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. Larger amounts were reported by observers affiliated with state-sponsored monitoring networks, but the value at Hokah is by far the largest 24-hour rainfall total ever recorded by an official National Weather Service reporting location in Minnesota, and stands as Minnesota's Official 24-hour rainfall record. 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. In southeastern Minnesota, approximately 7.75 inches over a 24-hour period is said to be a "100-year" ( or 1% annual probability) storm. The current official source for return-period estimation indicates that the 1000-year (or 0.1% annual probability) 24-hour rainfall in southeastern Minnesota is about 12.5 inches. The much higher values measured at Hokah and elsewhere nearby are literally off the charts.
Additionally, climatologists have no means by which to estimate return-periods for storms of remarkable spatial extent. With this event, 24-hour rainfall amounts equal to or exceeding the "100-year" value of 7.75 inches covered thousands of square miles. We have no way to estimate the frequency of a storm producing that much rain over such a large area; we only know that it is exceptionally rare. Other events of comparable spatial coverage and only slightly lesser magnitude during the 2000s include extraordinary rainfalls in northwestern Minnesota on June 9-10, 2002, and in southern Minnesota on September 14-15, 2004. More large-spatial-coverage events are documented here.
The applied science of extreme rainfall frequency estimation has made remarkable strides in recent decades, and now incorporates sophisticated and flexible spatial and statistical techniques that have much greater power than techniques used during the 20th century. These techniques do not, however, have any sort of "crystal ball," because they are based entirely on past data. Familiar phrases like "100-year storm," therefore, really refer to a storm with a 1% annual probability at a given location, assuming the climate of that location is the same as it was during the reference period. This is an important distinction, because if the climate regime shifts dramatically, then that would alter extreme precipitation probabilities too. Indeed, climate scientists agree that a significant hydroclimatic change is underway in the Upper Midwest. This change appears to be making heavy and extreme rainfall more likely. Unfortunately, knowing the rate at which the size of 1% probability event is increasing, or the rate at which return-periods are shortening (for example, from 100 years to something less), is all but impossible.
All that said, no matter how we choose to examine the southeast Minnesota event of August 18-20, 2007, it stands out in the official record.
Extreme Rainfall Climatology Resources:
- "100 Year Storms" defined
- Modern/improved precipitation frequency estimates for Minnesota
- Minnesota Flash Floods 1970-2012
National Weather Service Information:
- Historic Rainfall and Flooding Event of August 18-20, 2007 (National Weather Service Office - La Crosse)
- Major Historical Floods in southeastern MN and parts of neighboring states (National Weather Service Office - La Crosse)
Additional Maps and Data:
- Black and white version of rainfall map show above:
- Updated rainfall map for Southeastern Minnesota only:
- Location by location rainfall totals are available as comma-separated text files. The files provide location coordinates that are followed by a multi-day precipitation total in inches. The files include rainfall data for the entire state of Minnesota for the August 18-20, 2007 time period.
- ESRI ArcView ASCII file of grid used for analyzed map
- Hourly rainfall data are available from recording rain gauges in the vicinity of the flood event.
- Microsoft Excel Spreadsheet containing hourly precipitation data
- Imagery from the La Crosse National Weather Service Doppler radar site is available without charge from the National Climatic Data Center (NCDC). Radar-based precipitation estimates for the flood event can be obtained from this archive. One-hour precipitation total estimates were calculated by the La Crosse radar facility at five-minute increments during the storm. The imagery can be viewed using free visualization and analysis software available from NCDC. In addition to viewing the radar data, the software allows the user to export the data to common formats such Shapefile, Arc/Info ASCII Grid, and more. Level III one-hour precipitation data are provided at a resolution of two kilometers in radial distance, by one degree of azimuth arc. The La Crosse office of the National Weather Service points out to users of the radar imagery that a radar "fault" is evident southwest of the facility during the event. Data found in this arc should be treated carefully.
- Hydrographs for Southeastern Minnesota Rivers - August 2007 Flood Event
(Courtesy U.S. Geological Survey)
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.
Note: this entry was updated in October 2017 to reflect scientific advancements and to remove broken links. Previously viewed versions of this article are no longer available.