ACUS01 KWNS 171252
SWODY1
SPC AC 171251

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0751 AM CDT Wed Jul 17 2024

Valid 171300Z - 181200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
MID-ATLANTIC INTO CENTRAL/SOUTHERN NEW ENGLAND...

...SUMMARY...
Damaging thunderstorm gusts appear most likely and concentrated
today over parts of the Mid-Atlantic into central/southern New
England.

...Synopsis...
In mid/upper levels, a seasonally high-amplitude pattern will
dominate the CONUS through the period, with longwave ridging in the
West and troughing in the East.  Anchoring the ridge, a persistent
anticyclone -- previously over the Four Corners area including NM --
will reorganize and retrograde slightly westward, becoming centered
over the Monument Valley region of AZ/UT by the end of the period.
Meanwhile, a synoptic-scale trough -- apparent in moisture-channel
imagery from James Bay to southern MN and northern IA -- will pivot
across the Upper Great Lakes and adjoining states through this
evening.  By 12Z, this trough should extend from central QC to the
Lower Great Lakes and upper Ohio Valley region.

A large area of cyclonic flow -- generally weakening with outward
extent from jet cores over the Great Lakes States and Northeast --
should cover much of the CONUS east of the Rockies and north of the
Gulf Coast States.  On its southwestern periphery, strong difluence
will persist over the central/southern High Plains and much of TX.
Within and beneath that, a prominent MCV was apparent in radar-
reflectivity composites northwest of OKC.  This feature is forecast
to drift southeastward over central/southeastern OK today, then
pivot eastward into southern AR overnight.

At the surface, 11Z analysis showed a cold front from southern QC
across LAKE Ontario, northwestern OH, central IN, southern IL,
south-central MO, southeastern KS, northwestern OK, the TX
Panhandle, and central/southeastern NM.  The cold front is, and will
continue to be, diffused along segments of its extent by
baroclinicity from outflows of varying sizes.  Still, in general, it
should reach western New England, the Delaware Valley, western VA,
middle TN, southern AR, to a weak frontal-wave low over north-
central/northeast TX by 00Z.  By 12Z, the front should extend from
near Cape Cod to central/eastern VA, north-central MS, and southern
AR, with a low lingering over northeast TX near or even somewhat
behind the MCV.

...Mid-Atlantic to New England...
Scattered to widely scattered thunderstorms are expected to develop
initially during midday near the cold front, then through the
afternoon ahead of the cold front (especially over higher terrain
and near prefrontal troughing in the boundary layer).  Convective
coverage and intensity should increase as this activity moves
eastward to northeastward across the outlook area, forming a blend
of discrete storms, clusters and lines.  Damaging gusts will be the
primary hazard, and several gusts above severe limits are possible
as well.

As the mid/upper-level synoptic trough approaches, deep-layer
southwest flow and shear will strengthen over the area, with the
mean-wind and deep-shear vectors oriented along to slightly
rightward of the axes of convective forcing.  This will support the
mixed modes, which may include isolate supercell structures as well.
Overall organization will be aided by 30-40-kt effective-shear
magnitudes from central PA northeastward, and 20-30 kt farther south
and southwest.  Meanwhile, the greatest buoyancy should reside over
the coastal Mid-Atlantic amid rich moisture and strong diurnal
heating.  Although midlevel lapse rates are forecast to be very weak
(near or even less than moist-adiabatic in some layers), ambient
low-level theta-e and diabatic heating should drive MLCAPE into the
1000-1500 J/kg range over most of the region, and around 2000 J/kg
near the coast -- all atop well-mixed subcloud layers supporting
downdraft accelerations.

...Central/southern High Plains...
Widely scattered to scattered thunderstorms -- some quickly evolving
to outflow-driven, forward-propagating clusters -- should form this
afternoon over the Sangre de Cristos, Front Range, Laramie Range,
and attendant foothills/ridges.  Damaging to isolated severe gusts
will be the primary concern, though isolated severe hail also is
possible.

Activity should form first over the mountains, where heating of high
terrain and at least marginal moisture will combine to
preferentially erode MLCINH.  Lee troughing should develop over the
High Plains late tonight, but before then, a substantial, post-
frontal upslope/easterly flow component generally will prevail in
low levels.  This will aid in lift and storm-relative boundary-layer
winds available to convection moving generally southeastward out of
the higher terrain.  An axis of low-level moisture will correspond
well to a buoyancy axis as well, with peak MLCAPE around 1000-1500
J/kg in a corridor roughly from the NE Panhandle to western KS by
21Z, then gradually diminishing as nocturnal cooling occurs and
outflow impinges on parts of the area from the west.  Modest
midlevel winds and weak shear should exist over most of this area,
despite strong directional change with height in low/middle levels.
Steep low/middle-level lapse rates -- near dry adiabatic in the
boundary layer -- will support strong-severe gust potential in the
most vigorous cells/clusters.

...North TX to central AR...
Widely scattered to scattered showers and thunderstorms should
develop over a vast area along and southeast of the front this
afternoon, southwestward from the outlook in the Atlantic Seaboard
across the Ohio/Tennessee Valleys, Southeast CONUS, and Mid-South to
north/central TX.  Most of this activity will be in a weak-shear but
moisture-rich setting.

While isolated, pulse-severe downbursts cannot be ruled out with the
most intense activity anywhere in those regions, the greatest
potential concentration of both convection and at least marginal
severe potential appears to be over parts of north-central/northeast
TX into central AR, along and either side of the I-30 corridor.  A
combination of lift along the front and prefrontal outflows, with
enhanced convergence in the MCV's mass response, and strong warm
sector diurnal heating, will strip away MLCINH this afternoon and
favor relatively dense development.  Additional activity should form
along both colliding outflow/differential-heating boundaries and the
leading-edge/forced-ascent zone of any clusters' cold pools that can
surge into the high-theta-e preconvective boundary layer.  Upper 60s
to low 70s F surface dewpoints will support MLCAPE in the 2000-3000
J/kg range (locally higher), with well-mixed subcloud layers despite
the rich moisture.  Weak vertical shear will limit the overall
organization of the threat.

..Edwards/Broyles.. 07/17/2024

$$
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