SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0634 AM CST Tue Nov 21 2023
Valid 211300Z - 221200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SOUTHERN
ALABAMA...SOUTHWESTERN GEORGIA AND PARTS OF THE FLORIDA PANHANDLE...
...SUMMARY...
A few tornadoes and isolated damaging gusts are possible today
across southern Alabama, southwestern Georgia and parts of the
Florida Panhandle. Isolated damaging gusts are possible, along with
a marginal tornado threat, from there northeastward to the coastal
Carolinas.
...Synopsis...
In mid/upper levels, a split-flow pattern will develop this period,
as a strong shortwave trough now over northwestern MX digs
southeastward and evolves into a cut-off cyclone, centered over
Sinaloa at 12Z tomorrow. Meanwhile, moisture-channel imagery
indicates a vorticity max and formerly closed circulation over
northern IL, on the northern part of a trough extending
southwestward to southwest TX. Though now in phase with the MX
perturbation, the IL one will break northeastward across the Lower
Great Lakes, ahead of an amplifying shortwave trough now over MN.
The MN feature will dig southward to the Ozarks today, then pivot
eastward across the Mid-South and lower Mississippi Valley tonight
as a closed (or very nearly closed) 500-mb low. Ahead of all that,
and the associated mean trough, a broad belt of southwest flow aloft
will shift slowly eastward with height falls over parts of the
Southeast and southern Atlantic Coast.
The 11Z surface analysis showed a primary synoptic low -- associated
with the IL mid/upper perturbation. The associated cold front was
analyzed near LAF. A cold front was moving southeastward over
eastern/southern MS, southeastern LA, and the northwestern Gulf. A
warm/marine front was drawn from a secondary/triple-point low near
CBM southeastward across the PNS area and northeastern Gulf,
becoming quasistationary near the southwest coastline of Florida.
The warm front is forecast to shift diffusely northeastward across
eastern AL, southern/eastern GA and parts of the Carolinas through
the period, while being overtaken from north-south by a belt of
precip/thunderstorms.
The cold front is forecast to move eastward/southeastward by 00Z to
the Appalachians of western VA/NC/SC and northern GA, southwestward
over southeastern AL, the west-central FL Panhandle and central
Gulf. Overnight cyclogenesis should occur along this boundary,
moving northeastward up the Atlantic Seaboard, with the surface low
over NJ by 12Z. The cold front should extend southwestward across
eastern NC -- where early-stage/secondary frontal-wave low
development is possible ahead of the Mid-South shortwave
perturbation. The front then should extend from there across
northern FL to the south-central Gulf.
...Northeastern Gulf Coastal Plain...
An ongoing, rather nebulously organized swath of precip and isolated
embedded thunderstorms is apparent from near the mouth of the
Mississippi River northeastward across parts of southern/eastern AL,
the FL Panhandle and western GA. Within or behind this activity,
diurnal destabilization and low-level convergence should support
reorganization of the convective belt today, and especially from
midday through afternoon. This may come about as convection forming
nearer to the front moves eastward and merges with what is left of
the ongoing belt.
Either way, a better-focused corridor of thunderstorms is expected
to take shape over parts of AL and the FL Panhandle and move
eastward, while the warm front diffusely shifts inland, and moisture
transport and theta-e advection continue from the Gulf. Forecast
soundings today suggest a plume of 500-1200 J/kg MLCAPE within about
100 nm of the coast, weakening/narrowing northward. Before
prefrontal flow veers to southwesterly, southerlies are expected
through much of the boundary layer, veering with height above about
1 km, and contributing to large hodographs. Around 150-300 J/kg
effective SRH may develop, about 75-150 J/kg of which is in the
lowest 500 m. The associated parameter space should support some
tornado potential with any sustained supercells or the strongest
line-embedded mesovortices.
...Remainder of GA-Carolinas...
The main band of convection should extend from the Gulf Coastal
Plain northward into the southern Appalachians by mid/late
afternoon, the move eastward over more of GA/SC this evening into
tonight. Already modest buoyancy should diminishing northward and
with time for the most part, given less-unstable low-level
trajectories apparent in progs out of the northern FL Peninsula.
Large-scale forcing should weaken as the Great Lakes-area trough
aloft shifts away from the region, with mid/upper flow largely
parallel to the swath of low-level lift supporting the convection.
Yet favorable low-level and deep shear will persist. As such, a
more conditional and isolated severe potential is apparent northeast
of the 15%/"slight risk" area.
Though presently remaining in the range of unconditional marginal
probabilities, a secondary/conditional ramp-up in severe threat may
develop late tonight across eastern/coastal NC. Sufficient
southerly flow component in the lower boundary layer should be
maintained to yield a persistent fetch off warm Atlantic waters --
and especially if the frontal-wave development is a bit stronger
than currently forecast. This should yield a corridor of mid-60s F
surface dewpoints over eastern NC the last several hours of the
period, and perhaps mid/upper 60s along the immediate coast. That
should offset weak, nearly moist-adiabatic deep-layer lapse rates
enough to yield 300-800 J/kg MLCAPE. Shear will favor embedded
supercells and/or mesovortices with the main line, as well as
storm-scale rotation for some cells ahead of it moving off the
ocean. Forecast soundings generally depict about 30-40 kt
effective-shear magnitudes, and long lowest-km hodographs beneath a
50+ kt LLJ. At this time, weak instability and uncertainty on
coverage of favorable storm mode precludes higher probabilities.
..Edwards/Leitman.. 11/21/2023
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