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Thunderstorms rapidly fired in southern North Dakota between 10 PM and 11 PM local time on the evening of July 13th. These thunderstorms developed to the north of a surface low pressure that was located in central South Dakota. While the air mass near the surface over North Dakota was quite stable - drier and cooler than to the southeast - a warm, moist air mass had surged north, elevated a few thousand feet above the ground. This increasingly unstable environment aloft helped support thunderstorm growth well into the overnight hours.
In the early morning hours of July 14th, the storms in North Dakota formed into small lines and clusters, and began propagating faster to the east, as the surface weather features became more well defined. The surface low was pushing into northeastern South Dakota, with a surface warm front pushing north into central Minnesota. The storms eventually became concentrated into a squall line along and to the north of the surface warm front. This sort of pattern, and storm evolution, is relatively common in the summer months in the central United States. Thunderstorm clusters and lines often become focused in areas where advection of warm, moist air is maximized.
These conditions became focused over central Minnesota and into northwest Wisconsin on the morning of July 14th - as a surface warm front was situated just south of the NWS Duluth county warning area, and a very humid air mass was surging north into southern Minnesota. As such, the squall line continued to take shape and intensify as it crossed into the NWS Duluth area roughly around 7 AM local time. This line of storms probably reached peak intensity over far northwest Wisconsin as it started to bow out near the Spooner area. Widespread wind damage was reported from Burnett County, eastward through Washburn and Sawyer Counties.
More thunderstorms developed quickly in the wake of the initial line of storms across far eastern Minnesota where dewpoints were rising into the mid-70s and the atmosphere was destabilizing. These thunderstorms became focused in the afternoon across northwest and west-central Minnesota where rain-cooled air (thunderstorm outflow) was reinforcing the pre-existing warm front. Finally, thunderstorms eventually developed southward southern Minnesota, and central and southern Wisconsin, from this 2nd round of storms, and a few more storms also re-developed closer to the surface low over western and central Minnesota. All of these thunderstorms continued to push east through the afternoon and early evening along the periphery of a deep moisture plume and in an extremely unstable air mass.