Central Minnesota Extreme Rainfall, June 29-30, 2020

radar loop
200-frame radar loop from Chanhassen, showing intense thunderstorms "training" over the Little Falls area on the evening of June 29, 2020.
Courtesy of College of DuPage.

A nearly stationary band of strong thunderstorms in the afternoon and evening over central Minnesota produced very high rainfall totals and flash flooding in and around Little Falls.

The storms formed in a nearly ideal environment for heavy rain, just on the warm side of a stationary boundary separating muggy air to the south from relatively cool air to the north. The winds aloft were very weak too, meaning thunderstorms that formed had little support to move them away.

The thunderstorms formed initially on the Stearns/Morrision county line but then lifted northward slightly, towards the Little Falls area. As individual cells would mature and shift away, new ones would replace them almost immediately. Meteorologists refer to this as "training," because new new cells replace old ones much the way rail cars on a passing train appear to replace one another. Many of Minnesota's major flash floods are associated with training thunderstorms.

The heaviest rains and flash-flooding were centered on Little Falls area, where stream gauges and automated weather stations reported 7-8 inches of rain. Observers in the CoCoRaHS network measured 8.76 inches in Little Falls itself, 5.01 inches near Swanville, 4.58 inches in Pierz, and 3.27 inches near Longville. Spikes of relatively high totals, generally 3-4 inches, radiated away from this area up into northwestern Todd County, and then eastward towards Ogilvie and Braham. The National Weather Service cooperative observer in Brainerd reported 2.93 inches.

Fortunately, the extreme rainfall remained confined to a relatively small area. Using automated gauges, human observations, and radar estimates, the area receiving six inches or more appeared to be under 200 square miles. This limited both the size of the direct-impact area, and also reduced the downstream effects. Water washed over roads and flooded nearby streams in Morrison, Todd, and Mille Lacs Counties, but did not lead to major problems outside of those areas.


Last modified: July 1, 2020

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