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Rapid Mesoscale Environmental Changes Accompanying Genesis of an Unusual Tornado

Abstract

This study documents a very rapid increase in convective instability, vertical wind shear, and mesoscale forcing leading to the formation of a highly unusual tornado as detected by a ground-based microwave radiometer and wind profiler, and in 1-km resolution mesoanalyses. Mesoscale forcing for the rapid development of severe convection began with the arrival of a strong upper-level jet streak with pronounced divergence in its left exit region, and associated intensification of the low-level flow to the south of a pronounced warm front. The resultant increase in stretching deformation along the front occurred in association with warming immediately to its south as low-level clouds dissipated. This created a narrow ribbon of intense frontogenesis and rapid increase of Convective Available Potential Energy (CAPE) within 75 min of tornadogenesis. The Windsor storm formed at the juncture of this warm frontogenesis zone a developing dryline. Storm-relative helicity suddenly increased to very large values during this pre-tornadic period as a mid-tropospheric layer of strong southeasterly winds descended to low levels. The following events also occurred simultaneously within this short period of time: a pronounced decrease in mid-tropospheric equivalent potential temperature (θe) accompanying the descending jet, an increase of low-level θe associated with the surface sensible heating, and elimination of the capping inversion and convective inhibition. The simultaneous nature of these rapid changes in such a short period of time, not fully captured in Storm Prediction Center mesoanalyses, was likely critical in generating this unusual tornadic event.

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