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ACUS11 KWNS 271312
SWOMCD
SPC MCD 271312
MSZ000-LAZ000-TXZ000-271515-

Mesoscale Discussion 1735
NWS Storm Prediction Center Norman OK
0712 AM CST Thu Dec 27 2018

Areas affected...portions of southeast Texas and western/southern
Louisiana.

Concerning...Tornado Watch 447...

Valid 271312Z - 271515Z

The severe weather threat for Tornado Watch 447 continues.

SUMMARY...The threat for occasional wind damage, and a tornado or
two with embedded mesocirculations, will continue from a line of
thunderstorms moving slowly eastward across the watch area.
Additional watch issuance may be needed within the next few hours to
its east, across more of Louisiana and perhaps southwestern
Mississippi, as the air mass slowly destabilizes there.

DISCUSSION...As of 13Z, the northern part of the primary QLCS was
located in an area of elevated buoyancy and atop roughly 300 J/kg
MLCINH over north-central LA, between RSN-MLU. The complex extended southwestward into progressively weakening CINH and strengthening
buoyancy, crossing the diffuse surface warm front in Vernon Parish,
then extending across extreme southeast TX, becoming ragged/
disorganized over Brazoria County. Most of the TX part of the line
appeared anafrontal, with outflow undercutting convective elements
forming along the leading edge or crossing it from the warm sector.

Until and unless deep, relatively discrete warm-sector development
occurs and interacts with the warm-frontal zone, the QLCS will
remain the dominant storm mode and severe concern. The foregoing
warm sector will destabilize slowly, as modest low/middle level
lapse rates are modulated favorably by a combination of low-level
warm advection and diffused diabatic surface heating. This process
should allow roughly 1000 J/kg MLCAPE to develop eastward across
portions of southern/central LA through midday. Meanwhile low-level
and deep shear will remain favorable over the warm sector, with
effective-shear magnitudes 50-55 kt. Large low-level hodographs
near the warm front, and beneath the roughly 60-70-kt southerly LLJ,
will enable 300-500 J/kg effective SRH (locally higher). Given the
strong low-level shear, and despite generally unsupportive nature of
both storm mode and leading-edge outflow behavior, isolated,
transient, embedded mesocirculations may persist and strengthen long
enough to offer a brief tornado threat. Damaging gusts also remain
possible.

..Edwards.. 12/27/2018

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...JAN...LIX...LCH...SHV...HGX...

LAT...LON 29479463 30789346 31719291 32109182 31879135 31379108
31069138 30359178 29859241 29669284 29759341 29729382
29599382 29679400 29549438 29479463



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