Wednesday, 1 August 2001
A case study of a severe Midwestern pulse thunderstorm event
On 11 June 1999, the city of St. Peters, MO, was visited by a high wind event generated by a severe pulse thunderstorm. Extensive property damage was reported, including the removal of scaffolding around a water tower that was being prepared for painting and maintenance. The environment was largely benign, with a weak low over northern Iowa and a diffuse cold frontal boundary well north and west of eastern Missouri. Moreover, this isolated event occurred between 15 and 16 UTC, well before the diurnal heating peak. Still, the morning sounding from Lincoln, IL (ILX), depicted a weakly unstable environment (LI=-1.8; CAPE=740 J/kg) but more importantly, an inverted V structure in the boundary layer, with surface dew point depressions of ~12C decreasing to 0 at 750 hPa, thus suggesting a dry microburst-type scenario. Support aloft was generally weak, although warm, moist, southerly flow was present at 850 hPa over Missouri, with a 300 hPa negatively-tilted trough just to the southwest of the state. WSR-88D data from St. Louis, MO (KLSX) are presently being analyzed to further elucidate this storm's fine-scale structure and evolution.