Mesoscale Discussion 1669
   NWS Storm Prediction Center Norman OK
   0237 PM CDT Wed Sep 02 2020

   Areas affected...southeast Pennsylvania...northern
   Virginia...northern Maryland and western Delaware

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

   Valid 021937Z - 022030Z

   Probability of Watch Issuance...80 percent

   SUMMARY...Thunderstorms are expected to develop from northern
   Virginia into northern Maryland and southeast Pennsylvania by
   21-22Z. A few supercells capable of damaging wind gusts and isolated
   tornadoes will be the main threats. A tornado watch will likely be
   needed prior to 21Z.

   DISCUSSION...As of mid afternoon a progressive vorticity maximum was
   situated over southwestern PA and will continue east into the Middle
   Atlantic region during the late afternoon and evening. A band of
   low-topped convection has already developed in association with this
   feature extending into northern WV where instability is limited.
   Farther downstream, a lee trough resides just east of the central
   and southern Appalachian mountains. East of this feature
   temperatures have warmed to the upper 80s F with dewpoints in the
   low to mid 70s contributing to 1500-2000 J/kg MLCAPE despite very
   weak mid-level lapse rates resulting from warm temperatures aloft.
   As the corridor of deeper ascent attending the shortwave trough
   continues east and begins to overtake the western edge of the moist
   warm sector, additional storms will likely develop from northern VA
   into southeast PA. Winds in the 850-500 mb layer will undergo a
   modest increase with approach of the impulse, resulting in 35-40 kt
   effective bulk shear and 100-200 m2/s2 storm relative helicity. This
   environment should be sufficient for some organized storm structures
   including a few supercells capable of locally strong to damaging
   gusts and a couple of tornadoes through early evening.

   ..Dial/Grams.. 09/02/2020

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

   ATTN...WFO...PHI...AKQ...CTP...LWX...

   LAT...LON   38267785 39077790 40117770 40577673 40237576 39177567
               38457616 38047703 38267785