Valid 151300Z – 161200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM SOUTHERN
ARKANSAS TO THE SOUTHERN APPALACHIANS…
Damaging thunderstorms winds may occur this afternoon and evening
from southern Arkansas to the southern Appalachians.
In mid/upper levels, the initially quasi-zonal synoptic pattern
across the CONUS will amplify considerably through the period, in
keeping with three predominant processes:
1. Phasing of a series of shortwaves now located from the Canadian
Prairie Provinces and northern Plains to the Upper Great Lakes,
leading to a strengthening synoptic trough over the area from
southern ON to the Ohio Valley and central Appalachians by 12Z. The
leading shortwave trough — now evident in moisture-channel imagery
over the Dakotas and NE — will move eastward across the upper Great
Lakes and deamplify slightly through 06Z.
2. Amplification of upstream ridging as it shifts from the
central/northern Rockies and Canadian Rockies out across the
adjacent High Plains.
3. Evolution of a broad cyclone over coastal BC as a low now over
extreme east-central BC and a strong shortwave trough offshore from
the Pacific Northwest phase/merge near the end of the period. In
the meantime, the southern perturbation will move obliquely ashore
on the WA/OR coast overnight while weakening, and a broader trough
will develop well offshore from the CA coast, southward from the
coastal BC low.
At the surface, a weak frontal-wave low was analyzed at 11Z near
ICT, with quasistationary frontal zone northeastward across southern
Lower MI and a cold front west-southwestward over the Oklahoma
Panhandle. A reinforcing Arctic front was noted across eastern WY
and NE. These boundaries should merge as the low moves/redevelops
east-northeastward to OH by 00Z, with cold front across southern IN,
central AR, southeastern OK, to west-central TX and west-central NM.
By 12Z, the cold front should extend from the low over western New
England across VA, western NC, northern GA, central MS, and
…Southern AR to southern Appalachians…
Isolated to widely scattered thunderstorms are expected to form
along and ahead of the cold front and move east-northeastward to
eastward across the outlook area, in episodic fashion, through the
overnight hours. Greatest convective coverage should be from
mid-afternoon into early evening. The most vigorous among these
cells may produce damaging gusts.
A corridor of low-level warm advection and moisture transport will
juxtapose with diurnal/diabatic heating from midday through
afternoon. Forecast soundings suggest that the resultant
boundary-layer theta-e should offset weak deep-layer lapse rates
enough to permit around 800-1300 J/kg MLCAPE (locally/briefly
higher) to develop across the Mid-South part of the outlook area,
lessening to around 300-800 J/kg across the southern Appalachians.
This will occur in an environment of generally weak surface winds,
but sufficient mid/upper flow to support around 45-50 kt effective-
shear magnitudes. A few organized multicells, and perhaps a couple
thunderstorms with transient/marginal supercell characteristics,
will be possible within this plume.
Aforementioned weak lapse rates through most of the troposphere, and
related lack of substantial buoyancy, indicate severe hail potential
is too low/conditional to maintain categorical-level probabilities.