Image credit: National Weather Service, Twin Cities/Chanhassen
Overview
The extremely wet pattern that led to major flooding in northeast Minnesota on June 18th took one day off and then set its sights on southern Minnesota, where it dropped 6-9 inches of rain in three days in some areas, with widespread totals of over two inches. The rains caused significant flooding on local streams and parts of the Minnesota River.
Individual Waves of Heavy Rain
The first wave of heavy rain fell as slow-moving thunderstorms trudged out of South Dakota, Nebraska, and Iowa late on Thursday June 20th into Friday morning. The heaviest rains spread across the extreme southwest, along Interstate 90 and up towards the "elbow" of the Minnesota River, near Mankato. Moderate and steady rains enveloped much of the rest of southern and eastern Minnesota. These rains produced totals of an inch or more in the southern fifth of the state (roughly the three southernmost tiers of Minnesota counties), with totals of 3-4 inches from St. Clair and Good Thunder (just south of Mankato), southwestward to St. James, Worthington, and Luverne.
The area of precipitation moved off into Wisconsin on Friday morning, and within a few hours, the next wave developed, as a nearly stationary band of heavy thunderstorms formed a line nearly overlaying the axis of heaviest precipitation from earlier in the day. Heavy rains lasted an additional 90 minutes to over three hours as this line expanded, intensified, and eventually weakened.
Another wave of heavy rain was fast on its heels, however, as thunderstorms that had been clobbering the Sioux Falls, SD area marched back into Minnesota and expanded eastward and northeastward during the evening Friday. Other intense thunderstorms developed near the Iowa border and spread along parts of I-90. By Saturday morning, 24-hour rainfall totals of an inch or more were common in almost the exact same areas as the night before, although they expanded into the souther Twin Cities, with 3-5 inches reported in and around Mankato, Waterville, Rochester, Austin, Winona, Albert Lea, and Worthington.
The final wave of rain came on Saturday and was faster and therefore smaller than the previous rounds of rain and storms, but an additional 0.50 to 1.50 inches of rain fell across the southern third of Minnesota as a slow-moving complex of rain and thunderstorms spread eastward from late morning into the afternoon. In parts central Minnesota and the Twin Cities, Saturday's rains were among the heaviest of the three-day period.
Image credit: Scott Roemhildt
Total Rainfall and Impacts
Rainfall totals from Thursday through Saturday included 9.16 inches at the National Weather Service cooperative observing site in Windom, 8.86 inches at St. James, and 7-8 inches from CoCoRaHS observers in the Mankato area. The Rochester airport received 4.56 inches, the Twin Cities airport received 1.49 inches, and St. Cloud received just 0.27 inches. Duluth and International Falls received even less rain, and fortunately, the area hit hard on June 18th was largely spared during this event.
The intense rains occurred within a an extremely wet period lasting over a week and followed a wet spring in general, leading to significant and devastating flooding in parts of southern Minnesota. The Rapidan Dam on the Blue Earth River made national news when the raging waters carved a new channel out of the surrounding slopes, causing landslides, and damaging nearby structures. The Minnesota River at Henderson ultimately surpassed the record crests observed after an extraordinary rainfall in September of 2010, and many other stream and river gaging sites observed were expected to observe crests at or near record levels.
Historical Frequency and Significance
Three-day totals of 7 to 9 inches have between a 0.5% and 2% chance of occurring during any year in communities across southern Minnesota, meaning that the average recurrence interval is 50-200 years. These are not unheard of totals in Minnesota, but they are rare at any given location.
The rainfall quickly led to major flooding in towns, and along streams and rivers in southern Minnesota, including the Minnesota River. As of Monday June 24th, the Rapidan Dam near Mankato was near failure and the water had rushed around it, causing stream bank failures and sweeping away small structures.
Had these rains occurred within 24 hours, the event would have qualified as a "mega-rain," given that the six-inch totals covered about 1500 square miles. However, the rainfall event was spread over parts of three days, during which time only isolated locations received over six inches in 24 hours. The State Climatology Office reminds Minnesotans that the term "mega-rain" is useful for defining a particular type of extreme rainfall event, but does not represent the totality of extreme rains or damaging floods. This was a major rainfall and flood event for Minnesota, even if it did not make the list of mega-rains.
Was it Climate Change?
In general, the advancing field of "attribution science" (see here for some examples) has shown that our warming world, which increases access to both heat and moisture, often makes extreme precipitation events larger, more intense, more frequent, and therefore more probable over a given time interval at a given location. It does not necessarily cause extreme events to happen, but instead enhances their size, strength, and likelihood. We do not currently know the exact contributions that the changing climate may have made to the size and intensity of this extreme rainfall event, but we do know that even when accounting for the droughts of the early 2020s, Minnesota is in a decades-long trend towards wetter and more humid conditions, with extreme precipitation events becoming both more frequent and heavier on average.
June 27, 2024
KAB