Tornado! Watch Issued for Most of Louisiana - Nocturnal Tornado Threat for Louisiana

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SPC has highlighted a large portion of Louisiana for the risk of tornadoes. These may occur after dark.




Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1129 AM CDT Wed Oct 31 2018

Valid 311630Z - 011200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THROUGH TONIGHT
FROM SOUTHEASTERN TX TO SOUTHWESTERN MS...

...SUMMARY...
The greatest risk for severe thunderstorms, capable of producing
damaging winds, hail, and tornadoes, exists from southeast Texas to
the lower Mississippi Valley today through tonight.

...TX today to LA/MS overnight...
A midlevel shortwave trough now near El Paso will continue eastward
across TX through tonight. Weak cyclogenesis is expected by this
evening and continuing through tonight, along a pre-existing front
from central TX to the Mid South. A moist low-level air mass is
already present south of the front, with low 70s boundary-layer
dewpoints spreading across the northwest Gulf coast. Gradual mass
response to the cyclogenesis will result in strengthening low-level
flow/shear, especially tonight across LA/MS.

Ongoing convection in TX has largely remained elevated atop the
frontal surface, but may gradually transition to surface-based
storms along the front later this afternoon. Isolated strong-severe
storms will also be possible this afternoon/evening into southwest
TX, along the west edge of the richer low-level moisture and in the
zone of stronger low-midlevel ascent preceding the shortwave trough.
Farther east, the ongoing warm sector convection across the upper TX
coast should persist through the afternoon and spread slowly
eastward/northeastward. Additional warm sector storm development is
also expected tonight along the moist axis into southern LA.

Wind profiles are only marginally favorable for organized/supercell
storms at present across TX, with slow strengthening of vertical
shear expected later this afternoon through tonight farther east
toward the lower MS Valley. A few supercells could evolve from the
ongoing upper TX coast convection, as well as embedded
supercells/bowing segments along the front from central into east
TX. This frontal convection will likely evolve into a line that
will move eastward overnight, with an accompanying threat for
damaging winds and a couple of tornadoes with embedded circulations.
The more substantial supercell/tornado threat is expected overnight
into parts of LA (and possibly MS) within the moist sector, and
where low-level shear becomes the strongest. A strong tornado or
two will be possible, especially with any pre-frontal supercell
clusters, where effective SRH increases to greater than 400 m2/s2 in
a moist environment with sufficient surface-based buoyancy.