Here's the storm report from NOAA on the Oct 18 2007 system.
A powerful fall storm system affected a large part of the Midwest on October 18. Strong low pressure developed out of a deep upper level trough which moved from the Rockies into the Plains overnight, and was strengthened by mid-level jet stream winds in excess of 100 mph. Surface low pressure developed over the central Plains and quickly deepened to 982 mb and moved northeast into southern Minnesota by early evening.
Round #1 of severe weather struck central and southeast Illinois during the early morning hours as a 60 mph low-level jet stream brought deep moisture and warm air northward into the area. Additionally, a mid-level shortwave moved quickly northeast helping to produce widespread showers and thunderstorms. Intense storms developed just after midnight in west-central Illinois producing strong winds which damaged trees and power lines from Scott county eastward into Sangamon county. These storms weakened as they moved northeast into a more stable airmass. New storms erupted around 4 AM in southeast Illinois and developed quickly to the northeast and intensified over the next couple of hours. Between 4 and 6 AM winds in excess of 60 mph damaged trees and power lines from Effingham eastward through Lawrenceville, and northward through Martinsville, Robinson, and Paris. In addition to the strong winds these storms were very efficient rain producers, with between 2 and 4 inches of rain falling within one to two hours over a large part of southeast and east central Illinois.
The system’s warm front pushed north of the area by the early morning hours, and the "dry slot" moved in from the west, shutting off precipitation and bringing mostly sunny skies by morning. The morning sunshine allowed the atmosphere to destabilize ahead of the approaching cold front to the west. Early afternoon temperatures rose well into the 70s, with dewpoints in the 60s. With a moderate risk of severe storms for the eastern half of the state, a special 18Z weather balloon was launched from the Lincoln office. Data showed very strong low-level lapse rates and sufficient moisture in the lower levels to produce a moderately unstable airmass . Additionally, wind fields were very strong aloft. Surface winds of 25-35 mph increased to 75 mph at mid-levels, and 125 mph at upper levels. While the wind direction did not change much with height, the speed difference created an impressive amount of vertical shear which was sufficient to create supercell thunderstorms. By late afternoon the atmosphere had continued to destabilize as the cold front/dry line plowed in from the west. Thunderstorms erupted around 5 PM between the I-55 and I-57 corridor and moved to the northeast at 40 to 60 mph. Frequent lightning, very heavy downpours, hail, and damaging straight line winds accompanied many of these storms. The activity continued until around 7:30 PM. Many locations along and east of the I-57 corridor sustained damage to trees and power lines.
There were no tornadoes associated with this activity, likely due to a lack of directional wind shear locally. Violent tornadoes did develop later in the evening as these storms pushed into Indiana . Over 4 dozen tornadoes were reported over a 2 day period across the central U.S.
And just for something different - the difference between chasing with big bucks and being a 'spotter' reporting from fixed locations? Prime mover with 5cm doppler vs banged up SUV !!! (Photos courtesy Reed Timmer attending the National Weather Service open day in Norman OK.)
Mike
