Chicago Bow Echo case study
Posted: (August 12, 2008 02:03 pm)

A Bow Echo moved through Northern Illinois on August 4th producing widespread wind damage and 4-5 tornadoes.  The wind damage inclued wind speeds up to 100mph in some spots.  The impreesive MCS (Mesoscale Convective System) was fueled by a VERY unstable atmosphere with SBCAPE values ranging from 5000-8000j/kg of CAPE.  KILX sounding in Illinois reported a CAPE exceeding 8500j/kg in the 00z soundings for August 5th.  Lift indicies were also extreme with values approaching -14C. 

The winds with these storms cold easily be seen as thebow took shape.  Bow echoes are known to be big wind producers.  The reasons why the wind was so bad included very steep low level lapse rates and mid level lapse rates.  Dry air also played a role with dew point depressions exceeding 15C at 700mb.  Downdraft cape is a measure of the intensity of the downdraft and values were impressive exceeding 1500j/kg.  A few features in the bow echo such as a vortex that forms on the northern portion of the bow (bookend vortex) enhanced the rear inflow jet (RIJ).  These terms are found in the article below and are very advanced so do not worry about it unless your interested then you can e-mail any questoins you have.  

The bookend vortex led to the devleopment of a few tornadoes with the increased low level shear.  The tornadic storms formed on the shear created by the bow echo and perhapse the intersection with a lake breeze front.  The set up did not favor a tornado outbreak and that is why we only saw 4-5 tornadoes with these extreme parameters.  One thing I want to touch on for future forecasters is the EHI.  The EHI was showing values approaching 10j/kg with widespread 5-8j/kg across the area.  The charts shown online show this increases potential for f4-f5 tornadoes.  This did not happen and was not supposed to despite the high numbers.  This index is good when CAPE and HELICITY are balanced.  

For more information about the August 4, 2008 Bow Echo: http://www.crh.noaa.gov/news/display_cmsstory.php?wfo=lot&storyid=17139&source=0

If I have time I will do another write up about a research paper I did investigating this event below where CAPE did not exceed 1000j/kg for most parts and produced this.......




***All images used are from the Storm Prediction Center