SPC AC 231942
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0242 PM CDT Mon Oct 23 2017
Valid 232000Z - 241200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE CAROLINAS
INTO A PORTION OF VIRGINIA...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM A PORTION
OF THE SOUTHEAST STATES INTO THE MID ATLANTIC...
Isolated strong to severe storms should persist over the Carolinas
this afternoon into the evening, but also spreading into the
Mid-Atlantic during the evening. The main threats will be damaging
wind gusts and a few tornadoes.
...Carolinas through Mid Atlantic...
Only minor adjustments have been made to previous forecast. Bands of
low topped storms will persist this afternoon and evening within
corridor of deep ascent associated with a progressive, negative-tilt
shortwave trough. Latest objective analysis shows MLCAPE from near
300 J/kg over west central NC to 500 J/kg across SC. Storms are
embedded within strong winds aloft and vertical shear profiles with
large hodographs (400-600 m2/s2 0-1 km storm relative helicity).
Occasional organized storms including a few supercells and bowing
segments will persist as activity gradually spreads east and
northeast into the evening.
.PREV DISCUSSION... /ISSUED 1104 AM CDT Mon Oct 23 2017/
...Carolinas and vicinity through tonight...
A midlevel shortwave trough over the TN Valley this morning will
eject northeastward to the southern Appalachians this
afternoon/evening, and continue north-northeastward to the central
Appalachians and upper OH Valley tonight, in response to an
amplifying upstream trough over the MS Valley. An associated
surface cyclone in central IN will move north-northeastward and
deepen across Lower MI and the upper Great Lakes by tonight, as a
trailing cold front progresses eastward to the southeast Atlantic
coast and Mid-Atlantic region. Ahead of the cold front, the moist
warm sector will spread northward across NC today and VA by tonight,
with the more unstable warm sector limited to areas east-southeast
of a secondary low across western NC this afternoon/evening.
Regional 12z soundings and radar/satellite imagery show a widespread
band of clouds/rainfall preceding the cold front, where surface
heating will be limited and lapse rates will remain rather poor.
Still, the rich low-level moisture (boundary-layer dewpoints of
68-72 F) will contribute to weak surface-based buoyancy into the
western Carolinas (SBCAPE 250-500 J/kg), and somewhat greater
buoyancy into SC (1000-1500 J/kg). At the same time, low-level
shear will become quite strong (0-1 km SRH of 250-500 m2/s2), which
will support strengthening of convection in a band with some breaks
along the front this afternoon from eastern GA into the western and
central Carolinas. Embedded supercells and LEWP structures will be
capable of producing a few tornadoes and damaging gusts, likely
beginning by 18-20z in the vicinity of the Savannah River, and then
spreading eastward-northeastward through this evening.
Weaker low-level lapse rates and near-surface-based buoyancy will
tend to limit the severe-storm threat with northward extent tonight
into VA. Still, a line of storms in the presence of very strong
low-midlevel flow/shear and moist profiles will pose some threat for
isolated damaging winds and a tornado or two.
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