collapsed six homes and resulted in a number of evacuations, but fortunately no injuries were reported.  As much as 8 inches of rain fell across Lake County, IN, prompting officials to request affected towns be declared state disaster areas.  Later reports indicated that at least 1,000 homes experienced some sort of flood damage from this flash flooding event.  Of the 8 inches, over 6 inches of the rain fell in just two to three hours time, which is nearly the monthly average total for September.  An interesting complicating factor is that the development of a deep-tunnel project for the removal of excessive rain water, and flood waters of the Little Calumet River, was slated to be completed on Monday, September 18.  

Given the roller-coaster swing of temperatures across the Midwest this week, and incredible amounts of precipitation, what more could happen in the third week of September?  Find out in next week's edition of the Midwest Climate Watch.

Kruk