Update 9:45PM CDT...
Kankakee County in Illinois and Newton, Jasper, Lake and Porter
Counties in Indiana have been added to the Watch that expires at
Winnebago, Ogle, Lee, Boone and DeKalb Counties have been
dropped from the Watch.
Mesoscale Discussion 1357
NWS Storm Prediction Center Norman OK
0819 PM CDT Wed Jul 19 2017
Areas affected...Northern IL...Southern WI
Concerning...Severe Thunderstorm Watch 423...
Valid 200119Z - 200245Z
The severe weather threat for Severe Thunderstorm Watch 423
SUMMARY...Severe squall line will progress across central into
eastern portions of WW423 over the next two hours.
DISCUSSION...Long-lived MCS has cycled and matured into a
fast-moving complex as it progresses across southern WI and northern
IL. Latest radar data suggests the leading edge of a bow-shaped
squall line is surging southeast at roughly 50kt. Given this
speed/movement it appears the squall line will approach the
northwestern Chicago metro between 0200-0230z, and the lake shore
around 0230z. There is a possibility for widespread damaging winds
along the leading edge of this convection.