Mesoscale Discussion 1673
   NWS Storm Prediction Center Norman OK
   0206 PM CDT Thu Sep 03 2020

   Areas affected...the Mid-Atlantic

   Concerning...Severe potential...Tornado Watch likely 

   Valid 031906Z - 032100Z

   Probability of Watch Issuance...95 percent

   SUMMARY...A tornado watch will be needed this afternoon as a
   damaging wind and tornado threat increases.

   DISCUSSION...A broad area of shower activity extends from the Ohio
   Valley into Pennsylvania and New York amid a belt of strong
   mid-level flow. Areas south and east of this activity have slowly
   destabilized through the day with temperatures now in the upper 80s
   to low 90s in northern Virginia, eastern Maryland, and Delaware. In
   addition, the boundary layer is very moist with dewpoints in the mid
   to upper 70s. This has yielded moderate MLCAPE ranging from 1000 to
   2000 J/kg across the region despite very weak mid-level lapse rates
   (5 C/km per 18Z IAD RAOB). Expect at least a few strong storms to
   develop in the eastern periphery of the cloud shield in the next 1-2
   hours as destabilization continues and deep-layer ascent increases
   ahead of a shortwave moving out of West Virginia. 

   Once storms develop, they will likely become supercellular, at least
   initially, given 40-45 kts of effective shear (per KLWX VWP and 18Z
   IAD RAOB). 18Z IAD RAOB showed relatively weak flow in the lowest
   1.5 km, but upstream VWP from KRLX shows strong (40 kt) flow at 1km.
   Once this overspreads northern Virginia, Maryland, and Pennsylvania
   in a few hours, the damaging wind threat will increase and low-level
   shear will increase which should increase the tornado threat. The
   best tornado threat will likely be across northern Virginia,
   southern Maryland, and far southern Pennsylvania where there has
   been less mixing and surface winds remain southerly or
   south-southeasterly.  

   A tornado watch will likely be needed in the next hour or two as
   storm coverage/intensity increases.

   ..Bentley/Grams.. 09/03/2020

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

   ATTN...WFO...PHI...AKQ...CTP...LWX...RNK...PBZ...RLX...

   LAT...LON   38357996 39487908 40307746 40087507 39237465 38407489
               37397533 37107609 37187742 37677938 38357996