April 11-17, 2001:

At the beginning of the week of April 11-17, the upper atmosphere ridge that brought summer-like conditions over the Midwest was moving east, and a strong trough entered the region from the west. A very strong surface low pressure center traveled from the southern Rockies, through the Great Plains, to northern Missouri. Heavy rains fell as several waves of thunderstorms moved from west to east along a warm front stretched across the center latitudes of the Midwest. As the low entered the region, it turned slowly northward, bringing very heavy rain to Iowa, Minnesota, and Wisconsin, especially in areas where stream and rivers were already rising from previous rain and melting snow. The precipitation totals for the week exceeded 3 inches in places in Minnesota (Figure 12), and were more than three times the normal precipitation for the week in a large stretch of the Upper Mississippi river basin (Figure 13). As the low center pulled away, an unusually cold arctic air mass entered the region, causing some areas of northern Minnesota, the Great Lakes, and eastern Midwest to receive late measurable snowfalls during the days that followed (Figure 14). The Midwest presented a weekly temperature departure pattern that was cooler than normal in the northwest half and warmer than normal in the southeast half (Figure 15). However, this average pattern smooths out an extreme transition, from maximum temperatures in the 80's at the beginning of the week to minimum temperatures in the 10's and 20's at the end of the week.

With the heavy rains of the first half of April (Figure 16) on top of the wetter than normal snow pack melting in the northwestern Midwest and northern Great Plains, widespread flooding occurred during the April 11-17 period. The two major rivers in the Dakotas and Minnesota, the Red River of the North and the Mississippi River exceeded flood stage by considerable margins in many places (Figure 17, NWS). The Red River peaked at Fargo on April 14 at 36.7 feet, which is almost 20 feet higher than the flood stage of 17 feet (Figure 18, USGS). However, the largest communities along the Red avoided the extreme damage of the 1997 floods in the same area. The Mississippi River was slower to rise, reaching its peak stage at La Crosse, WI, and Winona, MN, on April 17 (Figure 19, USGS). A number of residential blocks in northern La Crosse were flooded, and thousands of acres of farmland were flooded. The Minnesota legislature is considering a $30 M package for short term needs and $60 M for infrastructure improvements. The flood wave is moving down the Mississippi toward Davenport, with levels already rising toward 1993 high water marks along the way (Figure 20, USGS). The governor of Iowa had already issued a disaster declaration for counties along the river, quickly followed by the governors of Wisconsin, Illinois, and Minnesota. The state disaster declaration cleared the way for National Guard assistance to flood-affected counties, and set the stage for later applications for federal disaster declarations.