SPC AC 281629
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1129 AM CDT Tue Sep 28 2021
Valid 281630Z - 291200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS
AFTERNOON/EVENING ACROSS THE MID-ATLANTIC...
Strong to severe gusts and hail are possible this afternoon and
evening across parts of the Mid-Atlantic region.
...Mid-Atlantic this afternoon/evening...
An initial midlevel shortwave trough near the PA/MD border will move
east-southeastward to the Mid-Atlantic coastline by mid afternoon,
as part of larger-scale trough amplification over the Northeast (in
response to upstream height rises over the northern Plains). The
timing of the lead wave is ahead of the primary diurnal cycle, and
additional storm development late this afternoon/evening will likely
depend on lift along the cold front which will move southeastward
from PA into the Mid-Atlantic by this evening. In the wake of
morning convection with the lead shortwave trough, gradual
moistening and surface heating in cloud breaks ahead of the surface
front will contribute to MLCAPE of 1000-1500 J/kg by late afternoon
from VA into the Delmarva/southern NJ. The moderate buoyancy and
effective bulk shear of 25-35 kt will support a mix of multicell
clusters and isolated supercells. The strongest storms will be
capable of producing damaging gusts given some storm organization
with DCAPE near 750 J/kg and 30-40 kt midlevel flow for downward
momentum transfer. Midlevel lapse rates near 8 C/km and relatively
cool midlevel temperatures will also support a threat for occasional
...Southern Plains this afternoon/evening...
A dryline will establish this afternoon along the lee trough from
near the western OK border to the Edwards Plateau, east of a weak
midlevel trough over the southern Rockies. Boundary-layer dewpoints
increasing into the 60s from south-to-north and strong surface
heating/mixing will support thunderstorm development along the
dryline this afternoon/evening from TX into southwest OK. MLCAPE of
1500-2000 J/kg and effective bulk shear up to 30 kt will favor a mix
of multicell clusters and transient supercell structures, with an
attendant threat for isolated large hail and outflow gusts of 50-60
mph given steep low-level lapse rates and DCAPE .
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