Severe Weather Hits Southern Illinois, Illinois State Water Survey

ISWS Press Release

For Immediate Release May 18, 2009
Severe Weather Hits Southern Illinois
Source:   
Contact:   
Water and Atmospheric Resource Monitoring Program
Bob Scott - (217) 333-4966, rwscott1@illinois.edu

A severe weather system moved across southern Illinois near mid-day on May 8, 2009, causing major wind damage to many roofs and other structures, and downing countless power lines and trees in its wake. The Illinois State Water Survey operates a network of weather stations across Illinois, one of which is located at the Southern Illinois University Agronomy Farm, approximately 3 miles southwest of Carbondale.

The charts below show both maximum and minimum values of weather data within each hour or hourly totals at the end of each hour for the entire day of May 8. For data consistency, Central Standard Time (CST) is maintained at our sites throughout the year. Therefore, add one hour for Central Daylight Time (CDT) in force on this day.

The significance of the event was not as much for its strength, but its longevity. Typically, damaging conditions from severe storms last several minutes, rarely tens of minutes. But this system created large wind gusts over a period of three hours: a gust of 31 mph was recorded at 10:44 a.m., 41 mph at 11:54, and 77 mph at 12:21 p.m., at which time the anemometer broke. Fortunately, the Carbondale site is solar powered with battery backup, thus, the rest of the site's data were recorded intact.

These wind gusts were caused by a significant change in barometric pressure during the event. Pressure values dropped by 2.8 millibars (mb) in 14 minutes between 09:57 a.m. and 10:11, then rose 5.9 mb by 10:42, and then dropped a whopping 9.7 mb (roughly 0.29 inches of mercury) by 12:16 p.m., before recovering by 6.3 mb by 12:54. Sudden pressure changes of this magnitude cause winds to accelerate in an attempt to "fill in" the air that has been removed by the storm. When this occurs over populated areas, wind damage is the typical result. Damage of this magnitude is not rare; wind damage is frequent near severe storms. However, typical sizes of severe storms are usually smaller, less wide-spread, and more short-lived than this event. Regardless, it is not common for data such as these to be recorded with such a sparse network of data recording devices.

The impact of the storm on most other weather variables was not atypical. Drops in temperature and dew point temperature were not out of line with severe storms conditions. Interestingly however, most of the heavy rain occurred well before the heaviest wind gusts, 1.18 inches fell in the hour prior to 11:00 a.m. CST. Similarly, the darkest skies occurred in the same hour when recorded solar radiation was near zero.

Weather conditions at the Illinois Climate Network site in Carbondale on May 8, 2009

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