June 4th Severe Weather Event
A warm front stretching west to east across northern Illinois and Indiana provided the
focus for severe convection and flooding Wednesday afternoon and evening. Dewpoints
in the lower 70s south of the boundary provided moisture to fuel these storms. As these
storms initiated during the late afternoon, strong vertical wind shear quickly
turned many of them into supercells capable of producing large hail and tornadoes.
Surface moisture convergence graphic from SPC (with front and low position
added) valid 10 PM CDT during the height of the event.
A tornado watch was issued by the Storm Prediction Center at 7:00 PM CDT for parts of eastern Iowa,
a large part of northern Illinois, and parts of northern Indiana. This was the easternmost of three
tornado watches stretching into eastern Colorado that evening.
Watch status as of 11:57 PM CDT on June 4, 2008.
At 7:11 PM CDT the NWS office in Chicago issued the first of 10 tornado warnings for the
evening. Interestingly, the first few tornadic circulations were very intense, yet
anticyclonic. Unlike the great majority of tornadic circulations, and thunderstorm circulations
in general, these first few storms were rotating clockwise, as seen in the following radar images.
Doppler radar storm-relative motion image from 7:33 PM CDT June 4.
Red values in the image show winds moving away from the radar, or toward the southeast.
Green values show winds moving toward the radar, or toward the northwest.
This particular image shows winds of 50-60 knots blowing in opposite
directions but very close together. This is an excellent indication that a storm
may be producing a tornado, though normally the circulation is cyclonic, meaning
counter-clockwise.
In the next graphic is the reflectivity image for the same storm.
Super-resolution reflectivity image from 07:33 PM CDT on June 4, 2008.
The image above depicts a very strong and somewhat unusual left-moving supercell. The rotation
in this storm, in what could be described as a hook echo, is located on the northern side of the
cell. The storm itself is moving to the northeast.
Shortly after these left-mover, or anticyclonic, supercells developed, traditional cyclonic supercells
also began to form in the area. The reflectivity and velocity patterns in these are more
familiar.
Super-resolution reflectivity image from 8:00 PM CDT on June 4, 2008.
The hook feature shown above is on the southern side of the storm, which is
moving almost due east.
Doppler radar storm-relative motion image for 8:00 PM on June 4, 2008.
In this image the cyclonic, or counterclockwise rotation is shown. It does
not appear to be as wide and as strong as the image shown earlier, but there still
were multiple tornado reports with this storm in southwest Livingston County, Illinois.
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