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Mesoscale Discussion 1387
NWS Storm Prediction Center Norman OK
0835 PM CDT Fri Jul 21 2017
Areas affected...Eastern IA...Southern WI...Northern IL...Northwest
Concerning...Severe Thunderstorm Watch 429...
Valid 220135Z - 220330Z
The severe weather threat for Severe Thunderstorm Watch 429
SUMMARY...Short-term severe threat is focusing across the eastern
half of WW429.
DISCUSSION...Numerous strong thunderstorms have developed within the
southern-southwestern flank of an old MCS from southern WI into
northern IL. This activity is maturing near/north of an early-day
outflow boundary that remains draped south of Lake MI across
northern IL. Several supercell structures are noted within this
activity but further storm mergers should lead to an expanding
cluster and new MCS. This MCS should mature over northern
IL/southern WI then propagate across southern Lake MI into parts of
lower MI and northern IN. Damaging winds are the greatest threat,
although a tornado can not be ruled out with supercells near the
Farther west across IA, there is some concern that a strengthening
southwesterly LLJ will initiate new convection atop reinforced
outflow late this evening. If this occurs large hail could become a
concern with elevated storms across this region.