Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point. Hatched Area: 10% of greater probability of wind gusts 65 knots or greater within 25 miles of a point.
Day 1 Wind Risk
Area (sq. mi.)
Some Larger Population Centers in Risk Area
Birmingham, AL...Montgomery, AL...Shreveport, LA...Mobile, AL...Little Rock, AR...
Houston, TX...Fort Worth, TX...Charlotte, NC...Oklahoma City, OK...New Orleans, LA...
SPC AC 091251
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0751 AM CDT Fri Apr 09 2021
Valid 091300Z - 101200Z
...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
SOUTHERN ARKANSAS...NORTHERN LOUISIANA...CENTRAL MISSISSIPPI...AND
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SEPARATELY OVER
PARTS OF SOUTHERN VIRGINIA AND NORTHERN NORTH CAROLINA...
Multiple episodes of severe thunderstorms are expected today from
parts of eastern Oklahoma and north-central/northeast Texas, across
the Mid-South and Mississippi Delta regions, to parts of Alabama and
Georgia. This includes a threat for a swath of severe wind from
Arkansas across the lower Mississippi Valley. Large, damaging hail
and tornadoes also are possible.
In mid/upper levels, a split-flow pattern will be maintained over
the central CONUS, as one cyclone ejects out of a broader-scale
trough causing the split, and another takes its place. The leading
cyclone -- initially centered near LSE -- is forecast to weaken
gradually to an open-wave trough and head northeastward over Upper
MI and Lake Superior overnight. Upstream, a strong shortwave trough
was apparent in moisture-channel imagery from eastern MT southward
to central CO. This feature will dig southeastward across the
central Plains today and this evening, before pivoting eastward
tonight as a closed cyclone, across parts of the lower Missouri
Valley. By 12Z, the associated 500-mb low should be located near
the KS/MO line south of MKC.
Farther south, a broad area of strong cyclonic flow aloft will
spread eastward across the outlook area, with embedded, closely
spaced perturbations now evident over southwestern/central NM and
the TX Panhandle. These perturbations will move across OK and south
TX today. Tonight, they should reach parts of TN/MS/LA, likely
blending into convectively generated vorticity fields. The broad
left-exit region of a cyclonically curved, 250-mb subtropical jet,
also exhibiting pronounced difluence, also will shift eastward over
much of central/north TX and LA today and tonight toward the Delta
At the surface, 11Z analysis showed an occluded, elongated area of
low pressure across northwestern WI to northern IA. An occluded
front was drawn from the northern part across Lake Superior and
Lower MI, with weakening/quasistationary front across western OH,
eastern portions of TN/KY, and northern AL. A warm front extended
across Lake Erie, eastern WV, and southwestern/southern VA near the
NC line. This boundary will move slowly northward over southern/
central VA today. A cold front extended across southern IA,
northwestern MO, central/western KS, and southeastern CO. A
marine/warm front extended from a low over northwest TX across
north-central TX, central LA, and southern portions of MS/AL. This
boundary will move northward across southern /eastern OK, AR, and
central MS/AL through the day.
The northern low should consolidate in form but continue to fill,
and move northeastward over ON. Meanwhile, a second low is expected
to develop along the front today near the dryline intersection,
perhaps evolving out of the initial/prefrontal low over northwest
TX. The resulting low should deepen as the next mid/upper cyclone
approaches, and move northeastward to near STL by 12Z tomorrow. By
that time, the cold front should extend from the low southward down
the Mississippi Valley to near BTR, then southwestward across
mid/upper TX coastal waters to deep south TX.
...Southern Plains to Arklatex/Mid-South/Delta regions...
Episodes of severe thunderstorms are expected today into this
evening between eastern OK, north TX and Mississippi, resulting in
large (locally very large) hail, severe gusts and several tornadoes.
