SPC AC 281959

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0259 PM CDT Sun Mar 28 2021

   Valid 282000Z - 291200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS SOUTHEASTERN
   VIRGINIA...

   ...SUMMARY...
   Scattered severe thunderstorms capable of wind damage and a tornado
   or two are expected across the Carolinas and Mid-Atlantic States
   this afternoon into the evening. Isolated wind and hail will be
   possible in the Southeast for the next few hours.

   ...Southeast Virginia into North Carolina...
   Storms have developed along a pre-frontal trough and will continue
   to progress eastward into eastern NC and parts of southeast VA.
   Additional development is expected to occur along the cold front,
   but dewpoints mixing into the low 60s/upper 50s F and early-day
   cloud cover has limited surface heating and hindered surface-based
   instability. The Enhanced risk area has been adjusted to where the
   best overlap of buoyancy and mid-level winds still exists.

   ...Southeast...
   Convection along the front continues to progress southeastward.
   Probabilities have been adjusted where the airmass has already been
   overturned. The expectation for the ongoing convection is that it
   will generally weaken as it moves east into a less unstable airmass.
   However, isolated wind damage and marginally severe hail will be
   possible for the next few hours.

   ..Wendt.. 03/28/2021

   .PREV DISCUSSION... /ISSUED 1125 AM CDT Sun Mar 28 2021/

   ...Synopsis...
   A band of rain and embedded thunderstorms persists along and just
   ahead of a surface cold front from AL/GA across the Piedmont of the
   Carolinas/VA, in advance of a midlevel shortwave trough over the
   OH/TN Valley regions.  The primary associated surface cyclone will
   continue to move northeastward near the Saint Lawrence Valley, while
   the cold front likewise progresses eastward to the Mid-Atlantic and
   southeast Atlantic coasts this evening into early tonight.  There
   will be a window of opportunity for severe storms in the zone of
   destabilization preceding the cold front/rain band this afternoon
   across the Carolinas into the Chesapeake Bay region.

   ...NC into VA this afternoon/evening...
   Cloud breaks are occurring across central/eastern NC and
   southeastern VA in advance of the frontal rain band and thicker
   clouds.  Temperatures are warming into the 70s with boundary-layer
   dewpoints in the mid 60s, which is driving MLCAPE of 500-1000 J/kg
   and some deepening cumulus near the SC/NC border.  Deep-layer flow
   and vertical shear will remain strong over the area this afternoon,
   per VWPs with 50-60 kt flow within the lowest 1-3 thousand feet
   above the ground.  The primary uncertainty will if there can be any
   substantial destabilization behind the lead cloud/rain band now
   moving across NC/VA.  Satellite imagery suggests that the clouds are
   rather thick and will be sufficient to limit surface heating through
   the afternoon to the north of southeastern VA.  In that scenario,
   the stronger storms would likely have to evolve from convection
   along or just ahead of the pre-frontal band which will cross
   southeastern VA and northeastern NC this afternoon, which is where
   the confidence in damaging convective gusts (and perhaps a tornado)
   is a bit higher.

   ...Mid-Atlantic into southern NY this afternoon/evening...
   As previously mentioned, the degree of destabilization is more in
   question with northward extent across VA/MD/DE toward eastern PA and
   NJ.  Surface temperatures in the 65-70 F range will be needed to
   much in the way of SBCAPE and a damaging-wind threat with a forced
   band of convection along the cold front.  Will maintain the
   character of the previous outlook, but the northward extent of the
   severe threat will depend on some cloud breaks this afternoon.  The
   Marginal area has been expanding some into southern NY to reflect
   the potential for just enough low-level warming/moistening to
   support minimal SBCAPE and the low chance for strong/damaging gusts
   with a shallow convective band along the cold front.

   ...SC/GA this afternoon...
   The convection from overnight persists in a band just ahead of the
   surface cold front.  Low-level forcing for ascent will be maintained
   along the convective band today, and in tandem with surface heating
   and modest low-level moisture/buoyancy could support a continued
   threat for at least isolated damaging gusts through the afternoon.

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