Day 1 Severe Weather Outlook

Valid 201300Z – 211200Z

…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
EASTERN OKLAHOMA…WESTERN ARKANSAS AND NORTH-CENTRAL/NORTHEAST
TEXAS…

…SUMMARY…
The greatest severe-weather threat appears to be this evening into
tonight across parts of eastern Oklahoma, western Arkansas and
north-central/northeast Texas. Large hail, damaging gusts and
tornadoes are all possible.

…Synopsis…
In mid/upper levels, a broad area of cyclonic flow will cover most
of the CONUS between the Appalachians and the western Intermountain
region this period. A broad area of height falls will spread across
the Plains and Mississippi Valley, ultimately eastward toward the
Appalachians, once amplifying ridging crosses the Appalachians
tonight. As this occurs, the remnants of former T.S. Nestor, and a
related midlevel shortwave perturbation, will eject across the
remainder of the Carolinas and over the Atlantic.

A mid/upper-level low already appears to be forming over north-
central/northeastern WY, along a strong trough apparent in moisture-
channel imagery from eastern MT to southern CO. This feature is
responsible for record to near-record cold 500-mb temperatures at
proximal RAOB sites this morning, including -28 C at GJT. The
resultant 500-mb cyclone should be centered over south-central SD
by 00Z, with troughing south-southwestward across northeastern NM.
By 12Z, this low should reach the FSD area, becoming nearly stacked
atop its low-level counterpart.

The related surface low was analyzed at 11Z between CDR-RAP, with
cold front across western NE and eastern CO. By 00Z, the low should
reach south-central SD north of VTN, with cold front arching across
east-central NE and central KS to a weak low over southwestern OK,
southwestward across southeastern NM. A dryline should develop
today and, by 00Z, set up from central/west-central OK to near an
SPS-DRT line. The front will overtake the dryline from north to
south overnight, reaching a 12Z position from central IA to
southeastern MO, southwestern AR, and south-central TX.

…Ozarks to north-central TX…Arlkatex…Mid-South…
Scattered to numerous thunderstorms, mainly this evening and
overnight, will offer the risk for large hail, damaging gusts, and
tornadoes over this part of the outlook area.

The most consistently progged and physically reasonable 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 southeastern OK, northeast TX, and western AR. Shear
and lift will diminish southwestward, while instability will
decrease northward. As such, only minor adjustments have been made
to the various probability lines based on newer guidance.
Indirectly, Nestor has contributed to some of the uncertainty here,
through offshore flow across parts of the northern/northeastern
Gulf, leading to a late-returning, somewhat narrowed, but still-
favorable moisture plume.

By this evening, the cold front is expected to impinge on a corridor
of increasingly favorable boundary-layer theta-e across eastern OK
into north-central TX. Forecast soundings show some lingering
low-level stability atop the boundary layer, related to a modest
EML. Still, 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 LLJ should strengthen to 55-65 kt between
00-06Z, leading to very large hodographs with effective SRH commonly
300-500 J/kg, amidst 50-60-kt effective-shear magnitudes.
Conditionally, this can support a significant-tornado risk as well,
but mode uncertainties still appear too great to draw such an area
at this time.

While that parameter space is quite favorable for all forms of
severe, 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 western Mid-South regions
by 12Z.

…Lower-mid Missouri Valley…
A narrow, strongly forced band of convection is expected to develop
close to the front late this afternoon and persist into early
evening, in an arc from central SD across eastern NE to northeastern
KS. Isolated/conditional potential exists for damaging to severe
gusts and marginal hail, and a tornado cannot be ruled out.

Intensifying deep-layer lift at all scales — including large-scale
DCVA/cooling aloft, lift related to the left-exit region of a
cyclonically curved upper jet, and low-level frontogenetic forcing
— is forecast as the cyclone moves eastward and deepens. This will
yield substantial strengthening of both low-level convergence and
midlevel lapse rates, building a vertical thermodynamic profile
supporting thunderstorms and enough low-level lift to maintain them.
Forecast soundings suggest steep low/middle-level lapse rates with
500-mb temperatures above the surface front ranging from around -18
C over northeast KS to the -22 to -25 C range in northeastern NE and
central/southeastern SD. This would yield a narrow corridor of
surface-based effective-inflow parcels and MLCAPE in the 200-700
J/kg range, despite cool surface temperatures in the mid 50s to mid
60s F. Favorable deep shear will spread across the region, with
isallobaric processes keeping near-surface flow backed and
hodographs enlarged. While the lack of greater buoyancy will be the
main limiting factor precluding a better-organized severe threat,
all forms of severe are at least marginally supported for a few
hours near the front.

…Southeastern NC…
A marginal tornado threat lingers this morning in the outer
eastern/northeastern fringes of the remnants of Nestor. By
afternoon, boundary-layer destabilization is expected behind the
early convective band and ahead of the surface low. Though
low-level winds will be veered relative to the environment near the
morning convective band, another low-topped arc of convection may
develop in a weakly capped, moist, marginally convergent environment
this afternoon. However, modest low-middle-level winds will keep
hodographs small, while warm midlevel temperatures (related to
trajectories from warm-core tropical-cyclone air) will restrict
buoyancy substantially, with MLCAPE less than 500 J/kg in most
areas. As such, the main severe concern is the marginal tornado
threat near the coast and southern Outer Banks for a few more hours.
See SPC mesoscale discussion 2083 for more details.

…Lower Mississippi Valley region…
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
preconvective 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, based on forecast
soundings. 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 appears too conditional for
an outlook, though strong gusts will be possible from the most
intense cells.

..Edwards/Dean.. 10/20/2019

$$

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