Day 1 Severe Weather Outlook

Valid 181300Z – 191200Z

…NO SEVERE THUNDERSTORM AREAS FORECAST…

…SUMMARY…
Isolated thunderstorms are possible from south Texas across the
central Gulf Coast States. Organized severe weather is not
expected.

…Synopsis…
The dominant upper-air feature for this period will be a pronounced
synoptic-scale trough, now evident in moisture-channel imagery from
a weak mid/upper low over MN to eastern OK. An initially separate,
strong shortwave trough — currently located from southern SK across
western ND and central SD — will phase with the synoptic trough
during the day, as the MN low crosses portions of Lake Superior.
Meanwhile, a basal shortwave trough — now over western MO and
northeastern OK, will eject northeastward and weaken. By 12Z, the
shortwave-reconfigured synoptic trough should extend from southern
QC across Lower MI to IL.

At the surface, an occluded cyclone was analyzed over the upper
Mississippi Valley at 11Z. Two cold fronts were drawn emanating
from its southern sector — the first across southeastern IL to the
Arklatex region, becoming quasistationary near a TXK-AUS-LRD line.
The trailing/reinforcing front extended across northwestern IL,
southwestern MO, south-central OK, and far west TX. A marine/warm
front was analyzed from between EYW and MUHA (Havana) northwestward
across the eastern Gulf to southeastern LA.

By 00Z, the low should move to Lake Huron, with the cold fronts
combining and reaching middle TN, central portions of MS/LA, the TX
Gulf shelf waters, to near BRO. By 12Z, the front should reach the
Carolinas, southern GA, and the central/southwestern Gulf. The
warm/marine front is expected to move slowly east-northeastward
across the central Gulf Coast and adjacent waters, reaching the
western/central FL Panhandle this evening before being overtaken by
the cold front.

…Gulf Coast region…
Isolated to widely scattered thunderstorms may develop near the
front today and tonight. Organized severe is not expected, given
the lack of upper-air support and modest flow/middle-level flow,
with the basal shortwave trough weakening and increasingly distant
from the region. Still, a few strong cells may develop, most
notably over deep south TX an near the warm front across the
northern Gulf near the MS/AL/western FL Panhandle coasts.

Over the lowest part of the Rio Grande Valley and Laguna Madre/Padre
Island region of deep south TX, strong diurnal heating is possible
just ahead of the front. The prefrontal boundary layer will remain
characterized by rich moisture, with surface dew points upper 60s to
low 70s F, and near-record PW around 1.75-2 inches. These
conditions will offset modest mid/upper-level lapse rates enough to
generate around 1500 J/kg deep MLCAPE, locally/briefly near 2000
J/kg. Gusts approaching severe limits may occur in the most
intense/water-loaded downdrafts, regardless of the weak low-level
winds/shear. However, severe potential appears too isolated,
conditional and uncertain for a categorical area. The more-
important threat should be locally heavy rain, per the marginal risk
of excessive rainfall in the WPC outlook.

Another area of interest is the coastal region from near the
Mississippi River mouth northeastward toward the western FL
Panhandle. Locally enlarged low-level hodographs, juxtaposed with
surface-based buoyancy, may occur in a narrow corridor corresponding
to the warm-frontal zone and related backed surface winds. However,
low/middle-level lapse rates, convective-scale forcing and frontal
convergence all appear weak, casting doubt on thunderstorm strength
and maintenance. Brief storm-scale rotation is possible in any
cells in this environment, but the tornado potential remains too
uncertain and low to assign unconditional/categorical-level
probabilities.

..Edwards.. 01/18/2020

$$

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