The Free Press
Details about the area’s heavy precipitation last weekend have been added to a Department of Natural Resources web page about similar weather events.
The DNR climate office has assembled a list of so-called “mega-rains” that have occurred since statehood to document events in which at least 6 inches of rain covers more than 1,000 square miles and the core of the rainstorm tops 8 inches.
Rainfalls of this magnitude and geographic extent have the potential to become catastrophic, according to DNR’s climatologists.
Even the early surveyors mapping out the state witnessed them in awe. Using newspaper accounts, diaries and the historical climate record, 22 such events in Minnesota’s post-settlement history have been identified.
The ability to detect these events has improved dramatically since the 1970s, when the number of daily rainfall observers in Minnesota exploded. Since that period, the state has benefited from an unusually dense network of observations. This network has remained intact, with some 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.
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.
Documented mega-rains include these regional events:
• Aug. 6, 1866 — The Wisel Flood. It killed 16 people, including three members of the Wisel family in Fillmore County; 10.30 inches of rain fell at the Sibley Indian Agency in Sibley County.
• July 17-19, 1867 — Climatologists and historians believe this to be Minnesota’s most extreme flash flood of the past 200 years. Torrential rains pounded portions of west-central Minnesota relentlessly. As a result of this event, catastrophic flooding occurred all down the Minnesota River.
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.
• Sept. 14-15, 2004 — More than 10 inches of rain fell in a 36-hour period in Faribault and Freeborn counties.
• Sept. 24-25, 2005 — Thunderstorms raged along I-90, with 8-9 inches of rain in Winnebago and Fairmont. This event held the Mega-rain record for just 11 days.
The current title-holder for the latest mega rain on record in Minnesota is a 2005 event that stalled over Pine City, North Branch, Cambridge and parts of the Twin Cities. Heavy thunderstorms began just before daybreak on Oct. 4 and continued to develop off and on for the next 30-plus hours. A large portion of central and southern Minnesota received more than 2 inches of rain — to put this in perspective, the normal monthly October rainfall in these areas is about 2 inches.
• Sept. 22-23, 2010 — The National Weather Service site in Amboy measured 9.48 inches on Sept. 23, with a 10.68-inch total for the event.
• July 25-26, 2020 — Repeat thunderstorms pounded southern Minnesota, producing up to 8.65 inches near Mankato. The highest known total for the storm came from near Winthrop, where a volunteer observer for Sibley County reported 11.5 inches. Other high totals included 10.7 inches near Lafayette and 9.15 inches in Gibbon. These amounts are near or exceed the threshold, for a 500-year threshold, for 24-hour rainfall in that part of Minnesota.
DNR climatologists who examined the period 1973-2019 found that Minnesota has seen 17 mega-rains, with a sharp uptick since 2000, despite a small decrease in observer numbers. Of these 17 events, three were in the 1970s, two were in the 1980s, one in the 1990s, but six occurred in the 2000s, with four more in the 2010s, and one so far in 2020.
The 21 years from 2000-2020 have seen almost two times as many mega-rains as the 27 years spanning 1973-99.
DNR climatologists believe that, although it is difficult to assess the statistical significance of that increase, they know these trends are consistent with the expectation that Minnesota and the Upper Midwest will receive more precipitation — and more precipitation from intense rainfalls — in response to increasing global temperatures and increased available moisture for passing storm systems.