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Latest SPC Mesoscale Discussion continues significant severe thunderstorm threat for northern Illinois and the Chicago Metro area through midnight
Mesoscale Discussion 1385
NWS Storm Prediction Center Norman OK
0628 PM CDT Fri Jul 21 2017
Areas affected...Eastern IA...Southern WI...Northern IL...Northwest
Concerning...Severe Thunderstorm Watch 429...
Valid 212328Z - 220100Z
The severe weather threat for Severe Thunderstorm Watch 429
SUMMARY...Severe threat continues across most of WW429.
DISCUSSION...Remnants of long-lived MCS, that evolved over the
central Plains before dawn, have progressed into the upper Great
Lakes region. Considerable amount of redevelopment is occurring in
proximity to the remnant center with the majority of strong updrafts
evolving along the southwestern flank of the mid-level debris shield
from eastern IA into extreme southwest WI. Early-day convection has
contributed to a well-defined outflow boundary that is draped across
the south side of the Chicago metro, westward into IA where a few
supercells have evolved. This new development will likely continue
to grow upscale with storm mergers likely contributing to another
mature MCS over southern WI/northern IL later this evening. Large
hail and damaging winds remain possible with most storms this
evening and a tornado can not be ruled out with supercells that
utilize more favorable shear near the outflow boundary.