ACUS01 KWNS 271252
SWODY1
SPC AC 271251

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0751 AM CDT Fri Sep 27 2024

Valid 271300Z - 281200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM
EASTERNMOST SOUTH CAROLINA OVER PORTIONS OF SOUTHERN/EASTERN NORTH CAROLINA...

...SUMMARY...
Tornadoes will be most likely through midday and remain possible
into afternoon across parts of the Carolinas and Virginia.

...Synopsis...
In mid/upper levels, a zonal to west-southwesterly belt of flow will
persist from the northwestern CONUS and western Canada into central
Canada, well-removed from a cut-off, cold-core low now evident in
moisture-channel imagery over northern MS.  This low is expected to
pivot slowly northeastward into middle TN by 00Z, then wobble
erratically over that area and/or south-central KY through the night
as it absorbs the remnant vorticity max from what is now Tropical
Storm Helene.

...SC/NC/VA...
The path of center of Tropical Storm Helene finally has turned
northward.  See SPC Tornado watch 688 and associated mesoscale
discussions for near-term coverage of the tornado threat over the
region.  Refer to NHC advisories for the latest track/intensity
guidance on Helene.

The NHC forecast shows that the center should bend northwestward
across southwestern NC, eastern TN and into southern KY through
tonight, weakening greatly and becoming more closely conjoined with
the mid/upper cyclone.  For several more hours, into at least
midday, a broad area of strong low-level flow above the surface, and
related large low-level shear/hodographs, should extend well east
and northeast of center across the outlook area, superimposed over:

1.  Relatively maximized low-level convergence, in an arc initially
located over eastern SC into southern NC, preceded by well-backed
surface winds, and forecast to shift northward to northeastward
today;

2.  A richly moist, high-theta-e and weak-MLCINH boundary layer
advecting inland from the Atlantic and characterized by surface
dewpoints commonly in the 70s F.

These conditions will yield a favorable environment for tornadoes
from any supercells that can cross it, whether within or ahead of
the main spiral/convergence band.  The most probable area for
several supercells to interact with this favorable airmass is over
the 10%/"enhanced" tornado-risk area, with potential decreasing
westward and northward from there due to weaker instability.

Although the supportive low-level airmass will continue to shift
northeastward over eastern NC and south-central/southeastern VA this
afternoon, tornado potential will diminish with time from
midafternoon into early evening.  This will occur as convection
decreases in coverage and intensity across the area due to a
combination of weakening lift and shear, as the remnants of Helene
decay rapidly and back well westward away from the area.  This
process will decouple the remaining stronger low-level flow and
isallobaric forcing away from the most-favorable boundary layer near
the coast, where hodographs should shrink by the hour this afternoon
into evening.

..Edwards/Broyles.. 09/27/2024

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