SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0650 AM CST Thu Feb 18 2021

Valid 181300Z - 191200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE
FLORIDA PANHANDLE INTO SOUTHERN SOUTH CAROLINA...

...SUMMARY...
Isolated severe thunderstorms, with a threat of damaging gusts
and/or tornadoes, are possible today from parts of the Florida
Panhandle into southern South Carolina.

...Synopsis...
A mid/upper-level longwave trough will begin to deamplify over the
central CONUS and shift eastward, as a basal shortwave perturbation
-- now over extreme northern MX and the TX Big Bend area -- pivots
northeastward and reorients its vorticity structure.  By 00Z, the
500-mb trough should be stretched, positively tilted, from the
Mid-South to south-central TX and northeastern MX.  By 12Z, this
feature should extend from eastern portions of KY/TN across MS to
the TX shelf waters of the Gulf.

At the surface, weak lows were analyzed inland in cool air near the
southeastern corner of AL, and over the Gulf southwest of PFN.  A
synoptic warm front -- separating a cold-air damming regime to the
north from a partly modified (but still mostly stable at the
surface) warm sector, was drawn from the northern low eastward then
east-northeastward across southern GA to near SAV.  This boundary
should drift northward through the day.  A marine warm front, south
of which more-completely modified Gulf and Atlantic air was evident
-- was drawn across northern FL then west-northwestward to an
intersection with a cold front near PFN.  The cold front extended
from there southwestward through the southern low-pressure area to
the central Gulf.  The marine/warm front is already showing signs of
becoming diffuse over northern FL amidst a patchwork field of
moisture/theta-e, and should blend northward into the northern
boundary through midday.  The cold front will move slowly eastward
over the outlook area, as the northern low slowly crosses southern
GA.

...FL Panhandle to SC...
A loosely organized, broken band of thunderstorms is evident mostly
ahead of the surface cold front, from clearly elevated convection
over central GA southwestward through the low-level frontal zone to
clearly-surface-based thunderstorms over the Gulf, and a broad
transitional zone in between.  Low-level shear remains favorable
near the southern/marine front, where backed flow enhances SRH, and
boundary layer theta-e supports surface-based effective-inflow
parcels.  For near-term information, refer to watch 10 and related
mesoscale discussions.

Through the remainder of the morning into the afternoon, gradual
inland destabilization (related to both diabatic heating and warm
advection) should occur with northeastward extent across the eastern
FL Panhandle, northwestern FL and GA.  This will lead to a gradual
increase in severe potential across those areas into early
afternoon, as:
1.  The main belt of convection and cold front proceed slowly
eastward (with embedded cells moving northeastward), and
2.  The marine warm front becomes shallower and diffuses
northeastward toward the synoptic boundary.  

The latter will lead to increasing boundary-layer lapse rates/
buoyancy and surface-based effective-inflow parcels, with dew points
increasing mostly into the mid/upper 60s F across the outlook area
prior to cold frontal passage.  Resulting near-surface theta-e
should offset modest midlevel lapse rates (including a "CAPE robber"
evident in the 12Z TLH RAOB) enough to yield peak MLCAPE in the
300-800 J/kg range, with near 1000 J/kg locally/briefly possible. 
Forecast soundings over southern GA show 200-300 J/kg effective SRH
and 40-45-kt effective-shear magnitudes, before flow veers near the
cold front.

..Edwards/Smith.. 02/18/2021

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