Mesoscale Discussion 1341
NWS Storm Prediction Center Norman OK
0300 PM CDT Sat Jul 24 2021
Areas affected...Parts of southern Lake Michigan...northern Illinois
and adjacent northwestern Indiana...into parts of southeastern Iowa
Concerning...Severe potential...Watch possible=20
Valid 242000Z - 242200Z
Probability of Watch Issuance...60 percent
SUMMARY...Widely scattered to scattered strong thunderstorm
development appears increasingly possible through 5-7 PM CDT,
particularly around the Greater Chicago metropolitan area, posing a
risk for a few strong downbursts.
DISCUSSION...Deepening convective development is ongoing along a
zone of boundary-layer confluence near/south of a slowly southward
advancing convective outflow boundary, with thunderstorms beginning
to initiate near/northwest of the Greater Chicago metropolitan area.
This coincides with a low-level thermal ridge axis, along which
strong daytime heating has contributed to steepening near surface
lapse rates and mixed-layer CAPE on the order of 2000+ J/kg. This is
also near the southwestern periphery of larger-scale mid-level
troughing shifting across and to the east of the Great Lakes
vicinity, where 30-40 kt northwesterly flow around 500 mb is
contributing to strong deep-layer shear.
Although forcing for ascent, particularly in mid/upper levels, may
only allow for widely scattered thunderstorm development to the
west-southwest of southern Lake Michigan late this afternoon, much
of the Greater Chicago metro may be impacted. This could include an
upscale growing line or clusters of storms with the potential to
produce at least a few strong downbursts.
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