SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0700 AM CST Sun Dec 29 2024
Valid 291300Z - 301200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE EASTERN
GULF COAST NORTHWARD INTO THE CAROLINAS AND SOUTHERN VIRGINIA...
...SUMMARY...
A few tornadoes and sporadic damaging to severe gusts are possible
today from the eastern Gulf Coast northward into the Carolinas and
southern Virginia. More-isolated severe storms may occur across
parts of the central Appalachians and Ohio Valley, and over
southernmost Florida.
...Synopsis...
In mid/upper levels, a progressive pattern will persist across the
CONUS. A strong shortwave trough will move ashore in the Pacific
Northwest and northern CA today. This will be preceded by some
thunder potential, as lapse rates steepen from large-scale ascent,
over areas where at least marginal low/middle-level moisture exists
to support convection. Downstream, a strong shortwave trough was
evident in moisture-channel imagery between the Mid-South and
Tennessee Valley. This perturbation will pivot northeastward over
TN, KY and IN today, forming a closed 500-mb low by 00Z over the
Michiana region, and phasing with an initially upstream perturbation
now over the central Plains. By 12Z, the resultant trough should
assume considerable negative tilt, extending from a low over eastern
Lake Huron to the Hampton Roads of VA/northeastern NC region.
The associated surface cyclone was centered at 11Z over southwestern
IN, with cold front across middle TN, central AL, past the
Mississippi River mouth, then across the northwestern Gulf. The low
is forecast to deepen and occlude today as it moves to near TOL.
The cold front should sweep across much of the eastern CONUS through
the period. By 00Z, the front should reach central parts of PA/VA/
NC/SC, southeastern GA, and the eastern Gulf, preceded by a
strong-severe line of thunderstorms. By 12Z, the front should reach
eastern NY, then offshore until central FL.
...Southern Atlantic Coast States...
An ongoing, prefrontal squall line, with sporadic/embedded LEWP/
bowing features and an occasional embedded mesovortex, extends from
northwestern SC across central/southern GA and the FL Panhandle, to
the adjoining Gulf. This convection should become more northeast/
southwest oriented as its northern portion races through the central
Appalachians/Piedmont region today, amid meager but still locally
sufficient buoyancy (MLCAPE mostly less than 400 J/kg), long and
curved low-level hodographs with 200-400 J/kg effective SRH, and
favorable deep shear. Damaging gusts and a few embedded QLCS
tornadoes will be the main concerns. See SPC Tornado Watches 723-724
and associated mesoscale discussions for the latest near-term
coverage.
...Central Appalachians/Ohio Valley...
Widely scattered low-topped thunderstorms are possible from midday
through afternoon in a pocket of marginally unstable air ahead of
the surface low and ejecting mid/upper trough. A marginal threat
exists for damaging gusts or a brief tornado from this activity. As
the trough approaches, midlevel flow will strengthen considerably,
associated large-scale ascent will steepen low/middle-level lapse
rates, while the mass response increases low-level and deep-layer
shear. Temperatures from the upper 50s F to mid 60s (southeastern
parts) would support MLCAPE in the 100-400 J/kg range amidst 40-50
kt effective-shear magnitudes and SRH of 100-200 J/kg in the lowest
1/2 km. Damaging gusts may be brought to the surface via momentum
transfer in downdrafts from the LLJ, and conditional potential
exists for mini-supercells to develop. Given the weak overall lapse
rates and lack of greater buoyancy, the already marginal threat
should diminish quickly by around 00Z.
...South FL/Keys...
A small but persistent MCS, with an embedded MCV on its northwest
side, has been moving slowly eastward across the easternmost Gulf
toward Florida Bay for most of the prior overnight hours. This
complex should affect at least parts of the mid/upper Keys, and
southern Everglades, through midday. The ambient/synoptic gradients
suggest only modest deep-layer shear this far from the ejecting
shortwave trough. However, mesoscale midlevel flow enhancement and
low-level mass response will promote some hodograph enlargement in a
small corridor southeast through east of the MCV. Accordingly,
sporadic and mostly short-lived mesocirculations have been observed
in the complex for several hours, amid rich inflow-layer moisture
and low LCL. An associated low-end tornado threat may extend into
the outlook area before the complex weakens substantially. See SPC
Mesoscale Discussion 2317 for near-term details.
..Edwards/Grams.. 12/29/2024
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