Day 1 Severe Weather Outlook

Valid 211300Z – 221200Z

…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE LOWER
MISSISSIPPI VALLEY TO PARTS OF ALABAMA AND THE FLORIDA PANHANDLE…

…SUMMARY…
The severe-weather threat will shift eastward today through this
evening from the lower Mississippi Valley to parts of Alabama and
the Florida Panhandle. Damaging wind gusts and a few tornadoes are
possible.

…Synopsis…
In mid/upper levels, a large area of cyclonic flow covers the
western/central CONUS to the Ohio/Tennessee Valleys, anchored by a
strong, stacked, occluded cyclone centered near YKN. The associated
500-mb low will pivot northeastward across the upper Midwest through
the period. A speed max and related vorticity banner now behind the
height trough — over the central Rockies — will pivot through the
trough and reach the Mid-South region by 12Z. This will lessen the
synoptic trough’s positive tilt and spread height falls across most
of the eastern CONUS, aside from ridging crossing New England
tonight.

At the surface, a warm front was drawn across central FL and the
northeastern Gulf to near MOB and JAN, northward to where it was
being overtaken by a squall line in western TN. The convection will
continue to overtake the warm front across western TN and northern
MS through the afternoon. Meanwhile, a cold front — initially over
northern AR to near TXK and SAT — will move southeastward across
most of the rest of TX and the lower Mississippi Valley by 00Z.
Weak frontal-wave cyclogenesis is possible late in the period over
portions of eastern AL or GA, with a resultant low over eastern GA
or SC by the end of the period. By then, the cold front should
extend from that secondary low southwestward across southwestern GA,
the western FL Panhandle, and the north-central/west-central Gulf.

…South…
Through the remainder of the morning, a band of thunderstorms should
continue to outpace the slow eastward shift of suitable warm-sector
air, from northwest-southeast across the Mid-South region, and into
parts of the Tennessee Valley. Isolated severe gusts are possible
in the meantime. For near-term details, refer to remaining portions
of SPC watches 681 and 682, and related mesoscale discussions.

Additional scattered thunderstorms should pose a risk of damaging
gusts today and tonight, along with a few tornadoes closer to the
Gulf Coast from portions of southern LA to the FL Panhandle. Some
spatial and timing uncertainty exists on onset of greatest severe
potential over the southern LA area, because of an expansive shield
of clouds/precip related to an MCS offshore from the TX/western LA
coast. Ultimately, the near-frontal convective band should overtake
the remnants of the MCS. The merged area of convection may
intensify as it proceeds eastward near the coast, the inland portion
encountering a destabilizing and favorably moist air mass.
Additional storms, perhaps including a few supercells, may develop
in a weakly capped, moisture-rich warm sector near the coast.
Damaging winds and a few tornadoes are possible, given the very
moist environment with strengthening vertical shear.

Forecast soundings suggest that the preconvective environment will
destabilize mainly from theta-e advection, with some diurnal
warming. This will offset modest mid/upper-level lapse rates enough
to yield warm-sector/preconvective MLCAPE 1000-2000 J/kg near the
coast, decreasing inland to around 300-800 J/kg over central
sections of MS/AL. Low-level mass response to the synoptic
troughing will lead to enlarged hodographs with effective SRH
commonly in the 150-400 J/kg range within about 100 nm of the
central Gulf coast from late afternoon into tonight, where buoyancy
should be greatest. Any relatively discrete cells sustained in this
environment will pose a tornado threat, in addition to localized
damaging wind, with better-organized wind threat from any Gulf
MCS/antecedent QLCS combination.

..Edwards/Smith.. 10/21/2019

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