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ACUS11 KWNS 111737
SWOMCD
SPC MCD 111737=20
MIZ000-INZ000-WIZ000-ILZ000-111930-

Mesoscale Discussion 1832
NWS Storm Prediction Center Norman OK
1237 PM CDT Mon Oct 11 2021

Areas affected...portions of north-central IL...eastern WI and
northwestern MI

Concerning...Severe potential...Watch possible=20

Valid 111737Z - 111930Z

Probability of Watch Issuance...40 percent

SUMMARY...Ongoing convection ahead of the upper low may pose a risk
for damaging winds and perhaps a brief tornado through much of the
afternoon. However, cloud cover and stratiform rain cast
considerable uncertainty on the convective evolution.

DISCUSSION...As of 1715 UTC, a well-defined upper low was located
near the Mississippi river along the border of MO and IL. To the
east, remnant convection and precipitation were ongoing across much
of central and northern IL. Despite dense cloud cover, low-level
warm advection ahead of the upper low has warmed surface
temperatures into the low to mid 70s as far north as the IL WI
border. With low-level moisture in place (mid 60s F surface
dewpoints) SPC mesoanalysis shows weak buoyancy (MLCAPE of 500-1000
J/kg) has developed. Continued low-level advection and the arrival
of strong dynamic lift ahead of a jet streak around the base of the
upper low may support some additional destabilization, though the
magnitude remains uncertain. The strong ascent timed with the
arrival of the upper jet will also support very strong shear
profiles across much of the upper Midwest and Great Lakes. Regional
model sounding and VWPs showing 0-6 km shear of 50-60 kts with
backed low-level flow also supporting large low-level hodographs.
Thus, supercells would be the expected storm mode with a risk for
damaging winds, hail, and perhaps a tornado or two.=20

Convective evolution of the ongoing cluster of storms across
north-central IL remains highly uncertain given the lack of stronger
buoyancy and continued development of showers and thunderstorms
ahead of the best kinematic environment northeast of the upper low.
While several weakly rotating embedded elements have been observed
over the last hour in central IL, the lack of greater buoyancy may
result in poor storm organization this afternoon. Strong low-level
shear (0-1 srh 200-300 m2/s2) and forcing will more than likely
support some severe risk, through the evolution maybe very gradual
and limited spatially.=20

Farther downstream across portions of northern lower MI, greater
clearing will likely support better buoyancy through much of the
afternoon. Somewhat removed from the better dynamics of the upper
low, a few strong/severe storms, including a supercell, may develop
with a risk mainly for damaging winds. Despite the better buoyancy,
the stronger flow aloft and better shear are forecast to arrive
later in the diurnal cycle poorly timed with destabilization. As
such, confidence also remains low on severe weather potential.
Trends are being monitored for a possible weather watch though
overall uncertainty remains very high.

..Lyons/Hart.. 10/11/2021

...Please see https://urldefense.com/v3/__http://www.spc.noaa.gov__;!!DZ3fj= g!oNjXWcWyJ0f5aG1gFRRv1P3DzroRKTnJoXnM4yyGk5eZ1FQE_qSz_ImN45CmkiBVkRF019h6$=
for graphic product...

ATTN...WFO...APX...GRR...GRB...LOT...ILX...MKX...DVN...

LAT...LON 43378880 44758718 45408578 45328511 44838441 44068494
42898648 41978722 41308775 40798810 40528883 40708955
41199045 41799017 42178982 43378880=20



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