ZCZC SPCSWOMCD ALL
ACUS11 KWNS 242221 
SPC MCD 242221 
MIZ000-OHZ000-INZ000-ILZ000-250015-

Mesoscale Discussion 1343
NWS Storm Prediction Center Norman OK
0521 PM CDT Sat Jul 24 2021

Areas affected...portions of central and southern Lower
MI...northern IN...northern IL...southernmost Lake
Michigan...west-central/southwestern Lake Huron.

Concerning...Severe Thunderstorm Watch 390...391...

Valid 242221Z - 250015Z

The severe weather threat for Severe Thunderstorm Watch 390, 391
continues.

SUMMARY...Bands of thunderstorms from northern IL to southern Lower
MI will continue to offer sporadic damaging gusts and large hail
through the 02Z and 03Z nominal expiration of WW 390 and 391,
respectively.

DISCUSSION...As of 22Z, bands of convection were evident across
central/southern Lower MI, with visible satellite imagery and radar
composited indicating linkage occurring to initially separate
activity over southern Lake Michigan and northern IL. The resulting
primary convective band should be the main severe-weather concern
over the remainder of the watch areas, with less (but nonzero)
threat from trailing activity southwest of the MBS area. 

The modified 18Z DTX sounding and model soundings indicate that
surface insolation and lower 70s F dew points are supporting MLCAPE
in the 1000-2000 J/kg range over southern Lower MI, transitioning to
2000-3000 J/kg over northern IL.  Convergence is (and should remain)
weaker over northwestern IL, reducing convective/severe potential
there, even with greater buoyancy.  Despite veered/southwesterly
surface winds, favorable deep shear will remain across northeastern
IL, northern IN and southern MI, with 35-45-kt northwesterly
effective-shear vectors being common over the region.  Though slow
evening cooling of the preconvective boundary layer is expected, the
rich moisture will make the process gradual, helping to maintain
surface-based effective-inflow parcels for several more hours. 

Five counties in southeastern Lower MI (including most of the DTW
area) have been added to WW 390.  Potential for severe thunderstorms
to extend past 02Z will be evaluated in the coming couple hours to
determine if any new watch(es) will be needed for this evening past
02Z and/or extending farther southeastward.

..Edwards.. 07/24/2021

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...DTX...APX...IWX...GRR...LOT...ILX...DVN...

LAT...LON   41578981 42078799 42118660 43478601 43978401 44658235
            43338227 41888332 41698480 41068603 40318882 40628982
            41578981 

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