A Meteorological Assessement of the Initial Development of the 19 July 2019 Wisconsin Derecho

Mesoscale convective systems (MCS) are common for the northern Plains and Upper Midwest during the summer months. These systems often produce swaths of wind damage for hundreds of miles, with occasional tornadoes and large hail. A derecho is a high end MCS which has historically been defined as producing a swath of wind damage extending for more than about 400 km (250 miles), wind gusts of at least 26 ms^-1 (58 mph) along most of its length, and also several, well-separated 34 ms^-1 (75 mph) or greater gusts.

An exceptionally rare combination of instability and shear developed during the afternoon of 19 July 2019 across portions of the Upper Midwest. Extreme buoyancy brought on by surface dew points as high as 28°C (83°F) and mid-level lapse rates around 7.5°C km^-1 led to an observed surface based Convective Available Potential Energy (CAPE) of 8,305 J/kg at KMPX during the early evening. Of the 46,714 rawinsonde launches conducted in central Minnesota between 1948 and 2014, the launch on 19 July was almost 1,200 J/kg more unstable than the previous record. In addition, an unusually strong mid-level jet for mid-summer translated eastward across central Minnesota during convective initiation. Explosive thunderstorm development was expected that day. A regional tornado outbreak was anticipated across central Minnesota, with some strong tornadoes possible, followed by upscale growth into a derecho with widespread, significant damaging winds across central Wisconsin. The anticipated tornado outbreak did not happen, but there were a few weak tornadoes embedded within the well-developed line of thunderstorms over northwest Wisconsin. Reasons behind the lack of tornadoes are explored. A measured gust of 38 ms^-1 (84 mph) was measured by a weather station in Polk County, Wisconsin with a sustained wind of 33 ms^-1 (74 mph) for five minutes. Hundreds of thousands of trees were toppled during the event with some structural damage as well.