Soaking rains over the southern half of Minnesota on Friday September 18th gave way to a narrow, regenerating, and nearly stationary axis of intense rainfall later in the evening and overnight in far southern and southeastern parts of the state, leading to extreme precipitation totals and significant flooding.
The day began with a large plume of moisture spreading east and north of an area of low pressure over Nebraska. An expansive mass of clouds, rain, and embedded thunderstorms spread out of the southwest corner of the state early in the morning, with reinforcing waves sweeping over much of the southern two-thirds of the state during the day.
For most of the day and in most areas, the rainfall was "steady," lasting 6-12 hours, with occasional surges of moderate to heavy rains falling south of US Highway 12 and south of the Twin Cities. Generally light to moderate rains fell over the majority of the Twin Cities area and central Minnesota. By early afternoon rainfall totals of 1 to 3 inches were common from near Madison, through the upper Minnesota River valley, to Mankato and eastward towards Red Wing and Wabasha. Most of the Twin Cities had between a half-inch and an inch of rain, with lighter amounts to the north.
Areas near Interstate 90 in the far south remained mostly rain-free during the day, but an area of higher humidity provided extra fuel for stronger storms that would develop during the evening. After dark, thunderstorms formed in an almost perfectly west-to-east line near the interstate, and then paraded eastward, with many storms passing directly over areas just hit by other ones. This process is called "training," named for the way train cars pass over the same section of track. Training thunderstorms can lead to excessive rainfall totals, and this case was no exception. The band of storms persisted for several hours with little north or south movement, and with rainfall rates of over two inches per hour at times.
The hardest-hit areas included far southern Waseca and Steele counties, and much of Mower, Fillmore, and Houston counties. Rainfall totals in these areas were generally five inches or more, with pocket of 6-8 inches, and even slightly more. An observer near Houston recorded 8.29 inches and one at Spring Valley recorded 8.17. Both of these reports exceed the 100-year return period for 24-hour rains. The deluge resulted in widespread flooding, which triggered mudslides and forced some roads to close. The town of New Richland in southern Waseca County was hit very hard, evacuations and dozens of residents displaced by structural damage from the inundation.
The DNR State Climatology Office estimates that the area receiving six inches of rain or more in 24 hours was about 700 square miles, which is short of the 1000 square mile minimum required to count as a "mega-rain." This was a damaging storm event in any case, and seems to have been at least as destructive as the actual mega-rain on July 1-2 that affected many of the same areas. We remind users that mega-rains are defined strictly by their dimensions (the total area receiving six inches or more) and not by their impacts.
Posted September 23, 2026
KAB