Heavy Rainfall - September 22-23, 2010

The largest flood event to hit southern Minnesota since the flood of August 18-20, 2007 began along a sluggish warm front draped across southern Minnesota. Wave after wave of heavy thunderstorms marched to the east along this front across the southern three tiers of Minnesota counties.

The most intense rains began in the early afternoon of September 22 and tapered off during the evening of September 23. Over four inches of rain fell in nearly all southern Minnesota communities. Six or more inches of rain fell in numerous locales. The highest two-day rainfall total reported by the volunteer network was 11.06 inches near Winnebago in Faribault County. The official National Weather Service observer at Amboy in Blue Earth County reported 10.68 inches for the event (including 9.48 inches for the 24-hour period ending 3:00 PM, September 23). Other higher amounts include 10.20 inches at Truman in Martin County, 9.26 inches at Windom in Cottonwood County, and 8.57 inches at Theilman in Wabasha County.

The combination of huge rainfall totals and a very large areal extent, make this episode one of the most significant flash floods in Minnesota's climate history. A six inch rainfall total for a given location in this region over a 24-hour period is said to be a 100-year storm. The area receiving six or more inches during this event encompassed over 5000 square miles in Minnesota alone. September flash floods are not without precedent. This event is similar to the storm that doused southern Minnesota on September 14-15, 2004. Over a 16 year period from 1970 through 1985, September flash floods made up 16% of all flash flood events studied. For more information on flash floods see: Minnesota Flash Floods: 1970-2008.

There were numerous reports of reports of urban and rural flooding as the rain fell on already soggy soil. Swollen creeks and rivers left their banks and flooded neighborhoods in communities such as Owatonna, Pine Island, Pipestone, Truman, St. James, Zumbro Falls, and many others. Basement flooding was reported throughout the southern one third of Minnesota. Numerous state, county, and township roads were closed due to flooding. Stream stage on many southern Minnesota rivers and streams approached or exceeded all-time highs. Record high river level values are uncommon outside of the spring snow-melt season. Agricultural interests were significantly impacted by the event. The fall harvest was slowed by wet fields and yield potential suffered from crop inundation.

The Mississippi River rose above flood stage (14 feet) at St. Paul on September 29. This is the first time that the Mississippi River has reached above flood stage in the autumn. Checking flood stage records back to 1893, the next closest autumn peak in the Mississippi at St. Paul was on September 30, 1986 with a peak stage around 12.8 feet.