SPC AC 291942

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0242 PM CDT Tue Sep 29 2020

   Valid 292000Z - 301200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM EASTERN
   VIRGINIA TO NORTHERN FLORIDA...

   ...SUMMARY...
   Isolated damaging winds will be possible through this evening from
   north Florida across the Carolinas to eastern Virginia. A brief
   tornado is also possible, mainly centered on eastern North Carolina
   tonight.

   ...20z Update...

   Only minor changes have been made to the ongoing outlook, mainly to
   adjust/trim the 10% General Thunder line from NY southward through
   FL based on the current position of the cold front. Elsewhere, 10%
   General Thunder was also removed from parts of the Upper Midwest and
   Great Lakes based on latest observational trends and calibrated HREF
   guidance. While a thunderstorm or two cannot be completely ruled out
   in these areas, coverage of any lighting flashes should be
   minimal/sub-10%.

   ...Southeast and Mid-Atlantic vicinity...

   The Marginal risk area has been trimmed on the western flank in
   NC/SC/GA as convection continues to shift east/northeast along/just
   ahead of the cold front. Otherwise, expect a few storms capable of
   gusty/locally strong winds will continue into the early evening
   period. For more details, reference MCD 1723.

   ..Leitman.. 09/29/2020

   .PREV DISCUSSION... /ISSUED 1111 AM CDT Tue Sep 29 2020/

   ...Delmarva to north FL...
   Within an amplified large-scale trough from the Great Lakes to the
   central Gulf Coast, a primary embedded impulse near the IL/IN border
   will eject east to the northern Appalachians through early
   Wednesday. Cyclogenesis will occur mainly tonight with the primary
   surface cyclone ejecting north from the DE Valley towards VT. A
   secondary lee cyclone over western NC should move towards the VA
   Tidewater, with an attendant cold front to its south that will
   progress east and reach the south Atlantic Coast by evening.

   Upper 60s to low 70s boundary-layer dew points will remain pervasive
   ahead of the cyclones/cold front, but boundary-layer heating will
   likely be more pronounced from eastern VA to north FL. Given the
   relatively meridional mid/upper-level flow regime and cooler
   temperatures aloft lagging well west, high-level lapse rates will be
   poor. This will likely hold MLCAPE to between 750-1500 J/kg north of
   FL. Deep-layer winds will increase with approach of the amplified
   trough, more substantially tonight, with relatively small low-level
   hodograph curvature expected to persist into early evening. The
   modest CAPE with an elongated hodograph due to the degree of
   vertical speed shear should favor weak/transient supercell
   structures within a broken line of convection along/just ahead of
   the surface cold front during the late afternoon and evening. The
   primary concern will be for locally strong gusts capable of
   producing sporadic damaging winds. A brief tornado is also possible,
   mainly centered on eastern NC tonight, with any deep convection that
   can linger amid meager buoyancy as low-level hodographs enlarge.

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