During the period of August 18th-20th, when the most intense rains fell, Stoddard, in northwest Vernon County, measured 12.41 inches, the south side of La Crosse, in La Crosse County, picked up 12.20 inches, Mukwonago, in south-central Waukesha County, measured 10.25 inches, and Viroqua, in central Vernon County gathered 9.73 inches of rain.
The image below shows the Wisconsin observed 48-hour rain amounts for the period from roughly 7 A.M. Saturday, August 18th through 7 A.M. Monday, August 20th:
During the period of Monday night, August 20th through Thursday morning, August 23rd, periodic, additional rounds of showers and thunderstorms with moderate to rainfall intensities, affected parts of the southern half of Wisconsin. This additional rain of 2 to 5 inches either made existing flooding problems worse, or initiated flood problems in those areas that were spared from the original weekend rains and floods.
The image below shows Wisconsin observed rain amounts for the period from roughly 7 A.M. Saturday, August 18th through 7 A.M. Thursday, August 23rd
Leading up to the rains and floods of August 18th through August 23rd, parts of southwestern Wisconsin received soaking or flooding rains that basically saturated the ground. The saturated soils were unable to handle the rain that would fall during the August 18th-23rd period. Specifically, parts of the counties of La Crosse, Monroe, Vernon, Crawford, Grant, Richland, Iowa, Dane, Lafayette, Green, and Rock had soils that were completely saturated prior to August 18th.
To put the August 18th-23rd extreme rainfall amounts into perspective, the normal August monthly rainfall is around 4 inches across southern Wisconsin. Locations in the broad swath mentioned above received 100% to 250% of their normal August rainfall in just 6 days! As of August 22nd, Madison measured 13.94 inches of rain for August, 2007, and La Crosse has received 12.32 inches. Consequently, through August 22nd, both Madison and La Crosse have experienced their wettest Augusts ever, and wettest month ever.
The reason for the historical, flooding rains during the August 18th-23rd period is related to periodic rounds of thunderstorms with heavy rains training through the same areas. Normally, warm fronts and cold fronts move through Wisconsin and the Upper Midwest and help trigger non-flooding, but beneficial rains. However, during the August 18th-23rd period, the fronts tended to stall on a west-to-east line from northern Iowa through northern Illinois. A very moist and warm air mass rose up and over this frontal boundary, providing fuel for showers and thunderstorms. Due to the depth of the warm layer, and considerable amount of moisture, the stage was set for heavy rainfall. Hourly rainfall rates reached 1 to 2 inches per hours, leading to ponding of water, floods, etc.