Minnesota is no stranger to heavy rain events. The early surveyors mapping out the state witnessed such events.
The DNR climate office has assembled a list of so-called "Mega-rain" events that have occurred since statehood. These are events in which six inches of rain covers more than 1000 square miles and the core of the event topped eight inches. Rainfalls of of this magnitude and geographic extent have the potential to become catastrophic. Using newspaper accounts, diaries, and the historical climate record, 15 such events in Minnesota's post-settlement history have been identified. However, our ability to detect these events has improved dramatically since the 1970s.
The number of daily rainfall observers in Minnesota exploded in the early 1970s, thanks to the foresight and ambitions of Dr. Don Baker, and then-State Climatologist Earl Kuehnast. Since that period, the state has benefited from an unusually dense network of observations. This network has remained intact, plus or minus year-to-year changes, and has enabled climatologists to identify mega-rainfall events that undoubtedly would have been missed during periods of much sparser observations.
Thus, the State Climatology Office considers the "stable" period of record to stretch from 1973 through present. Any given year during that period has roughly the same chance of capturing (or missing) an actual mega-event. The years prior to 1973, however, are likely to have some number of missing events.
If we examine the period 1973-2016, Minnesota has seen eleven mega-rains, with a sharp uptick since 2000, despite a small decrease in observer numbers. Of these 11 events, two were in the 1970s, one was in the 1980s, none were in the 1990s, but four occurred in both the 2000s, and the 2010s (still underway). Thus, the 18 years from 2000-2017 have seen nearly three times as many mega-rains as the 27 years spanning 1973-99. Although it is difficult to assess the statistical significance of that rapid increase, we do know that these trends are consistent with the expectation that Minnesota and the Upper Midwest will receive more precipitation, and more precipitation from large events, in response to increasing global temperatures and increased available moisture for passing storm systems.
Documented Mega-Rains in Minnesota
(There may be other events prior to 1973 that require further investigation such as September 11-15, 1903.)
August 6, 1866 Southern Minnesota
Also known as the Wisel Flood, this event killed 16 people, including 3 members of the Wisel family in Fillmore County. 10.30 inches of rain fell at the Sibley Indian Agency located in Sibley County. The story of the Wisel family in peril appeared in the Harmony/Mabel/Canton News Record Newspaper.
July 17-19 1867 Central Minnesota
Climatologists and historians believe this to be Minnesota's most extreme flash flood of the past 200 years. In his Minnesota Weather Almanac, Mark Seeley referred to this event as "Minnesota's Greatest Thunderstorm." Torrential rains pounded portions of west-central Minnesota relentlessly. Unfortunately, the rains escaped direct measurement, but astute observers of the time estimated from unobstructed upright barrels and other such containers, that 30-36 inches of rain fell in 36 hours. No official observation in Minnesota has come anywhere near those magnitudes. The few surviving details of the storm back up the claims, however, as the flooding that resulted was unimagineable and catastrophic. Most of what we do know about this event comes from a paper that was read before the Minnesota Academy of Sciences on March 7, 1876. Climate Historian Tom St. Martin summarized the event as well.
July 20-22, 1909 Northern Minnesota
Extensive flood event from Northwest Minnesota to the UP of Michigan. Highest one day rainfall total was 10.75 inches at Beaulieu in Mahnomen County (11.10 inches for the three day total.) This storm also did extensive damage in Duluth and killed two children in the city when they were swept out of their mother's arms.
September 9-10 1947, Iron Range
24 hour totals of 6 inches or more at Hibbing, Ely and Winton. Unofficial report of 8.60 inches in five hours at Hibbing. Extensive damage over the Iron Range district.
July 21-22, 1972 Grand Daddy Flash Flood
10.84 inches fell in 24 hours was set at Fort Ripley. This was the state record for a highest 24 hour total at a National Weather Service station until Hokah broke the record in 2007.
June 28-29 and July 1-2, 1975, Northwest Minnesota
Geographically extensive and intense rains fall on eastern North Dakota and Northwest Minnesota in two separate events.
July 23-24, 1987 Twin Cities Superstorm
June 9-10, 2002 Northern Minnesota
48 hour rainfall totals topped 12 inches in a some areas of Roseau and Lake of the Woods counties.
June 22-23, 2002 Northern Minnesota
This event was so large, two different parts of northern Minnesota met the mega-rainfall definition used here.
September 14-15, 2004 Southern Minnesota
More than ten inches of rain fell in a 36 hour period in Faribault and Freeborn Counties.
August 18-20, 2007 Southern Minnesota
Although the 1867 storm detailed above likely produced higher totals, the 15.10 inches measured one mile south of Hokah stands as the official record for 24-hour rainfall at a Minnesota National Weather Service Cooperative station. The three day total for this station was 16.27 inches.
September 22-23, 2010 Southern Minnesota
The National Weather Service site in Amboy measured 9.48 inches on September 23, with 10.68 inches for the event.
June 19-20, 2012 Northeast Minnesota
The two day total at Duluth was 7.24 inches. The St. Louis River at Scanlon set a new record crest at 16.62 feet, rising 10 feet in 24 hours.
July 11-12, 2016, East-central Minnesota
Extreme rainfall affected a swath from the Brainerd Lakes area, eastward into Pine County (and also well into Wisconsin). Cloverton in Pine county recorded 9.34 inches.
August 10-11, 2016, Central Minnesota, Southeastern Minnesota
Two distinct areas received over 6 inches of rainfall: one near Willmar, and another in Wabasha County. The highest total of 9.74 inches was recorded just east of Willmar.