Analysis of the 21 July 2006 Greater St. Louis and Southwest Illinois Bow Echo event

Severe thunderstorms rapidly developed across central Missouri during the morning of 21 July 2006, and evolved into a classic bow echo as they crossed the St. Louis metropolitan area. This occurred less than 48 hours after a line of severe thunderstorms caused significant damage and a large power outage to the same area on 19 July.

Trained weather spotters estimated wind speeds in excess of 30 ms-1 across eastern St. Charles and northern St. Louis Counties, causing additional widespread tree and power line damage. This exacerbated the power outage, and caused the total number of households and businesses without electricity to rise over 750,000. This stands as the largest and most widespread power outage ever to occur in the St. Louis metropolitan area.

A number of mesovortices was observed north of the apex of the bow echo as the convective system accelerated across southwest Illinois. Storm surveys conducted by the National Weather Service revealed five non-supercell tornadoes and five microbursts occurred within the larger downburst footprint of the bow echo. Wind speeds as high as 40 ms -1 were estimated within the microburst regions, while the tornadoes were classified as F0 or F1 on the Fujita Scale.

A detailed radar analysis showed the development of the bow echo was closely tied to the evolution of the mesoscale rear inflow jet. Furthermore, radar analysis along with storm surveys identified the close correlation between storm mergers and damaging winds over eastern St. Charles County and northern St. Louis County, and tornadogenesis with observed mesovortices across southwest Illinois. Time height cross section analysis of radial velocity (Vr) showed a lead time of less than one volume scan, or roughly 5 minutes, between mesovortex identification and tornadogenesis.