SPC AC 210525

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1225 AM CDT Wed Apr 21 2021

   Valid 211200Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
   THE MID-ATLANTIC INTO NEW ENGLAND...

   ...SUMMARY...
   Scattered strong to severe storms capable of producing
   gusty/damaging winds will be possible across parts of the
   Mid-Atlantic into portions of New England today.

   ...Synopsis...
   An upper-level trough will shift eastward from the Great Lakes to
   the Northeast through the period and amplify. As it does, a surface
   low, located in the Ohio Valley this morning will deepen and move
   northeastward from somewhere near southern Pennsylvania to southeast
   Maine during the day. A cold front will extend southward from this
   surface low and will provide the focus for scattered strong to
   severe storms from the late morning to early evening hours. 

   ...Mid-Atlantic to Northeast...
   Mostly clear skies are anticipated ahead of the cold front as it
   moves across the Appalachian mountains during the morning hours.
   This will allow surface temperatures to warm into the 60s which,
   given the cool temperatures aloft, will be sufficient to erode
   inhibition across the region. Therefore, expect storms to form along
   the cold front during the late morning. The severe threat may start
   to increase as early as 15Z given the limited heating needed for
   surface based storm development and the strong forcing for ascent.
   Forecast soundings show a well mixed boundary layer ahead of the
   cold front with moderately steep low and mid-level lapse rates. This
   will provide a favorable environment for damaging wind gusts,
   especially given the 30 to 40 knot low-level jet. The cool
   temperatures aloft and strong shear may support some marginally
   severe hail, but wind will likely be the primary threat. While most
   CAM guidance shows storms becoming better organized near the coast
   in the eastern Virginia to Delmarva region, the threat could begin a
   few counties further inland given the potential for late morning
   initiation. HREF mean temperature fields show temperatures
   increasing into the low 70s ahead of this front which would yield
   MLCAPE around 750 J/kg even if surface dewpoints mix out ahead of
   the front. Therefore, would expect storm activity to pose a severe
   threat given the unstable and moderately sheared (40-45 kts
   effective shear) environment. Farther north, expect storm activity
   to reach maturity by early afternoon with a line of storms moving
   from northeast Pennsylvania across southeast New York, New Jersey,
   and into portions of New England. This region, in close proximity to
   the surface low, may have the best chance for a more organized
   severe threat. Storm mode will favor wind damage as the primary
   threat, but strong speed shear and some weak low-level veering flow
   supports at least some threat of a brief tornado.

   ..Bentley/Jewell.. 04/21/2021

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