Courtesy of Iowa State University, Environmental Mesonet
A busy evening with relatively wide coverage of strong thunderstorms led to many reports of large hail, and by far the most Severe Thunderstorm Warnings issued on any day so far in 2023.
The storms formed on Thursday July 13, ahead of a weak disturbance and along a cold front dropping southeastward into Minnesota. Several different pockets of mostly isolated thunderstorms formed all over the region, from southern Manitoba, into North Dakota, South Dakota, and eventually Minnesota. These storms generally remained apart from one another, existing mostly as semi-persistent, single-celled thunderstorms drifting eastward and southeastward across the state, as opposed to the familiar arced "squall lines" that often speed through Minnesota on summer evenings, or the messy conglomerations of storm clusters that form in high-humidity environments with weak winds aloft.
These storms may have remained somewhat isolated because of the lack of deep moisture in the region resulting from building drought conditions. This allowed storms to use the available local energy without "sharing," and to develop some limited "supercellular" characteristics. Supercell thunderstorm develop in unstable conditions with favorably strong winds aloft, allowing them to isolate and sustain themselves individually for two hours or more. Supercells can be the most severe class of thunderstorms, often with a rotating updraft that produce violent weather including very large hail and tornadoes. Thursday's storms could have qualified as borderline supercells, as they were self-sustaining, but only contained weak and fleeting rotation. They were able to produce very large hail, however.
Indeed, large hail was the main severe weather hazard associated with these storms. The National Weather Service received 61 reports of large hail, including 27 that were golf ball-sized (1.75 inches in diameter) or larger, and 13 that were at least as large as tennis balls (2.50 inches in diameter). Hail estimated to be 3.25 inches inches across was reported near Belgrade in Stearns County. The large hail battered houses, cars, and crops in different parts of the state, with heavy damage in Winona County, as well as in central and far southwestern Minnesota.
The storms led to 49 different Severe Thunderstorm Warnings, by far the most of the year so far (the next-most is 17). For perspective, through the same period in 2022, this would have been the fifth-highest number of warnings.