Minnesota’s latest mega-rain event drenched parts of southern Minnesota Saturday night.
A band of persistent thunderstorms along a slow-moving frontal boundary dumped torrential rains across a large area roughly 40 to 60 miles southwest of the Twin Cities. The radar loop shows persistent heavy rain cells regenerating over the same areas as they crawled slowly south and east Saturday night.
Near-record tropical moisture levels Saturday
The amount of deep tropical moisture in the air mass over Minnesota Saturday was at near-record levels. Precipitable water was over 2-inches above the Twin Cities. Weather systems can pool that moisture into local areas and produce much higher rainfall totals with slow-moving summer storms. Here’s more perspective from the Minnesota DNR Climate Working Group.
The thunderstorms formed in an air mass that contained some the highest total moisture measured in southern Minnesota this season. The 2.25 inches of "precipitable water" tallied from the 7 PM weather balloon launch at Chanhassen was just below record record levels for the day and was above the 95th percentile for this time of year. The excessively humid air mass led to dew point temperatures in the middle and upper 70s throughout Minnesota by Saturday afternoon.
Widespread multi-inch rainfall totals
The system produced a huge area of 3 to 6 inch-plus rainfall totals. The heavy rain zone blanketed a zone from Mankato northwest along and north of the Minnesota River Valley.
For a system to be classified as a mega rain, it must produce 6 inches of rain across an area of at least 1000 square miles, with a maximum of at least 8 inches.
Saturday’s system meets those criteria. The Mankato area picked up 6 to more than 8 inches. Doppler rainfall estimates suggest around 10 inches fell between rain gauges in Sibley County.
Here’s more on the extreme rainfall totals from the Minnesota DNR Climate Working Group.
The highest known total for the storm came from a CoCoRaHS observer just south of Mankato, who reported 8.65 inches. Other totals of 8 inches or more were found in the area, with numerous 6-7-inch reports near St. Clair, St. Peter, Kasota, Morristown, Gibbon, and Fairfax. A DNR automated rain gauge at Fort Ridgely State Park recorded 7.77 inches, with the same type of equipment reporting 6.37 inches in Morristown. These values are likely "under-catches" by up to 10%, because automated tipping buckets cannot keep up with intense rainfall rates. According to radar estimates, rainfall totals closer to 10 inches may have hit an area with no observations, from southeastern Renville into southwestern Sibley County.