Mesoscale Discussion 1323
   NWS Storm Prediction Center Norman OK
   0932 AM CDT Wed Jul 21 2021

   Areas affected...Parts of northern Virginia...the District of
   Columbia...central/eastern Maryland...Delaware...southeastern
   Pennsylvania...New Jersey...southeastern New York including Long
   Island...Connecticut...Rhode Island and southeastern Massachusetts

   Concerning...Severe potential...Watch likely 

   Valid 211432Z - 211530Z

   Probability of Watch Issuance...80 percent

   SUMMARY...Scattered intensifying thunderstorm development is
   possible through Noon-2 PM EDT, posing a risk for at least
   marginally severe hail and locally strong surface gusts.  With
   storms likely to become more widespread thereafter, a severe weather
   watch probably will be issued.

   DISCUSSION...A slowly southward advancing surface cold front has
   reached southeastern New England and the northern Mid Atlantic
   coast, with large-scale ascent likely to increase in a corridor
   along and ahead of it through early afternoon.  This appears likely
   to be aided by forcing ahead of substantive mid-level short wave
   trough gradually turning east of the lower Great Lakes region.

   The strengthening lift will coincide with a narrow pre-frontal
   corridor of rapidly steepening near-surface lapse rates aided by
   daytime heating.  Lower/mid tropospheric lapse rates are also
   relatively steep and, despite seasonably modest low-level moisture,
   it appears that this will contribute to mixed-layer CAPE in excess
   of 1000 J/kg (up to around 2000 J/kg).

   Although low-level wind fields are generally weak, 30-50+ kt
   cyclonic flow in the 500-300 mb layer is contributing to sufficient
   shear to support organizing clusters of storms.  Isolated supercells
   are possible initially, and these storms may pose a risk for
   (mostly) marginally severe hail, before the risk for potentially
   damaging wind gusts become more prominent later this afternoon.

   Areas of deepening convective development are already evident along
   this corridor, and a severe weather watch will probably be issued
   within the next hour or two.

   ..Kerr/Hart.. 07/21/2021

   ...Please see www.spc.noaa.gov for graphic product...

   ATTN...WFO...BOX...OKX...ALY...PHI...AKQ...CTP...LWX...

   LAT...LON   41397389 42227140 41367090 40097343 38967544 38087720
               38937754 39757718 40267570 41397389