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Weekend storm in Minnesota classified as rare 'mega-rain' event

It's happened just 17 times since 1973.

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The long-duration rain storm that caused flash flooding in south-central Minnesota last weekend has been classified as a relatively rare "mega-rain" event. 

The dubious classification comes from the Minnesota State Climatology Office, which explains that a large thunderstorm developed Saturday evening over Sibley County and essentially crawled to the east while back-building to the west, resulting in torrential rainfall for hours. 

"The expansive, regenerating, and long-lasting intense thunderstorms activity resulted in a wide swath of very high precipitation totals. Automated rain gauges and human observers in six different counties reported rainfall totals of six inches or greater," the Climate Journal explains. 

While radar estimates indicate some areas received around 10 inches of rain, the highest official recording submitted to the National Weather Service was 8.65 inches just south of Mankato.  

More from the Climate Journal explaining how automated rain gauges are known to underestimate rain totals by up to 10% because they can't 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. The State Climatology Office is working with local observer networks to try to understand the extent and intensity of rainfall in this area."

Because the storm dropped six inches of rain or more over an area of "roughly 1,000 square miles," it qualifies as a "mega-rain" event. 

Here are the radar replays from the storm system. 

Radar loop from around 6 p.m. Saturday to just before midnight. 

Radar loop from around 6 p.m. Saturday to just before midnight. 

Radar loop from approximately 2 a.m to 8:30 a.m. Sunday. 

Radar loop from approximately 2 a.m to 8:30 a.m. Sunday. 

There have been 17 mega-rains in Minnesota since 1973, according to data from the Climate Office. Three were in the 1970s, two in the '80s, one in the '90s, six in the 2000s and five since 2010. 

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