Storm surveys from Kankakee and Watseka Aug 4/5th

A storm survey was conducted by NWS meteorologists on Wednesday afternoon at Watseka and Kankakee in northeast Illinois.  It was determined that straight line winds of 80 to 110 MPH occurred near the intersection of county roads 2100N and 2100E approximately 2 to 3 miles north of Watseka.  A machine shed was completely destroyed and damage witnessed from the survey was suggestive of straight line winds. Another pocket of significant straight line wind damage was also noted near Route 1 just south of Route 24 approximately 1 mile east of Watseka where significant tree damage was noted and debris from this tree damage resulted in broken windows to a glass patio area. Based on damage across this location, wind gusts were estimated to be in excess of 80 MPH.  Tree damage continued eastward to Sheldon.  Most of the wind damage that occurred across these areas was in association with thunderstorms that moved through Iroquois county during the early morning hours on Tuesday, just after 6am.  The map below highlights the areas of most significant wind damage that were surveyed in the Watseka area.



The following is a radar image of storm relative velocity at 616am Tuesday morning from the storm that affected Watseka.  The radar from the NWS in Romeoville is located to the northwest of this location.  The green colors represent velocities toward the radar, and the red colors represent velocities away from the radar.  The combination of these colors suggests the presence of a mesovortex, or a small scale counter-clockwise rotation in the low levels of the atmosphere.  This mesovortex was likely responsible for the straight line wind damage observed near Watseka as it moved from north of Watseka to the Sheldon area in eastern Iroquois county. 




An area of widespread wind damage also was observed in the southern portions of Kankakee.  Wind damage across southern portions of Kankakee was more widespread in nature.  The following image shows an approximate location of the more extensive wind damage:

The extensive wind damage began along W. Jeffrey Street and extended to east of the Cobb Park area.  Many roads were impassable across these locations on Tuesday due to downed trees.  Several very large trees were uprooted and power lines were downed.  One very large tree landed on the roof of a home across the western portion of the main damage area. The damage across southern Kankakee came in two rounds, the first during Monday evening and then another round of storms affected this area early Tuesday morning.  The most significant damage was reported to have occurred with the intial round of storms on Monday evening. Based on the observed damage, wind gusts to 80 to 100 MPH were estimated. 

The following is a radar image of base velocity from 922pm Monday evening from the storms that affected the southern parts of Kankakee.  In this image, the NWS Romeoville radar is located to the north northwest.  Note the presence of velocities toward the radar (green colors) north of the velocities going away from the radar (red colors).  This signature suggests divergent winds near the ground indicative of a microburst.  During a microburst, strong downburst winds hit the ground and spread out in circular fashion. The damage observed in Kankakee is consistent with a microburst.

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