SPC AC 271728

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1228 PM CDT Tue Jul 27 2021

   Valid 281200Z - 291200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS EAST-CENTRAL
   MINNESOTA INTO CENTRAL AND SOUTHERN WISCONSIN...

   ...SUMMARY...
   Thunderstorms associated with severe wind gusts, large hail to very
   large hail, and a couple tornadoes will be possible Wednesday
   afternoon into Wednesday night across parts of the Upper Midwest and
   extending into the southern Great Lakes region.

   ...Upper Midwest into the southern Great Lakes...
   A cluster or two of showers/thunderstorms may be ongoing Wednesday
   morning from northern MN southward into perhaps the IA/WI border
   vicinity within a warm advection regime.  This activity will likely
   dissipate by the late morning.  A subtle mid-level disturbance,
   cresting the mid-level ridge centered over the central Great Plains,
   will move eastward into western MN by late afternoon.  A warm
   frontal zone will become draped from the southwest Great Lakes
   northwestward through WI into northern MN.  Strong heating and a
   very moist boundary layer will lead to a very unstable airmass
   developing by mid afternoon over the Upper Midwest (2500-4000 J/kg
   MLCAPE, locally higher).  Suite of convection-allowing models show
   an initial cluster of supercells will probably develop during the
   late afternoon into the early evening.  Large to very large hail,
   severe gusts, and a couple of tornadoes are possible.  Enlarged
   hodographs near the warm front will favor strong mesocyclones but it
   is unclear how much of a tornado risk will ultimately develop due to
   storm-scale interactions.  Nonetheless, an initial cluster of storms
   will likely grow upscale into a forward-propagating severe MCS over
   the western Great Lakes during the evening with localized intense
   gusts (75+ mph) possible with HP supercell to QLCS transitions or
   with the more pronounced bowing segments.  It seems plausible this
   MCS will move across southern Lake Michigan into the southern Great
   Lakes during the overnight.  The coverage/intensity of gusts will
   probably lessen with time as the squall line encounters weaker
   buoyancy during the pre-dawn hours.  

   ...Central Appalachians/Mid-Atlantic...
   An upper-level trough is forecast to move southeastward across the
   Northeast on Wednesday.  A moist airmass will likely be in place
   across parts of the central Appalachians, where moderate instability
   is forecast to develop by afternoon.  Although large-scale forcing
   will be weak, isolated storms are possible.  In addition to the
   instability, deep-layer shear will be strong enough to contribute to
   a marginal severe threat.  A few marginally severe gusts will be
   possible in the mid to late afternoon as low-level lapse rates
   steepen.

   ..Smith.. 07/27/2021

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