SPC AC 170558

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1258 AM CDT Fri Jul 17 2020

   Valid 181200Z - 191200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE UPPER MIDWEST INTO THE GREAT LAKES VICINITY...

   ...SUMMARY...
   Scattered severe storms should occur Saturday across portions of the
   Upper Midwest and Great Lakes vicinity. Damaging winds and large
   hail appear to be the main hazards with these storms, some of which
   could be significant.

   ...Upper Midwest/Great Lakes into the Northern/Central Plains...
   A large, well-organized MCS occurring in the Day 1/early Saturday
   morning time frame should be weakening by the start of the Day 2
   period somewhere in the vicinity of WI/Upper MI. This convection may
   still pose an isolated threat for damaging winds until it dissipates
   later Saturday morning. In the wake of this activity, strong airmass
   recovery is expected by Saturday afternoon across the Upper Midwest
   along/south of a cold front. An upper trough initially over the
   Canadian Prairie Provinces and northern Plains is forecast to shift
   eastward across the Upper Midwest and upper Great Lakes through the
   period. Surface dewpoints in the low to mid 70s across the warm
   sector in MN/WI, robust diurnal heating, and steep mid-level lapse
   rates will all support very strong to potentially extreme
   instability (MLCAPE 3000-5000+ J/kg) by late Saturday afternoon.

   Strong capping across the warm sector will probably limit convective
   development through much of the afternoon. Still, at least scattered
   storms should initiate along the front across parts of MN and
   perhaps southeastern SD by early Saturday evening, as the
   large-scale ascent associated with the upper trough begins to
   overspread this region and erode the cap. 30-40 kt of effective bulk
   shear will likely support organized storms, including the potential
   for some supercells initially. Both large hail and damaging winds
   may occur. Given the large degree of instability forecast, isolated
   very large hail (2+ inch diameter) is a possibility.

   A fairly rapid transition to one or more bowing clusters appears
   probable Saturday evening/night across eastern MN and WI, with
   damaging winds becoming the primary threat. Have included a small
   significant wind area across parts of east-central MN into western
   WI, where high-end severe gusts appear possible, based on the
   forecast instability/shear and likely linear storm mode. A
   severe/damaging wind threat may continue overnight across much of WI
   with this MCS before eventually weakening over MI late in the
   period. Greater severe hail and/or wind probabilities may need to be
   introduced once concerns about capping and placement of greatest
   storm coverage are reduced.

   ...North Dakota...
   Although instability will be more limited across ND behind the front
   Saturday afternoon, it should still be sufficient in tandem with
   30-35 kt of effective bulk shear to support some strong to locally
   severe storms. Both isolated large hail and strong/gusty winds
   appear possible with any storms that can form in this regime through
   early Saturday evening.

   ..Gleason.. 07/17/2020

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