August 19, 2009

Severe thunderstorms impacted parts of central Illinois on Wednesday, August 19th...bringing damaging winds, heavy rainfall, and several possible tornadoes.  The storms developed along a warm frontal boundary during the early afternoon, then tracked east-northeast through the area.  The hardest hit locations were along the I-72 corridor...with the bulk of the damage occurring across parts of Scott, Morgan, Sangamon, Logan, and DeWitt counties eastward to the Indiana border.

Weak high pressure moved into northern Illinois on Tuesday, August 18th, allowing slightly cooler and drier conditions to settle southward into central Illinois.  As this high moved off to the east, the warm and very humid airmass that had been temporarily suppressed southward into the Ohio River Valley began making its northward return on August 19th.  Scattered showers and thunderstorms began developing along the advancing warm front around midday, then became much more numerous during the afternoon as an upper-level disturbance approached from the west.  In addition, a Mesoscale Convective Vortex (MCV) associated with the remnants of a large thunderstorm complex across Kansas and Oklahoma during the previous night tracked into central Illinois.  This feature provided an extra kick, which enhanced lift and created vigorous updrafts within the developing thunderstorm cells. 

The storms continued to strengthen, as they drew energy from a moderately unstable atmosphere.  An upper-air sounding conducted by National Weather Service Lincoln at 1 PM on August 19th, revealed a CAPE value of 2791 J/kg and a Lifted Index (LI) of -6.6C.  Despite the modest instability parameters, ample low-level wind shear was present along the warm front...with southeasterly winds at the surface, gradually turning to southwesterly at 4000 to 5000ft aloft.  This veering of the winds with height helped many of the thunderstorm cells exhibit rotation.

Damage reports began to come in at 2:14 PM, when powerlines were blown down in Morgan County.  Over the next several hours, numerous wind damage reports were received as a line of strong thunderstorms with embedded supercells marched across central Illinois.  Most locations along the advancing line of storms only experienced brief heavy rain and gusty winds: however, those areas impacted by a supercell within the line reported much stronger winds and possible tornadoes.  The National Weather Service Lincoln is currently investigating the tornado reports and will release a statement with much more information as soon as possible.  Please refer to our homepage at:  for the latest news on the tornadoes.  Elsewhere around the area, estimated 60 to 70 mph winds brought trees, tree branches, and powerlines down from the Illinois River Valley eastward to the Indiana border.

Later in the afternoon, additional severe thunderstorms impacted locations along and close to I-70.  At around 5:30 PM, a storm with estimated 60 to 70 mph winds knocked trees down and damaged semi trailer tankers in Mattoon.  Shortly afterwards, another cell brought trees down in Neoga in Cumberland County.  In addition to the wind damage along I-70, training thunderstorm cells dropped 1 to 3 inches of rain in a short amount of time, producing flash-flooding from Effingham northeastward toward the Wabash River Valley.  

Here is a listing of some of the severe weather reports received from the SWOP network:






65 to 70mph winds, trees down



65 mph winds, tree branches down



55 mph winds



75 mph winds, tree branches down



powerlines down, trees/tree branches down



60 mph winds



45 mph winds



tree branches down



quarter-sized hail



tree branches down, powerlines down



60 to 70 mph winds, trees down, powerlines down

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