SPC 1200Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1148 PM CST Sat Dec 12 2020

Valid 131200Z - 141200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
MIDDLE/UPPER TX COAST INTO SOUTHEAST TX AND CENTRAL/SOUTHERN LA...

...SUMMARY...
Isolated strong to severe thunderstorms may impact parts of the
Upper Texas Coast and southern/central Louisiana on Sunday. Damaging
wind gusts and perhaps a tornado or two are the main threats.

...Middle/Upper Texas Coast into Southern/Central Louisiana...
Shortwave trough currently moving through the Four Corners region is
forecast to continue quickly southeastward, reaching the southern
High Plains by later this morning. This shortwave is then expected
to pivot more eastward across the southern Plains and Lower MS
Valley, while also obtaining a more neutral tilt. This transition to
a more neutral tilt will also be accompanied by strengthening
mid-level flow as well as increasing low-level flow throughout the
warm sector and surface cyclogenesis. 

Surface low resulting from this cyclogenesis will likely develop
along the middle TX Coast before moving northeastward across central
LA and into central MS. This path will limit the inland penetration
of the warm sector, with low 60s dewpoints likely remaining confined
from the middle TX coast in southeast TX during the day. Later this
evening and overnight, low 60s dewpoints will likely reach central
portions of the Lower MS Valley and Southeast states.

Primary forecast concern is whether or not surface-based storms will
be able to establish within the warm sector over southeast TX/Upper
TX Coast. Based on forecast soundings, surface temperatures likely
need to reach the low/mid 70s for surface-based storms, which
appears most probable from the middle TX Coast into southwest LA in
the 18 to 21Z time frame. Given the impressive kinematic profiles,
the expectation is that any warm-sector storms that are able to
persist and maintain updraft continuity should be able to organize
into supercells. Primary threat would be damaging wind gusts, but a
tornado or two is also possible. One additional factor working
against tornadogenesis (in addition to limited instability and a
potentially elevated storm structure) is the anticipated veering of
the low-level winds during the afternoon ahead of the cold front.

A strongly forced, but narrow line of storms is anticipated along
the front in the wake of the earlier warm-sector activity. A few
stronger gusts are possible from the middle TX coast into southern
LA as this line moves through. Low-level stability will likely limit
strong gusts at the surface across much of east TX and
northern/central LA, but potential exists for a few gusts to be 
strong enough to penetrate the stable layer. Overnight, storms
should have a tendency to become more elevated with eastern extent
into the Southeast as the system outpaces the low-level moisture
along the coast.

..Mosier.. 12/13/2020

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