The historic flash flood event of 18-19 August 2007 in the Upper Mississippi River Valley: impacts of terrain and societal response

A flash flood event of historic proportions took place in the Upper Mississippi River Valley during the night of 18-19 August 2007, with the National Weather Service (NWS), La Crosse, Wisconsin (WFO ARX), forecast area experiencing both record 24-h rainfall amounts and a terrain-changing flash flood. A backbuilding mesoscale convective complex (MCS) north of an unseasonably strong late summer baroclinic zone, which had been immediately preceded by an all-day stratiform rain event, led to the flash flood. Seven fatalities and tremendous damage to infrastructure resulted.

The Minnesota state record for 24-h rainfall (previously 275.3 mm – Fort Ripley, MN in 1972) was shattered by an official NWS Cooperative Observer measurement in Hokah, MN, of 383.5 mm. Several other unofficial 24-h measurements over 432 mm were reported in the vicinity of the Hokah measurement. The event was so rare, and falls so far outside the statistical probabilities of occurrence, that the Minnesota Climatology Working Group (State Climatology Office) estimated the return period of such an event at over 2000 years. Based solely on areal coverage and amounts of rainfall, the Minnesota State Climatology Office called this event one of the most significant flash floods in state history. The heavy rain band (>152 mm) stretched across much of southern Minnesota into southwest Wisconsin and directly affected the population centers of Rochester, MN, and La Crosse, WI.

Compounding the very heavy rainfall, however, was the fact this event occurred at night, primarily over the unglaciated areas of southeast Minnesota and southwest Wisconsin. With terrain changes of 200 m or more common over short distances, this area is vulnerable to flash floods and mudslides given excessive values of rainfall. The combination of varied terrain and excessive rainfall created some unique challenges not only to warning and forecast operations at WFO ARX, but also to rescue workers and emergency responders. Societal response to the flash flood event was varied. Some citizens heard warnings and chose not to act, while others refused to evacuate when asked to do so by law enforcement. This jeopardized not only themselves, but also the rescue personnel asked to save them later in the night when flash flooding began. The steep and varied terrain of the WFO ARX forecast area also led to unfathomable river and stream response, when combined with nightfall, presented a widespread threat to life and property for those living along any waterway.

After a brief meteorological review of the flash flood, this presentation will focus on the impacts of terrain in the WFO ARX forecast area and how extreme rainfall events can alter the perception of threats to life and property. In addition, victim accounts relating to societal response of the well-forecast event will raise important questions for the meteorological community on how to communicate threats effectively when an event of epic proportions is occurring.