The 19 July 2006 Midwest Derecho: A Meteorological Perspective and Lessons Learned

An historic wind storm occurred during the late afternoon and evening of 19 July 2006 over the Greater St. Louis Metropolitan area. The Mesoscale Convective System (MCS) formed during the late morning over parts of southeast Minnesota and southwest Wisconsin and subsequently moved southeast then south across central and parts of southwest Illinois before hitting the St. Louis area during the early evening. The convective complex produced a damage swath extending from parts of northwest Illinois through the heart of the St. Louis metropolitan area, and then continued to move south-southwest into south-central Missouri. The complex of severe storms which moved through the city of St. Louis during the Cardinal baseball game injured thirty fans at the ball park. Ground surveys revealed clusters of downburst swaths with intense microbursts embedded within the larger swaths. The width of the overall damage area ranged from 50 km over parts of southwest Illinois to 75 km over the Greater St. Louis Metropolitan area. Some of the greatest structural, tree and power line damage occurred from the Bethalto – East Alton Illinois areas through the city of St. Louis and southward into parts of Jefferson County Missouri. Witnesses over parts of southwest Illinois also observed two tornadoes which briefly touchdown. Damage intensity from the wind ranged from EF0 – EF1 over much of the metropolitan area.

The first part of this presentation will show an overview of the environmental setting over the region. Storm Prediction Center (SPC) mesoanalysis fields and RUC sounding data will be used to show the extremely unstable atmosphere and weak low-level shear and moderate deep-layer shear. These fields will be used to show how the north-south axis of highest instability shifted eastward during the afternoon from central Missouri to east-central sections of the state and over parts of southwest Missouri enhancing the convective system as it moved into the Greater St. Louis metro area. .

The second part will highlight the radar analysis for this case. WSR-88D reflectivity and Doppler velocity imagery from Weather Forecast Office (WFO) St. Louis (KLSX) will show an advancing outflow boundary approximately 15 km downshear from the strong convective towers. Surface gusts behind the surging outflow exceeded 20 m s-1 at several locations across the St. Louis metropolitan area. This feature was persistent throughout much of the event. Plan-view and cross-sectional reflectivity and Doppler velocity data will capture multicellular evolution with developing convective towers rapidly forming between 5 to 8 km AGL aloft and 5 to 10 km downshear from the mature convective cells. Damage assessment findings of this case will reveal that the strongest surface winds (35 – 40 m s-1) occurred during the period when individual convective cores collapsed similar to that of an intense microburst storm. A well defined bow echo and trailing stratiform rain region was not observed with this system suggesting the absence of ascending (descending) mesoscale airflow branches typically observed with forward propagating MCSs. This convective complex will briefly be compared to the 10 August 1992 southwest moving MCS which also produced severe wind damage over parts of east-central through south-central Missouri. Lessons learned from the warning forecaster's perspective will briefly be discussed at the end of this presentation.