Heavy Rains Fall on Southeastern Minnesota:
August 18-20, 2007

rainfall map

A series of thunderstorms moving along a stalled frontal boundary dropped extremely heavy rain on much of southern Minnesota on August 18, 19, and 20, 2007. The most intense precipitation rates occurred during the afternoon and evening hours of Saturday, August 18, and the early morning hours of Sunday, August 19. Over the course of the event, all or portions of 28 counties received at least four inches of rain. Six-inch totals were common across the region, and portions of southeastern Minnesota reported astounding rainfall amounts ranging from 8 to 20 inches. The heaviest rainfall reports came from Winona, Fillmore, and Houston counties, where 36-hour totals exceeded 14 inches. The largest multi-day rainfall total reported (through Monday, August 20) was 20.85 inches observed near the town of Houston in northern Houston County. An official National Weather Service climate observer near Hokah in Houston County reported a storm total of 16.27 inches. Of the 16.27 inches, 15.10 inches fell within the observer's 24-hour observation cycle ending at 8:00 AM on Sunday, August 19. This is the largest 24-hour rainfall total ever recorded by an official National Weather Service reporting location in Minnesota. The previous Minnesota record was 10.84 inches, measured at Fort Ripley in Crow Wing County on July 22, 1972.

The deluge produced flooding tied to seven fatalities. Major flood damage occurred in many southeastern Minnesota communities. Hundreds of homes and businesses were impacted. Reports of stream flooding, urban flooding, mudslides, and road closures were numerous throughout southern Minnesota.

The combination of huge rainfall totals and a very large geographic extent, make this episode one of the most significant rainfall events 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" (1% probability) storm. The area receiving six or more inches during a 24-hour period in the midst of this torrent encompassed thousands of square miles. Other heavy rainfall events during this decade of comparable magnitude and spatial coverage include extraordinary rainfalls in northwestern Minnesota on June 9-10, 2002, and in southern Minnesota on September 14-15, 2004.

Further Discussion of Rainfall Return Period:

When asked to provide a recurrence interval estimate for this event, an investigator should note the enormous gap between the 100-year 24-hour threshold (six inches), and the fifteen or more inches that fell during a 24-hour period on that ill-fated weekend. Although a precise recurrence interval cannot be determined, we can safely state that the probability of being subjected to such an occurrence was considerably below one percent. What are, from a risk management perspective, the meaningful distinctions between a 1.0% (100-year) probability, a 0.1% (1000-year) probability, and a 0.01% (10,000-year) probability of occurrence? That is to the judgment of the investigator.

Extreme Rainfall Climatology Resources:

National Weather Service Information:

Media Accounts:

Additional Maps and Data:

A Word of Thanks:

The State Climatology Office thanks the Soil and Water Conservation Districts, the National Weather Service, and all of the diligent volunteer precipitation observers who make analyses of these events possible.

For more information contact: climate@umn.edu