Mesoscale Discussion 1550
   NWS Storm Prediction Center Norman OK
   0746 AM CDT Wed Aug 18 2021

   Areas affected...Parts of the eastern West Virginia
   Panhandle...northern Virginia...central Maryland and adjacent south
   central Pennsylvania

   Concerning...Severe potential...Watch possible 

   Valid 181246Z - 181445Z

   Probability of Watch Issuance...40 percent

   SUMMARY...The development of occasional isolated supercell
   structures with the potential to produce tornadoes (generally
   short-lived) may increase through 11 am-1 pm EDT, mainly west
   through northwest of the District of Columbia into the higher
   terrain around Martinsburg, WV and Hagerstown, MD.  It is not yet
   certain a watch will be needed, but trends are being monitored for
   this possibility.

   DISCUSSION...The remnant surface low of Fred has migrated northward
   into areas south/southeast of Parkersburg, WV during the past few
   hours.  As this has occurred, very moist boundary layer has advected
   northwestward across the Mid Atlantic region.  This includes dew
   points as high as the mid 70s F into the higher terrain centered
   around the Martinsburg, WV vicinity, where low-level convergence is
   becoming focused, and an area of enhanced mid/upper forcing for
   ascent is forecast to overspread through 15-17Z.  Beneath a narrow
   belt of 30+ kt southerly flow around 850 mb, this also appears
   likely to coincide with where clockwise-curved low-level hodographs
   will become maximized, with the terrain contributing to the
   maintenance of backed/southeasterly near surface flow.  As breaks in
   the overcast allow for insolation to contribute to boundary-layer
   destabilization, it appears that the environment may become
   increasingly conducive to supercell structures with the potential to
   produce relatively short-lived tornadoes.

   ..Kerr/Edwards.. 08/18/2021

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

   ATTN...WFO...CTP...LWX...

   LAT...LON   39777813 39787740 39007724 38347764 38807819 39777813