Image credit: National Weather Service, Duluth
Slow-moving thunderstorms with torrential downpours led to some of the highest rainfall totals on record in northeastern Minnesota on Tuesday June 18, 2024, closing roads and flooding towns from the Iron Range to the north shore of Lake Superior.
The rains formed as an intense surge of moisture from the Gulf of Mexico streamed into the region. Thunderstorms with heavy rain had pushed out of the eastern Dakotas into northern Minnesota early in the morning on Tuesday, and as those dissipated, a warm and humid air mass took over. A cold front moving in from the west helped ignite new thunderstorms during the early afternoon, which mushroomed quickly into a massive, slow-moving area of storms with heavy rain.
Rainfall rates during the afternoon evening in northeastern Minnesota were 1-2 inches per hour, with the heaviest rains lasting over two hours in some places. Even after the most intense rain subsided, waves of moderate to heavy rain continued affecting the area off and on through the evening, with the final wave of cells exiting northeastern Minnesota during the night.
A wide area from Walker to Grand Rapids, northeastward to the Canadian border and eastward to the the shore of Lake Superior, received at least three inches of rain, with many areas seeing 4-6 inches and several locations reporting 6 to almost 8 inches of rain.
The extreme rainfall inundated Iron Range communities from Hibbing through Hoyt Lakes and eastward towards Silver Bay and Lutsen, as well as those around Lake Vermilion. Many roads in and out of town were impassible because of deep standing or flowing water. Segments of US Highways 53 and 169 had to be closed, as did parts of Minnesota Highways 1 and 61. Major flooding contributed to dangerous conditions and closures of facilities at Gooseberry Falls, Lake Vermilion-Soudan Underground Mine, and Tettegouche State Parks.
The heaviest rainfall totals included 7.60 inches to the northwest of Tower, 7.33 inches about 20 miles north of Two Harbors, and 7.30 inches near Marble in Itasca County. Totals near Lake Vermilion may have exceeded eight inches, although no observers in any of the three major networks serving Minnesota reported such amounts. At the National Weather Service cooperative station at Brimson, the total of 5.18 inches was the largest in over 65 years of reliable records. The Wolf Ridge Environmental Learning Center recorded its largest rainfall event in over thirty years of record, with 5.32 inches.
Based on an extensive "frequency analysis" conducted by NOAA during the early 2010s, an average community in northeastern Minnesota has about a 2% chance of seeing a 5-inch daily rainfall total in a given year, less than a 1% chance of seeing a 6-inch rainfall, and a less than 0.5% chance of a 7-inch rainfall. These probabilities correspond to average recurrence intervals of about 50 years, just over 100 years, and just over 200 years, respectively. In other words, these storms left behind unusually high rainfall totals, of the sort generally observed just once or twice in a lifetime, if at all.
Despite the severity and extent of this storm, it was not a "mega-rain," because the area receiving six inches or more did not exceed 1000 square miles (it appears to have been about 400 square miles), and there were no confirmed 8-inch rainfall totals. This was, nevertheless, one of the largest rainfall events on record to strike northeastern Minnesota.
June 20, 2024
KAB