Day 1 Severe Weather Outlook

Valid 202000Z – 211200Z


Severe thunderstorms are expected to affect parts of eastern
Oklahoma, northeast Texas, Arkansas, and southern Missouri this
evening and overnight. Damaging winds, large hail, and tornadoes are
all possible.

No change have been made to the ongoing forecast. Storms are
expected to form along/ahead of the approaching cold front after 00z
over east-central OK and north-central TX, with an initial threat of
large hail and damaging winds. The tornado threat will increase by
late evening as storms move into far eastern and southeastern OK
where greater low-level moisture and continually increasing vertical
shear will be present.

Have extended the ENH and SLGT risks farther east/northeast for the
overnight squall line. Trends in surface dewpoints over AR have
increased confidence that surface-based convection will be likely
farther north into southern MO, with a risk of damaging winds and a
few tornadoes. The ENH now includes much of northern AR where
recent CAM runs are bullish on a mature and fast-moving squall line
after midnight, posing a risk of significant wind damage and a few
supercellular and QLCS tornadoes.

..Hart.. 10/20/2019

.PREV DISCUSSION… /ISSUED 1146 AM CDT Sun Oct 20 2019/

…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.


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