Image credit: National Weather Service, Twin Cities/Chanhassen forecast office.
Minnesota's first significant, rainfall-driven flood event of 2023 unfolded near the Cottonwood and Minnesota rivers after a slow-moving weather system produced multiple waves of moderate to heavy rain over a period of several days.
A sprawling, crawling area of low pressure over the western US began affecting Minnesota on Wednesday, May 10, 2023. The system produced heavy rains over the Colorado Front Range, and in a swath from northwestern Kansas through the western Dakotas, eastern Montana, and eastern Wyoming. As it moved eastward into the Plains, its humid circulation reached into parts southern and western Minnesota, leading to daily rounds of showers and thunderstorms from Wednesday into Sunday.
Although rains fell somewhere in southern Minnesota every day for five straight days and some areas received rain each of those days, two periods in particular contributed to the excessive water problems. The first was from Wednesday May 10th into Thursday May 11th, as thunderstorms developed in mostly west-to-east formations and moved slowly over parts of southern Minnesota, with totals of 2-3 inches in isolated areas, especially near and to the west of Mankato. The second period of concentrated heavy precipitation was from late on Friday May 12th into Sunday May 14th, as a more widespread area of rain and heavy thunderstorms overspread the area, with additional rains of 2-4 inches along the central Minnesota River and through much of south-central Minnesota.
When including not just the two main episodes, but also the other periods of rain and thunderstorms, six-day precipitation totals (through the morning of May 15th) of four inches or more were common in the southern three tiers of Minnesota counties. The highest totals were concentrated 20-60 miles west of Mankato, with one CoCoRaHS observer east of Comfrey recording 9.16 inches, and several others between Sleepy Eye and St. James reporting 6.5 to nearly eight inches of rain. The St. James Wastewater Treatment Plant recorded 5.84 inches, and the University of Minnesota observer at Lamberton recorded 5.39 inches. In this part of Minnesota, May precipitation for the entire month is generally 4 to 4.5 inches.
The heavy and excessive rains produced flooding in Comfrey, caused the Cottonwood River to reach Major Flood stage, and led to a damaging mudslide that closed parts of Minnesota Highway 68 near Courtland.
May 15, 2023