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Characteristics of Warm Season Heavy Rainfall in Minnesota


Warm season heavy rainfall in Minnesota can lead to flooding with serious impacts on life 10 and infrastructure. Situated in a transition zone between humid eastern and semi-arid western 11 conditions in the U.S., Minnesota experiences large spatial variability in precipitation. Previous research has often lacked spatiotemporal detail important for heavy rainfall analysis for Minnesota. This research used Stage IV hourly precipitation data with 4-km grid spacing during May-September 2004-2020 to analyze Minnesota spatial, seasonal, and event-based characteristics. Rain event frequency, accumulation, hours, and intensities were compared for all rain events (>2.5 mm) and heavy rain events (>36 mm). For all rain events, results showed the highest regional median monthly rain event frequency (>6 events) in June and the lowest (<5 events) in September. Median monthly accumulations were largest (∼75 mm) in June, followed by July and August. Monthly total rain event hours at a point peaked around 20 h in May in southeastern Minnesota. Smaller event accumulations occurred more frequently than larger accumulations, and event mean intensities were higher in summertime (June-August) than in May and September for rain events and heavy rain events. Heavy rain event region-based analyses showed monthly peaks for frequency in July-August, accumulation in July, and event hours in June-July and September. Median heavy rain event durations were shorter during June-August than in May and September. Monthly heavy rain event accumulation as a percent of all rain event accumulation was greatest in September (24%). These results establish a foundation for future research into precipitation patterns and trends.