Overview of the 22 July 2013 Northwestern Minnesota Nocturnal Tornado Event

A strong, long track tornado occurred across northwestern Minnesota during the early morning hours of 22 July 2013. Of multiple tornado or damaging wind events which occurred throughout the episode, this tornado produced high end EF1 to EF2 damage along most of its 21 mile path across portions of Mahnomen and Clearwater Counties. Despite occurring during a relative climatological minimum for significant (EF2 or greater) tornadoes over the geographic area, both the synoptic and mesoscale environments remained quite favorable for supercell tornado formation as a cluster of thunderstorms moved across the Red River into Minnesota during the late evening and early morning hours. During that time, several factors favored a re-intensification of the existing deep convection into a broken line of supercellular thunderstorms with leading stratiform precipitation. Boundary layer stability decreased, likely associated with moistening of the lower troposphere. Concurrently, low-level vertical wind shear increased in association with a strengthening low level jet stream and an intensifying low pressure system across south-central Canada.

This study will examine the radar evolution of the event, with particular focus on the rapid re-intensification into more identifiable individual convective cells in the vicinity of a surface warm front along the back edge of the leading stratiform precipitation during the early morning hours. This study will also include analyses of the synoptic and mesoscale environments that supported such a transformation.