Valid 131300Z – 141200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE CAROLINAS LATE THIS AFTERNOON INTO THIS EVENING…
…SUMMARY…
Thunderstorms may produce strong to marginally severe gusts over
parts of the Carolinas late this afternoon into this evening.
…Synopsis…
The upper-air pattern will become more progressive across the CONUS
this period, though the dominant feature will remain a large,
occluded, stacked cyclone initially centered over northwestern Lake
Superior. The associated mid/upper low will move slowly east-
northeastward across northern ON through 12Z. In the cyclonic flow
to its south, a strongly positively tilted, elongated, but weak
shortwave trough was apparent in moisture-channel imagery from the
Ozarks across OK to the TX Panhandle. This perturbation should move
across the Mid-South and TN Valley region today, crossing the
southern Appalachians and parts of the Carolinas overnight.
Upstream troughs initially were located over coastal/northwestern BC
and offshore from WA/CA/OR. These perturbations will move eastward
nearly in phase with each other, reaching the northern Rockies,
northwestern NV, and central/southern CA by 12Z.
At the surface, 11Z analysis showed a mostly stationary frontal zone
from south-central PA across western portions of VA/Carolinas,
southeastern AL, western FL Panhandle, to a weak frontal-wave low
offshore from BRO. The Carolinas portion of the baroclinic zone
should undergo slow net eastward movement later today and tonight,
aided by convective processes, while the GA/AL part remains
quasistationary, and the northwest Gulf segment retreats slowly
northward.
…Carolinas…
An initial round of non-severe, elevated convection, containing
scattered showers and isolated thunderstorms, should continue to
move northeastward across a corridor from northern GA across the
western Carolinas through midday. This activity will reinforce the
baroclinic zone with both outflow and differential-heating
processes. The resultant boundary, along with a weak surface
trough/confluence line to its east over the Piedmont, should focus
another round of convection this afternoon and evening, in the form
of widely scattered to scattered thunderstorms. Activity should
form as weak large-scale lift related to the shortwave trough
overlies a diabatically destabilized near-surface layer, with
minimal MLCINH.
The prospective inflow air mass will be well-heated, with peak
afternoon surface temperatures in the 80s F and dew points generally
in the lower 60s. This will offset modest mid/upper-level lapse
rates enough to yield a corridor of 500-1000 J/kg MLCAPE, atop a
well-mixed subcloud layer conducive to strong downdrafts. Weak but
backed surface winds east of the trough will aid low-level shear,
and some forecast hodographs suggest 100-200 J/kg effective SRH —
marginal for supercells. Messy, upscale-aggregating storm modes may
occur rather quickly, and lack of stronger deep shear should temper
a greater threat. Still, but a few cells may organize enough to
pose a risk of damaging gusts near severe limits late this afternoon
into early evening.
..Edwards/Goss.. 10/13/2019
$$