On Tuesday, September 12, 2006, a series of thunderstorms with heavy rains moved across south-central and southeast Wisconsin. Nearly saturated soils and additional rounds of heavy rains resulted in scattered reports of flash flooding across south-central and southeast Wisconsin...in the counties of Jefferson, Waukesha, Milwaukee, Racine, and Kenosha. The flash flooding consisted of flooded roads with water depths of 1 to 5 feet, flooded basements, closed roads and even a couple road washouts.
The worst flooding problems occurred in and near Watertown, the Lake Mills-Johnson Creek area north along the Crawfish River, Elm Grove, Wauwatosa and in or near the cities of Racine and Kenosha.
Jefferson County picked up the greatest amounts of rain. A NWS gage site near Milford on the Crawfish River measured 6.21 inches for the 24-hour period ending about 7 A.M. CDT Thursday, September 13, 2006. Other measured amounts in Jefferson County include - 5.10 at the Watertown WWTP, 4.69 inches 1.2 mile southwest of downtown Watertown (severe weather spotter), 3.59 inches at the Lake MIlls WWTP, 2.70 inches at a river gage site on the south side of the Jefferson, 2.42 inches at the Jefferson WWTP, 2.02 inches at the NWS office about 3.8 miles southeast of Sullivan, 1.27 inches at a location about 3 miles southeast of Palmyra in the extreme southeast corner of Jefferson County, and 1 inch at the Ft. Atkinson WWTP. Below is an unofficial image showing rainfall amounts across Jefferson County.
In response to the heavy rains, the Root River near Raymond went over flood stage, while all of the other larger rivers had with-in bankfull rises. No doubt, some of the smaller creeks and streams had lowland flooding, many yards had brief ponds, and roadside ditches ran full.
The several rounds of thunderstorms were the result of low pressure moving northeast along a front through the Chicago area, while an upper-level low pressure moved southeast across northern Illinois. These low pressure pulled and focused moisture over south-central and southeast Wisconsin in the form of several short lines of thunderstorms that moved northeast through southern Wisconsin. The thunderstorms were very efficient at converting moisture into rain, with the result being large rain drops and heavy rain. Hourly rainfall rates reached 2 to 5 inches during some of the more intense downpours.
Below is an hourly accumulative rainfall plot for Tuesday, September 12th, produced by a severe weather spotter who lives about 1.2 miles southwest of downtown Watertown. He measured 4.69 inches of rain for the day. This spotter has a home weather station. Note the large amount of rain that fell between 1:00 and 2:00 P.M.
From late Sunday (9-10-06) until early Wednesday (9-13-06), about 60 hours, I recorded a total of exactly 2.00 inches of rain. Most of that fell in the 24 hour period between 7am on 9-11-06 to 7am on 9-12-06 with a total of 1.32 inches. Prior to that it was the lack of rain here that was the story. The total amount of rain to fall in August was 0.75 inches; most of that (0.54”) fell in the first 6 days of the month. From 8-7-06 until 9-7-06 only 0.22” fell. There was a stretch of 9 days, 8-28-06 to 9-6-06, when no measurable rain fell. There was another stretch of 17 days when only 0.04” fell, 8-7-06 to 8-23-06.