Climate Summaries and Publications


Historic Mega-Rain Events in Minnesota

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 ten mega-rains. However, of these ten, two were in the 1970s, one was in the 1980s, none were in the 1990s, but the 2000s saw three and the 2010s, still underway, has seen four. Indeed, the frequency of these potentially disastrous events has been increasing sharply, and 2016 became the first year on record with two mega-rains in the state. 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.)

For more information contact: climate@umn.edu