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ACUS01 KWNS 181313
SWODY1
SPC AC 181311
Day 1 Convective Outlook CORR 2
NWS Storm Prediction Center Norman OK
0811 AM CDT Thu Apr 18 2019
Valid 181300Z - 191200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF LOUISIANA...MISSISSIPPI...ALABAMA...AND THE WESTERN FLORIDA
PANHANDLE...
CORRECTED FOR UNMATCHED CATEGORICAL AND PROBABILITY LINES
...SUMMARY...
Severe thunderstorms may produce damaging gusts, a few tornadoes
(possibly strong), and isolated large hail today across the lower
Mississippi Valley through the central Gulf Coast region.
...Synopsis...
In mid/upper levels, a synoptic-scale trough will proceed from MN
and the central/southern Plains to the Upper Great Lakes and
lower/mid Mississippi Valley through the period. The two most
important associated smaller-scale perturbations for convective
purposes will be:
1. An initially strong, basal shortwave trough -- now evident in moisture-channel imagery over western/central OK and west-central
TX. This feature is forecast to pivot eastward then northeastward
while losing amplitude, and to reach portions of AR, northern LA and
southeast TX by 00Z. This feature will eject northeastward toward
the Ohio Valley and weaken markedly overnight.
2. An upstream trough and associated speed max in northwesterly
flow -- currently apparent over the northern High Plains. Its
leading portion will dig southeastward across KS/OK by 00Z, then
become the new strong/basal shortwave trough over LA by 12Z.
At the surface, beneath height falls related to the features aloft,
a surface low should form along a cold front over northeast TX or
the Arklatex region and shift erratically to the Mid-South region by
00Z, perhaps catching up to the preconvective warm sector this
afternoon. The cold front was analyzed at 11Z in a convectively
modified air mass across the MO/northwest AR Ozarks, southeastern
OK, north-central TX, to near DRT. The leading convective/outflow
boundary extended from southeastern AR across northwestern LA and
extreme southeast TX. By 00Z the cold front should reach western
portions of TN/MS and southeastern/south-central LA. The low should
deepen considerably overnight and move across central/northeastern
KY. By 12Z the front should reach eastern TN, the GA/AL border
vicinity, and western FL Panhandle.
A marine/warm front was drawn at 11Z over western FL Panhandle
coastal waters to near the MS/AL coastlines, then inland across
southeastern to northern LA. This boundary, which denoted the
northern rim of the best-modified, richly moist Gulf air mass,
should become gradually more diffuse, and move east-northeastward
across portions of southern/central MS through the afternoon. This
evening and overnight, it should cross southern AL and western FL
Panhandle.
...Lower Mississippi Valley, Gulf Coast States...
Scattered to numerous thunderstorms are expected to cross the area
through the period, generally from west to east in the form of a
quasi-linear convective system (QLCS), but with some warm-sector
supercells also possible. Damaging winds and a few tornadoes
(possibly strong) are expected, and isolated hail is possible as
well.
As the shortwave perturbation rounds the base of the broader-scale
trough and the surface low strengthens, low-level isallobaric/mass
response will contribute to backed warm-sector flow across the
Central Gulf Coast and Delta regions, in addition to the backing and
low-level vorticity enhancement expected along the warm/marine
front. These factors will contribute to a 50-65-kt LLJ atop
favorably large low-level hodographs (i.e., effective SRH 300-500
J/kg) and a CAPE/shear parameter space ordinarily well within ranges
typically associated with significant tornadoes. Forecast soundings
show a combination of boundary-layer theta-e advection and muted
diurnal heating helping to boost MLCAPE into the 1000-1500 J/kg
range near and south of the inland-penetrating marine front.
Strengthening southwesterly mean-wind/deep-shear vectors will spread
over the outlook area today into this evening. The QLCS largely
responsible for the enhanced damaging-wind threat, beneath that
strong flow, may arise from an increasingly organized band of
convection now located over the Gulf -- south of a nearshore outflow
boundary, east of CRP, and southwest of GLS.
A modulating influence, however, may be storm mode. The bulk of
convection should be quasi-linear, with only conditional potential
of warm-sector supercells surviving long enough to become tornadic
in that favorable environment before being overtaken by the MCS.
QLCS tornadoes (which occasionally can become significant also,
though not to the frequency and duration of supercellular ones) also
are possible as the convective band impinges on the high-SRH and
marginally to moderately unstable boundary layer. For now, will
maintain the significant-tornado area with some peripheral
modifications.
...South-central Plains...
Isolated to widely scattered thunderstorms are possible mainly this
afternoon, as diabatic heating of the boundary layer removes MLCINH
beneath very cold air aloft. Hail and gusts approaching severe
limits are expected in the most intense cells.
Even with surface temperatures only in the mid/upper 60s F, modified
forecast soundings show some favorable conditions: -20 to -24 C
500-mb temperatures, lapse rates 7-8 deg C/km from surface-500 mb,
and enough residual low-level moisture to support 500-800 J/kg of
uncapped MLCAPE. Buoyancy also should extend into icing layers
suitable for production of both lightning and at least small hail. Additionally, well-mixed subcloud layers with almost dry-adiabatic
0-3-km lapse rates may enable gusts approaching severe limits. A general-thunder area is being added with this outlook package. If
confidence increases in large hail and severe gusts, severe
probabilities may be added in a succeeding update.
..Edwards/Goss.. 04/18/2019
$$
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