Day 1 Severe Weather Outlook

Valid 021300Z – 031200Z


Strong to severe thunderstorms may impact parts of the Mississippi
Delta and central Gulf Coast States, offering damaging wind gusts
and a tornado threat.

In mid/upper levels, a high-amplitude, positively tilted synoptic
trough is taking shape from MB across the northern Plains, central/
southern Rockies, to west-central MX. A series of perturbations
will traverse the associated cyclonic-flow field today, including a
basal shortwave trough now evident in moisture-channel imagery
between Los Cabos and mainland west-central MX. The latter
perturbation will deamplify and accelerate northeastward in broadly
confluent flow, its 500-mb vorticity axis reaching near a DFW-LRD
line by 00Z, then losing definition over the lower Ohio Valley
region by 12Z. Meanwhile, a couple of smaller perturbations
initially over AZ and the Great Basin will phase into a better-
defined shortwave trough, reaching west-central TX and north-central
MX by the end of the period.

At the surface, 11Z analysis showed a cold front extending from a
low over northwestern ON across southeastern MN, eastern NE, central
KS, to the southeastern TX Panhandle and southeastern NM. By 00Z,
the cold front should extend from eastern Upper MI across northern
MO, to a weak frontal-wave low over north-central TX near the Red
River, west-southwestward across west-central far west TX. The wave
low should move slowly eastward into northeast TX/Arklatex region
overnight, with the southern segment of the front reaching south-
central TX and northern Coahuila. To its south, a weak baroclinic
zone, accompanied by a surface trough, will shift eastward across
the TX coastal plain and western LA after being quasistationary for
much of the day.

…MS Delta/central Gulf Coast States…
Scattered thunderstorms are expected to form today well ahead of the
main cold front and near the weaker front/trough, in a band from
southwestern/south-central LA northeastward across western/northern
MS. The middle/southern parts of this band will pose a threat for
damaging gusts, and a couple tornadoes also are possible, as it
moves eastward to northeastward across the outlook area into this

Supporting destabilization today will be gradual, and progressive
from southwest to northeast across the outlook areas, in the wake of
many hours of antecedent clouds/precip inland. With abundant cloud
cover tempering direct diabatic heating, the bulk of the boundary-
layer instability will arise from theta-e advection. Mesoscale
trends, modified RAOBs and model soundings suggest weakly inhibited
MLCAPE of at least 500 J/kg will spread across southern LA and into
southwestern/south-central MS through midday, reaching western AL
mid-late afternoon. Peak preconvective MLCAPE may reach around 1000
J/kg MLCAPE near an axis from HUM to south of MEI.

Kinematically, deep shear will slowly strengthen as well, with
effective-shear magnitudes potentially reaching the 65-75-kt range.
This will support storm organization and potential for storm-scale
rotation — be it in the form of embedded supercells or inflection/
LEWP structures attached to small bows. However, deep-shear and
mean-wind vectors will be aligned strongly parallel to the zone of
convective lift, indicating a tendency for messy/quasi-linear storm
mode. Despite the weakness of MLCINH in forecast soundings,
potential for relatively discrete, deep convective development ahead
of that boundary appears isolated, but possible, rendering the
unconditional tornado risk more than marginal. The severe threat
overall should diminish with time overnight.

..Edwards/Jewell.. 01/02/2020


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