SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0651 AM CST Sat Dec 28 2024
Valid 281300Z - 291200Z
...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
EAST TEXAS...CENTRAL/NORTHERN LOUISIANA...CENTRAL MISSISSIPPI...AND
WESTERN ALABAMA...
...SUMMARY...
Tornadoes (some strong with at least EF2 damage possible), severe
gusts and hail are expected today into tonight. The threat area will
shift eastward from parts of north and central Texas early, across
east Texas and the lower Mississippi Valley today, then over the
Tennessee Valley, parts of Georgia, and the central Gulf Coast
tonight.
...Synopsis...
In mid/upper levels, broadly cyclonic flow will persist and shift
eastward across the central CONUS through the period. The most
important feature aloft will be an embedded shortwave trough, now
apparent in moisture-channel imagery over the southern High Plains,
lowest Pecos Valley and northern Coahuila. This perturbation will
move eastward across much of the remainder of TX today, reaching a
position from eastern OK to east TX to the northwestern Gulf by 00Z.
Overnight, this trough should pivot northeastward, reaching
southern IL, western parts of KY/TN, and MS by 12Z tomorrow.
The 11Z surface analysis showed a wavy, mostly warm to locally
quasistationary frontal zone across the southeastern Hill Country of
TX, eastward over central/east TX and central LA to western/central
MS, then southeastward diffusely into outflow air over coastal areas
near MOB/PNS, related to offshore activity. A low was drawn over
northwest TX between SPS-ABI. The low is expected to migrate
east-southeastward across north TX this morning, meet the returning
warm sector, then pivot east-northeastward to the northern LA/
southern AR area by 00Z, when the attached cold front should extend
southwestward over portions of east and south TX. By 12Z, the low
should deepen considerably and reach western KY, with cold front
across middle TN, northern AL, southeastern MS, southeastern LA, and
the northwestern Gulf. Well-organized convective band(s) should
precede the cold front from this afternoon through tonight. Prior
to the low's arrival, the warm/marine front should advance northward
into the Arklatex, and across parts of southern AR and northern MS.
The warm front should move northeastward across AL and the FL
Panhandle tonight, bounding the northeastern rim of a narrowing warm
sector.
...Southern Plains to Southern Appalachians and central Gulf
Coast...
Convection is increasing in two main regimes either side of the
warm/marine front:
1. Elevated convection to its north over north-central TX,
predominantly of a hail concern, and forecast to move/expand into
the Arklatex over the next few hours. See Severe Thunderstorm Watch
717 and related mesoscale discussions for near-term details.
2. Warm-sector thunderstorms building gradually from east TX across
parts of LA to southern MS. This activity will pose a threat for
tornadoes, isolated hail, and occasional severe gusts. See SPC
Mesoscale discussions 2294 and 2295, with watches likely over these
areas today as well.
Peak tornado and severe-gust potential should be from this afternoon
into tonight, in and near the "Moderate" and "Enhanced" areas.
Perhaps the greatest uncertainty at this time is coverage of
supercells in the warm sector during peak instability this afternoon
and into evening, following the morning episode, given 1) the need
for recovery behind earlier activity and 2) subtle and gradual
boundary-layer forcing/convergence would be the main factor
contributing to their development. A broad area of favorable
moisture, represented by mid-60s to near 70 F surface dewpoints,
will contribute to peak MLCAPE in the 2000-2500 J/kg range from east
TX to southwestern MS, and around 1000-1500 J/kg from central MS
into central/southern AL where diurnal heating will be less a factor
than theta-e advection for preconvective destabilization.
Hodographs will favor supercells and tornado potential during the
day -- especially near the marine/warm front -- but largest after
00Z east of the Mississippi River where a 45-60-kt LLJ will overlap
at least marginally surface-based inflow parcels. SRH of 150-250
J/kg in the lowest 1/2 km, and 200-400 J/kg in the effective-inflow
layer, should be attainable.
Any sustained, discrete supercell in this environment will pose a
threat for cyclic, strong tornado production. As the primary
convective band organizes and moves into increasing moisture and
LLJ-enlarged hodographs in low levels this evening, embedded
LEWP/bowing features may produce both severe gusts and tornadoes.
The cumulative tornado threat suggests maintaining "moderate" level
probabilities with this outlook cycle, but enough uncertainty
remains to preclude a greater unconditional risk area. As the event
proceeds through the overnight hours, one or two dominant QLCSs
should continue across AL and into portions of GA and the FL
Panhandle, with a threat persisting for embedded severe gusts and at
least a few tornadoes.
..Edwards/Grams.. 12/28/2024
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