Day 1 Severe Weather Outlook

Valid 201630Z – 211200Z


The greatest severe-weather risk will be this evening into overnight
across parts of eastern Oklahoma and north-central/northeast Texas
to Arkansas. Large hail, damaging wind gusts and tornadoes are all

…Eastern OK/North-central TX to Ozarks/ArkLaTex/Mid-South…
Scattered to numerous thunderstorms, mainly this evening and
overnight, will offer the risk for large hail, damaging gusts, and
tornadoes over the region.

An upper-level trough centered over the central High Plains, and
accompanied by a strong cyclonically curved polar jet, will continue
to amplify and dig east-southeastward through tonight. A moist air
mass, which continues to make its way northward across
south-central/southeast Texas and much of Louisiana at midday, will
steadily stream northward and become established across much of the
region by tonight ahead of a southeastward-advancing cold front.

Accordingly, the most favorable overlap between returning moisture
(supporting surface-based, favorably buoyant parcels), convective
forcing, and strong shear still appears to be this evening into the
overnight hours across the region encompassing
east-central/southeast Oklahoma, north-central/northeast Texas, into
Arkansas. Deep-layer shear and lift will diminish southwestward,
while instability will decrease northward.

By this evening, the cold front will impinge on a corridor of
increasingly favorable boundary-layer theta-e across eastern
Oklahoma and north-central/northeast Texas. Surface-based
effective-inflow parcels will be possible beneath steep
mid/upper-level lapse rates, with MLCAPE ranging from around
2000-2500 J/kg over north-central/northeast TX to 500-1000 J/kg in
the Ozarks. The low-level jet will strengthen to 55-65 kt between
00-06Z, leading to very large hodographs with effective SRH commonly
300-500 m2/s2, amidst 50-60-kt effective-shear magnitudes.
Conditionally, this could support a strong-tornado risk, but
convective mode uncertainties and late-day arrival of richer
maritime air are points of uncertainty.

While that parameter space is quite favorable for all forms of
severe weather, storm type and duration of any supercell-linear
transition will be the biggest factors influencing change of hail to
wind as the main coverage threat, and of sourcing (supercell vs.
QLCS) for the tornado threat. A quasi-linear arc will evolve
overnight, shifting a severe-wind threat and at least isolated
potential for tornadoes across parts of the Arklatex, and
Ozarks/Mid-South regions by 12Z.

…Lower/Middle Missouri Valley…
A narrow, strongly forced band of low-topped convection should
develop close to the front late this afternoon and persist into this
evening. Isolated/conditional potential exists for damaging to
severe gusts and marginal hail, and a tornado cannot be ruled out.
Scenario will be supported by ample deep-layer ascent and cooling
temperatures aloft/steepening lapse rates in conjunction with a
narrow corridor of modest moisture/near-surface-rooted buoyancy.

…Lower Mississippi Valley including SE TX to LA/MS…
Widely scattered to scattered thunderstorms may develop this
afternoon in a somewhat strongly heated, weakly capped air mass
signifying the northeastern rim of the richer return-flow moisture,
from southeast TX across LA, shifting into central MS. Forcing for
convective-scale ascent away from the southeast TX/southwestern LA
sea breeze appears rather nebulous, aside from diurnal heating, and
perhaps subtle boundary-layer confluence/convergence lines. This
still may be sufficient to initiate convection off the southeast
edge of the EML, with around 1500-2000 J/kg MLCAPE developing in the
pre-convective warm sector. This regime will be very peripheral to
effects of the western/central CONUS troughing, though 35-45 kt
effective-shear magnitudes are possible. Low-level flow will be
modest, keeping vector shear and hodograph sizes small in the
boundary layer despite veering with height. At this time, severe
potential still appears too conditional/uncertain for an outlook,
though strong gusts will be possible from the most intense cells.

…Coastal North Carolina…
Any severe-weather threat for the Outer Banks will quickly diminish
as the modestly unstable warm sector moves away from the coast.

..Guyer/Nauslar.. 10/20/2019


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