Midwest Weekly Highlights - July 25 - 31, 2007

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Overview ...

The Midwest saw some much needed precipitation (Figure 1) early in the week as a cold front pushed through the area (Figure 2) triggering showers and thunderstorms. These rains were especially beneficial to some drought stricken areas of Indiana, Kentucky, Michigan and Ohio (Figure 3). However, following the frontal passage, high pressure built back in (Figure 4), dry conditions returned and warmer, more seasonal temperatures came to the area. The northwest portion of the Midwest experienced the most extreme temperatures, with some locations at almost 10 degrees above average for the week (Figure 5), while the south and central portions continued their below- to near-normal trend for the month.
 

Supercells & Microburst Strike

Thursday, July 26th saw severe weather in Illinois, Indiana, Iowa, Kentucky, Michigan, Minnesota and Ohio as storms struck across the Midwest (SPC storm reports), including microbursts in Chicago, a supercell in northeastern Indiana / northwestern Ohio and heavy rains in Minnesota that turned deadly.

During the morning of July 26th, microbursts struck in areas of suburban Chicago - Burr Ridge and Homer Glen. A microburst is a strong localized downdraft (downburst) less than four kilometers wide that occurs beneath a thunderstorm. They can produce winds as strong as those in weak tornadoes, but can be differentiated from a tornado by the wind damage pattern. Microbursts typically have an outward (or starburst) damage pattern, while tornadoes will scatter debris in a variety of directions due to the rotating winds. In Burr Ridge, the winds from the microburst were estimated at 60-80 miles per hour. Numerous large limbs and branches were downed and two buildings had portions of their roofs peeled off. In Homer Glen, there was also damage to trees and roofs from 70 to 100 mile per hour winds.

Heavy rains also hit the Chicago area that morning with over four inches of precipitation occurring along the lakefront (Figure 6). A portion of northbound Lake Shore Drive had to be closed due to flooding and tens of thousands of people lost power. Flights at O'Hare International Airport were delayed for one to two hours and at Midway Airport for 30 to 90 minutes. Please note that the Illinois recently became a member of the Community Collaborative Rain, Hail & Snow Network (CoCoRaHs) and that observations from volunteer CoCoRaHs observers enabled the above figure to be generated. Official National Weather Service Cooperative Observer sites are fewer in number than CoCoRaHs locations and were not located in the core of the heavy rain (Figure 7). The four inch precipitation amount would only have been determined from radar estimates without the ground truth provided by the high density community observations. CoCoRaHs is active in much of the Midwest (Illinois, Indiana, Iowa, Missouri and Wisconsin). For more information on CoCoRaHs, please see their website.

Strong thunderstorms also occurred over southeast Michigan, northeast Indiana and northwest Ohio on the 26th, with some of the most intense cells developing into supercell thunderstorms. Widespread wind damage occurred with numerous trees and power lines downed. Near Fayette, OH (Fulton County), the top of a barn was blown off and in Fremont, IN (Steuben County) a strip mall was significantly damaged. Additionally, golf ball sized hail was reported in Steuben County, IN and Williams and Defiance Counties, OH.

In St. Paul, MN, heavy rains on July 26th turned deadly when two workers in a storm sewer were drowned in a surge of storm water. Seven men were deep below the city's streets working on a sewer restoration project when severe weather hit the area. Following a call to evacuate, two men were safely lifted 150 feet out the main shaft to the surface by a crane operated bucket while the others fled to a staircase. As they began their ascent, the water was already waist high and only three of the five workers made it to the top. The storm sewers, which are six to eight feet in diameter, were flooded when 0.44 inches of rain fell in 30 minutes. Rescue operations began immediately, but the missing men's bodies were not located until Friday and Saturday. They were found in the Mississippi River downstream from a spillway that feeds into the river.
 

Rains Were Helpful, But Drought Continues

Despite the thunderstorms early in the week, much of the Midwest continues to suffer due to hot, dry conditions.

In Minnesota, portions of the central and western part of the state were downgraded over the past week from moderate drought to severe drought (Figure 8) and Governor Tim Pawlenty has asked the Farm Service Agency to begin assessing drought damage as a first step toward agricultural disaster designation. Impacts are also starting to be felt in urban areas. In Chanhassen, MN (Carver County), the city has had to enact a complete watering ban as three of the city's ten water pumps went out due to low water levels in their aquifer. They have banned all irrigation so as to focus on the priorities of fire prevention and drinking water. Other Twin Cities communities such as Blaine, Brooklyn Park, Burnsville, Eagan, Eden Prairie, Edina, Forest Lake, Inver Grove Heights, Lakeville, Minnetonka, Oakdale, Plymouth, Rosemont, St. Louis Park and Shoreview have also had to enact watering restrictions, although none as severe as Chanhassen. Tourism is being affected by drought as well as the Minnehaha Creek Watershed District, in effect, shut off flow to the Minnehaha Falls last week. Lake Minnetonka, which feeds the Minnehaha Creek and the Minnehaha Falls has fallen to the point at which the dam that feeds the creek had to be closed. The Minnesota Department of Natural Resources is also cautioning that many of the state's 1200 irrigating permits for farmers & golf courses may be suspended in early August due to low running rivers. In neighboring Wisconsin, approximately 95% of the state is categorized as either abnormally dry or experiencing drought.

One exception to the intensification reported above is Ohio. Most areas received rain last week - the state as a whole received 1.48 inches, 0.63"above normal. This helped to improve all categories of crop & livestock conditions and resulted in the D1 in northwest Ohio being upgraded to D0. However, more precipitation is still needed to combat long term dryness. Nearby in Kentucky, most of the rains on July 26 and 27 were of low intensity and long duration and were exactly what was needed to provide drought relief. According to the Kentucky Weekly Crop & Weather Report, this was only the third week this entire growing season with above normal rainfall for state as a whole.

Michigan missed many of the beneficial rains that hit Ohio and Indiana this past week and the lack of moisture has continued to increase concerns for crop condition and quality. Over 90% of the state is very short/short for both topsoil and subsoil moisture. In the southwest and along the Lake Michigan shoreline, moderate and severe drought regions were expanded. According the the Grand Rapids National Weather Service, there has been abnormal mixing of extremely dry air down to the surface in that region this summer, causing afternoon relative humidity values to to drop and disrupting the Lake Michigan breeze. Without this lake to land wind flow, there isn't a trigger for convection and the area is missing out on a typical summer source of precipitation. Fire danger is also becoming serious concern in Michigan as the fire danger ranges from moderate to high across central Lower Michigan and from very high to extreme across much of southern Lower Michigan. Typically fire danger is not significant problem in the state after green up occurs in spring, however this summer's lack of widespread significant rains has caused the atypical risk.
 

Daily Records during July 25 - 31

Date	Location	Record Type	New Record	Old Record & Year
July 25	South Bend, IN	Precip	2.15	1.67"/1985
July 25	Marquette, MI	High Max	91°	91°/1989
July 26	Mason City, IA	Precip	2.26"	1.93"/1940
July 27	Lexington, KY	Precip	2.32"	1.61"/1992

MCP