Mesoscale Discussion 1521
   NWS Storm Prediction Center Norman OK
   1254 PM CDT Fri Aug 18 2017

   Areas affected...Southeast NY...Eastern
   PA...NJ...DE...MD...Northeast VA

   Concerning...Severe potential...Watch possible 

   Valid 181754Z - 182000Z

   Probability of Watch Issuance...60 percent

   SUMMARY...Thunderstorm coverage is expected to increase over the
   next few hours. Some strong to severe storms are anticipated, with
   damaging wind gusts as the primary severe threat.

   DISCUSSION...Latest surface analysis indicates a broadly confluent
   frontal zone extending from the Adirondack mountains southwestward
   through eastern PA and western VA. Visible satellite shows extensive
   cloud cover across central and eastern NY/PA while the pre-frontal
   areas of southeast PA, central MD and central/eastern VA remain
   relatively cloud free. Temperatures have warmed into the low 90s in
   these cloud free areas, which, in conjunction with a very moist
   low-level airmass, supports MLCAPE greater around 2000-3000 J/kg. 
   Cooler surface temperatures exist farther north with a corresponding
   drop in instability. However, better forcing for ascent is expected
   farther north as the shortwave trough continues northeastward
   through northern New England. Stronger mid-level flow and, as a
   result, stronger bulk shear, are also anticipated across northern
   portions of the discussion area (i.e. southeast NY, northeast PA,
   and northern NJ).

   Overall expectation is for storm coverage to gradually increase over
   the next few hours as forcing for ascent associated both with the
   shortwave trough and approaching cold front interact with the very
   unstable airmass across much of the region. Wet microbursts capable
   of damaging wind gusts are possible within the strongest updrafts
   with some stronger wind gusts also possible along the leading edge
   of any convective lines or bowing segments. Additionally, given the
   enhanced low to mid-level flow and very moist airmass, a brief
   tornado is also possible. Trends across the region will be monitored
   for watch issuance.

   ..Mosier/Goss.. 08/18/2017

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

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

   LAT...LON   41157643 42397483 42387377 41527335 40007438 38207660
               38357887 41157643