This is a complex scenario with an assortment of mesoscale
possibilities more diverse than the number of models and ensemble
members available. Progs somewhat inconsistently depict early
initiation and evolution of the convection across this region,
whether near the front in OK (a very plausible consensus theme), on
the dryline (more isolated/conditional due to strong capping), and
in the warm sector (highly variable). Multiple episodes of
strong-severe thunderstorms may move over any given spot through
tonight. This outlook represents both:
1. The potential for overlapping areas of severe at different times
through the day and evening as different areas of convection evolve,
2. A scenario that most progs reasonably do converge upon despite
early differences: a dominant QLCS/MCS evolving out of convection
initiating in OK and/or northeast TX, then moving east-southeastward
to southeastward over the Arklatex to Delta regions, organizing a
well-developed cold pool, and producing a swath of severe wind.
Whether or not this event will qualify as a derecho is a semantic
exercise, and likely regulated by mesoscale convective/boundary
processes yet to become apparent. Regardless, hurricane-force
convective gusts are a threat with any such complex, in the Enhanced
and Moderate areas. Also, because both the probable QLCS and any
preceding supercell(s) will encounter an environment of moderate-
strong buoyancy, steep midlevel lapse rates around the eastern rim
of an EML, enlarged low-level hodographs, and favorable vertical
shear, tornadoes also are expected. Peak preconvective MLCAPE
across the warm sector commonly should be in the 2000-3000 J/kg
range, amidst 40-55-kt effective-shear magnitudes. As one or more
MCSs plow cold pools into this environment, they should be
maintained southeastward toward southern MS/AL tonight, continuing a
wind-damage and sporadic tornado threat.
...Central Gulf Coast to AL/GA this morning/afternoon...
Widely scattered to scattered thunderstorms should develop from
mid/late morning through this afternoon, with a threat for sporadic
large hail and damaging gusts. A conditional, localized tornado
threat also may develop. For near-term coverage, please see SPC
mesoscale discussion 338.
Midlevel lapse rates will be less than in the more EML-influenced
air mass of the Plains/Delta area. Still, rich low-level moisture
spreading inland, combined with areas of diurnal heating (tempered
in pace by mid/upper cloud cover from upstream convection), will
weaken MLCINH and yield favorable surface-based buoyancy.
Meanwhile, low-level shear will be maximized near the marine frontal
zone, though weak absolute speeds will limit hodograph size
somewhat. Modified 12Z RAOBs and forecast soundings from midday
through afternoon indicate around 1500-2000 J/kg MLCAPE is possible,
with 35-45 kt effective-shear magnitudes. Effective SRH will range
from around 200-250 J/kg on the front to around 100-1-50 J/kg
coastward in the warm sector. With upper support meager in this
area, but also modest MLCINH, convective coverage along and south of
the front is uncertain.
Widely scattered to scattered thunderstorms should form this
afternoon over higher terrain near the eastern WV Panhandle and VA
Blue Ridge, with additional convection developing later in the
afternoon and evening southward across portions of western NC. This
activity is expected to move eastward across much of NC and VA,
offering isolated large hail and the potential for damaging to
marginally severe gusts. A tornado cannot be ruled out, especially
with any convection that can interact with the front, where weak but
also backed surface winds will enhanced low-level hodograph size and
boundary-layer bulk shear. Modest low/middle-level winds will limit
deep shear, with effective-shear magnitudes generally 20-35 kt in
most areas, perhaps around 40 kt along the front. As such, mostly
multicell modes are expected, with isolated, transient supercell
character possible near the front.
Convection in the northern parts of the outlook area will be tied
more strictly to orographic processes, and will have a much narrower
zone of favorable instability to access, north of the warm front.
That convection should weaken within a short distance eastward of
their initial flare-up. However, activity across central/southern
VA into central NC should persist well into the evening, perhaps as
far eastward as the Tidewater areas from Hampton roads toward the
northern sounds of NC, before weakening in the ongoing nocturnal/
boundary-layer stabilization process. Surface dew points mid 50s to
low 60s F, beneath steep low/middle-level lapse rates, will support
pockets of 1000-1500 J/kg MLCAPE along and south of the front, atop
a well-mixed afternoon boundary layer supporting potential for
strong/isolated severe gusts and hail to reach the surface.
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