Minnesota's biggest rainfall events become generational stories and defining weather experiences for those who encounter them.
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 in 24 hours or less, with at least eight inches falling somewhere in that area. Rainfalls of of this magnitude and geographic extent have the potential to become catastrophic. Using newspaper accounts, diaries, and the historical climate record, we have identified 21 such events in Minnesota's post-settlement history. However, our ability to detect these events has improved dramatically, beginning in the 1970s, and so only the period since 1973 can be used for serious analysis.
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, valid for analyses, 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.
Note: observer numbers from a different but similar network called CoCoRaHS have begun increasing in recent years, and if the total number of daily observers in Minnesota exceeds historical ranges, that could make it easier to detect mega-rains, which would have to be considered as new events are identified. As of 2022, however, the total number of observes is within historical ranges.
Mega-rains more common since 2000
If we examine the period 1973-2021, Minnesota has seen 16 mega-rains, but 11 of these 16 events have been in the most recent 22 years (2000 through 2021), compared to five confirmed events in the 27 years from 1973 through 1999. Put another way, these major rainfall events have been over 2.5 times more common during the first few decades of the 21st century than they were during the last few decades of the 20th century. Although it is difficult to assess the statistical significance of that increase, we do know that these observations are consistent with observed increases in the frequency and intensity of heavy rainfall events at historical observing stations, and also 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
Prior to 1973, before modern record-keeping
(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 unimaginable 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.
1973 to present (through 2021), after establishment of MnGage network
June 28-29, 1975 & July 1-2, 1975, northwest Minnesota (two events)
Geographically extensive and intense rains fall on eastern North Dakota and Northwest Minnesota in two separate areas.
June 30-July 1, 1978 southeast Minnesota
A long and narrow swath of heavy rains and flooding ran parallel to the Mississippi River in southeastern Minnesota.
June 21, 1983, central Minnesota
Torrential rains drenched and flooded an area including Litchfield, Paynesville, Buffalo, Clearwater, and Big Lake, with totals exceeding 8 inches covering parts of five counties.
September 7, 1991, central Minnesota
This widespread, drenching rain (see p. 3) produced a total of up to 11 inches near Glencoe, with 6 inches or more in another swath extending from Paynesville, northwestward to Glenwood and Hoffman.
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.
September 24-25, 2005 southern Minnesota
Thunderstorms raged along I-90, with 8-9 inches of rain in Fairmont and Winnebago. This event became the latest (on the calendar) mega-rain on record, but held that title for just 11 days (see next).
October 4-5, 2005 east-central Minnesota
This exceptional event produced six-inch-or-greater totals over Pine City, North Branch, and Cambridge, as well as over parts of the Twin Cities, becoming the new (and current) title-holder for the latest mega rain on record in Minnesota.
August 18-20, 2007 southeastern 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.
July 25-26, 2020, southern Minnesota
Repeat thunderstorms pounded southern Minnesota, producing up to 8.65 inches near Mankato, with potentially more from near Fairfax to Winthrop, and at least 6 inches from St. Peter to Morristown.
Major damaging events that do not qualify
The true damage potential of a flash-flood event is determined by more than just the size of the area of intense rain. Topography, drainage potential of the landscape, and hydroclimatic conditions prior to the event also influence the likelihood of flooding. A small number of events listed above occurred during relatively dry periods and/or in well-drained areas and resulted in less flooding than would have been expected at other times or in other areas.
Similarly, some rainfall events that did not qualify as a mega-rains nevertheless produced extreme, even devastating flash-flooding. Many such events are cataloged in our ongoing collection of Minnesota flash-flood events.
July 23-24, 1987 Twin Cities superstorm
Greatest calendar day precipitation on record for Twin Cities International Airport with 9.15 inches, and over 10 inches in suburbs west and southwest of Minneapolis. This rainfall cataclysm produced the worst flash-flooding on record in the Twin Cities. Its extraordinary magnitude and impact had granted it automatic status as a mega-rain previously. However, its footprint of 6-inch rainfall was estimated to be 574 square miles--well short of the 1000 square-mile requirement. The event was undoubtedly worsened by the high-runoff metropolitan landscapes, as well as the fact that an intense rainfall of 3 to 9 inches had affected much of the same area three nights earlier.
September 21-22, 2016
This major flash-flood event produced extensive damage in the northwestern Twin Cities suburbs, and also in the Waseca area, and capped off a wet summer that already had produced two qualifying mega-rain events. This storm had the intensity, but not enough areal coverage to qualify.
Modified August 10, 2022