Mesoscale Discussion on threat of severe storms this Monday afternoon/evening across the Chicago area

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 

   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 12:15 CDT, 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. 

   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. 

   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.

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