Valid 221630Z – 231200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
SOUTHEAST STATES…
…SUMMARY…
A few strong/severe thunderstorms are possible from northern Florida
to central and eastern North Carolina and the Tidewater region of
Virginia this afternoon into early evening.
…North Carolina/southeast Virginia this afternoon/evening…
The potential exists for a few thunderstorms capable of locally
damaging winds, and possibly a tornado, across the region this
afternoon into early evening, although the overall magnitude/extent
of today’s severe risk should remain limited.
Associated with the base of a prominent east-central CONUS longwave
trough, a vorticity maxima and the entrance region of a strong
cyclonically curved polar jet will quickly transition
east-northeastward from the Tennessee Valley and Lower Ohio Valley
to the Appalachians by tonight. Modest cyclogenesis will occur in
the lee of the southern/central Appalachians as a cold front
continues to spread east of the mountains, and generally exits the
coast by early/mid-evening. A warm/moist sector with 70F dew points
will precede the cold front, with a north-northeastward expansion
across coastal portions of South Carolina/North Carolina today.
Even with this moist influx off the Atlantic, the relatively
small-inland warm sector remains mired by broken multi-layer cloud
cover at midday. Its potential persistence will temper lapse rates
and limited upward parcel accelerations even with near 70F surface
dewpoints, which casts uncertainty on how many mature/sustained
surface-based storms will develop and intensify within the warm
sector. That said, strengthening/gradually backing mid-level winds
and resultant 45-55 kt effective shear will be conditionally
supportive of fast-moving line segments capable of damaging winds,
with some supercell/non-zero tornado potential particularly across
eastern North Carolina in proximity to the warm front, should
adequate destabilization and storm development/maturation occur.
…Northeast Florida to South Carolina this afternoon…
A moist air mass and 80 F temperatures are contributing to moderate
destabilization (MLCAPE of 1000-1500 J/kg) early this afternoon
ahead of the east/southeast-moving cold front. Sufficient
instability exists for a potential increase in storm
coverage/intensity, although pre-frontal winds continue to veer with
weakening near-frontal convergence. Isolated strong outflow gusts
may occur with the strongest storms along or ahead of the front this
afternoon, although the overall magnitude/coverage of the risk is
likely to remain limited.
…Central Montana this afternoon/evening…
A midlevel shortwave trough over southern BC will crest the midlevel
ridge and begin to dig southeastward over MT later this
afternoon/evening. Though low-level moisture is limited, cooling
midlevel temperatures and steepening low-midlevel lapse rates will
support at least weak surface-based buoyancy this afternoon across
central MT. Given strong wind profiles, the weak buoyancy and steep
low-level lapse rates could allow some downward momentum transfer
with low-topped convection.
..Guyer/Nauslar.. 10/22/2019
$$