SPC AC 231243

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0743 AM CDT Sun Aug 23 2020

   Valid 231300Z - 241200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
   ACROSS PARTS OF EASTERN NY/NEW ENGLAND...THIS AFTERNOON INTO TONIGHT
   ACROSS PARTS OF THE NORTHERN PLAINS...AND EARLY MONDAY MORNING NEAR
   THE SOUTHEAST LA COAST...

   ...SUMMARY...
   Isolated large hail and damaging winds will be possible this
   afternoon into tonight across parts of the northern Plains. 
   Isolated damaging winds will be possible across eastern New York and
   parts of New England this afternoon.  An isolated tornado may occur
   early Monday morning near the southeast Louisiana coast.

   ...Northern Plains this afternoon into tonight...
   A surface cold front will drift southward across ND today, in the
   wake of a midlevel trough moving over SK/MB.  Given minimal
   larger-scale forcing for ascent and only shallow lift along the
   surface front, the prospects for surface-based convection should be
   limited zone of deep mixing across western SD along a surface
   trough, where gusty outflow winds will be possible.  Otherwise,
   expect elevated thunderstorms to form by this evening over
   central/south central ND, with buoyancy rooted above the 850 mb
   pressure level atop the frontal surface.  This elevated convection
   will then likely persist into tonight while spreading
   east-southeastward, supported by increasing low-level warm
   advection/frontogenesis, and the approach of a subtle shortwave
   trough now over the ID Panhandle.  Some organized/supercell
   structures may occur within the band of elevated storms, which would
   locally enhance the threat for large hail.  Isolated strong/damaging
   gusts may also occur despite the elevated nature of the updrafts,
   given DCAPE approaching 1000 J/kg.

   ...Eastern NY into New England this afternoon...
   A low-amplitude shortwave trough over Lake Huron this morning will
   move over New England tonight.  Prior to then, forcing for ascent
   will be nebulous from NY into New England, within a warm sector
   characterized by boundary-layer dewpoints mainly in the low-mid 60s
   with afternoon surface temperatures of 85-90 F.  Still, daytime
   heating/mixing and terrain influences should be sufficient to
   initiate at least widely scattered storms this afternoon from
   eastern NY eastward across MA/VT/NH and southern ME.  Midlevel lapse
   rates near 6.5 C/km and weak vertical shear are not supportive of
   organized severe storms (vertical shear will be a little stronger on
   the north edge of the warm sector).  However, steep low-level lapse
   rates, SBCAPE of 1000-1500 J/kg, and DCAPE approaching 1000 J/kg
   will favor multicell/pulse storms capable of producing isolated
   damaging downburst winds for a few hours this afternoon before
   ending this evening.  

   ...Southeast LA coast early Monday...
   Tropical Storm Marco is forecast to move north-northwestward and
   make landfall along the southeast LA coast by midday Monday (early
   Day 2 period) as a category 1 hurricane (see latest NHC advisories
   for details/forecast reasoning).  The storm is located to the east
   of a mid-upper trough over the northwest Gulf of Mexico, and the
   convective structure is quite asymmetric as a result of
   south-southwesterly shear aloft.  The small size of Marco suggests
   any zone favorable for supercells will likewise remain small and
   will not approach the coast until late in the period, with expansion
   of the favorable envelope unlikely given only modest deepening of
   the storm.  Thus, will maintain low tornado probabilities for late
   in the period near the mouth of the MS River.

   ..Thompson/Leitman.. 08/23/2020

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