FLOOD082007.WX

Lay of the land opens door to disaster

With its scenic hills and deep valleys, southeastern Minnesota is particularly vulnerable to flash flooding.

By Bill McAuliffe, Star Tribune

Last update: August 20, 2007 - 12:31 AM

Sunday's devastating floods in far southeastern Minnesota were the product of a stalled-out weather system meeting the region's rugged topography.

Houston, Fillmore and Winona counties had just slipped out from under a "moderate drought" designation in recent days, according to the National Drought Mitigation Center. But dry or not, this area is particularly prone to flash flooding because of its unusual topography.

"You get a lot of rain in this bluffy area, and you're going to have flash flooding," said Jeff Boyne, meteorologist in the La Crosse, Wis., office of the National Weather Service.

Known as the "driftless zone" because it escaped the flattening effects of the last glaciers, southeastern Minnesota and southwestern Wisconsin are characterized by gently rolling farmland that drops rapidly over steep bluffs into narrow hollows or "coulees."

Heavy rain can wash quickly from uplands to low areas and convert the small, rocky streams and rivers in the coulees into thundering torrents. Rains that penetrate the soil can also move quickly through the fractured limestone underlying the area into waterways.

The nine counties in the southeastern corner can take a maximum of 1½ inches of rain over six hours before flash floods occur, making them the most vulnerable in the state.

Boyne said the extremely heavy rain over the area Saturday and into Sunday gushed from a warm front that had been wandering around the Midwest for much of the past two weeks, drifted north from Missouri and Iowa and finally bumped up against a high-pressure system over the western Great Lakes.

Now stalled, the system is expected to drop another inch to 3 inches of rain into today across a wide reach of southeastern Minnesota and southwestern Wisconsin. Rain could continue for several more days.

Some preliminary and unofficial rainfall totals this weekend could top the Minnesota single-day rainfall record of 10.84 inches, set at Fort Ripley on July 22, 1972. That depends on when the observations are taken and whether the observer is part of a National Weather Service network.

One observer near Witoka reported 17 inches falling starting Saturday into Sunday. Three Rochester stations reported more than 10 inches each, though the National Weather Service recorded 5.15 at the airport Saturday and 1.7 Sunday.

One of the earliest recorded fatal floods in Minnesota killed 30 settlers in August 1866 along the Root River, tucked in the southeast corner of the state, according to University of Minnesota Extension meteorologist and climatologist Mark Seeley in his book "Minnesota Weather Almanac."

In July 1978, Rochester was inundated overnight when the Zumbro River and its tributaries, which converge in the city, were pushed out of their banks by 7 inches of rain. Five people drowned.

From 1970 through 2006, Minnesota has seen 114 flash floods, as defined by the State Climatology Office. At least 23 have occurred in the relatively small area from Austin to Red Wing to the Iowa border.

Bill McAuliffe • 612-673-7646

Bill McAuliffe • mcaul@startribune.com