Figure 1: The red area represents estimated wind speeds of 70 to 80 mph. The damage occurred in St. Clair county along I-55, near Centreville on August 25th, 2001
Figure 2: The red area represents estimated wind speeds of 70 to 80 mph. This is also in St. Clair county, where the brunt of the storms fury was felt. This location is just west of Mascoutah on August 25th, 2001
Figure 3: Tree damage in the area of Cahokia represented on the first damage map. The photo was taken just to the northwest of Interstate 255.
Figure 4: Tree damage in the area of Mascoutah represented on the second damage map. The photo was taken just to the south of IL Route 177.

NWS
St. Louis, Missouri

Downburst Winds in Metro East August 25th, 2001

Thunderstorms developed across parts of eastern Missouri and southwest Illinois on Saturday afternoon, August 25, 2001.  One of these storms produced damage over parts of St. Clair and Clinton counties in Illinois.

This storm developed in an area where skies had cleared earlier in the day.  The combination of increased sunshine and the humid air already over the region resulted in very unstable conditions.  The instability allowed strong upward motion in a few of the thunderstorm updrafts.  As is often the case with thunderstorms in the summer, the updraft quickly weakened causing the rain and associated cold air within the atmosphere to quickly drop to the earth's surface.  The result was a downburst.

Downburst winds are caused by air rushing out from the base of a severe thunderstorms to the surface and spreading outward over a wide area, similar to a "starburst" type pattern.  Microburst winds are similar but occur over a smaller area and are often more intense.

A damage survey was conducted the following day by Ron Przybylinski, Science and Operations Officer at the National Weather Service in St. Louis.

Damage Map Near Centreville, IL August 25th, 2001
Damage Map Near Mascoutah, IL August 25th, 2001
Tree Damage in Cahokia, IL August 25th, 2001

Tree Damage in Mascoutah, IL August 25th, 2001

       

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