SPC 1630Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1028 AM CST Thu Dec 31 2020

Valid 311630Z - 011200Z


Severe thunderstorms with tornadoes and damaging winds as the
primary hazards will initially be possible across the Upper Texas
coast and southwest Louisiana through this afternoon, before
expanding across the rest of Louisiana into Mississippi and Alabama
this evening and persisting through tonight.

...Upper TX coast to MS/AL...
A mid-level closed low over western Coahuila will amplify as it
advances north into OK by early Friday, in response to an upstream
shortwave trough digging south-southeast from southern CA to Sonora.
At the surface, a 1004 mb cyclone should track north from the
Middle/Upper TX coast to northeast TX. By 06Z, this cyclone will
become increasingly occluded, yielding a T-shaped surface pattern
with the apex of the warm sector across the Ark-La-Miss as a cold
front sweeps east across the western Gulf and a west/east-oriented
warm front advances north across central portions of MS/AL. The warm
sector characterized by boundary-layer dewpoints in the mid-upper
60s will spread inland to the east of the cyclone and contribute to
MLCAPE of 500-1000 J/kg across extreme southeast TX/southern LA
through this afternoon, and into tonight across central/southern
portions of MS and AL.

Thunderstorm development has been focused near and offshore of the
Upper TX coast this morning, with multiple supercells noted
offshore. Convection will spread northeast through tonight along and
just ahead of the surging cold front. An additional swath of
convection will likely develop separately downstream across the
central Gulf coast by early evening and persist north over eastern
MS and AL through tonight within a robust low-level warm advection

Greater low-level shear and enlarged hodograph curvature will
initially be confined to the TX/LA portion of the outlook region
this afternoon before spatially expanding this evening as flow
fields strengthen downstream of the amplifying low. The combination
of linear forcing for ascent along the cold front and substantial
cross-boundary flow of deep-layer shear vectors will support broken
bands of supercells and line segments, capable of producing
tornadoes and damaging winds. However, the magnitude of the tornado
threat should be tempered some by rather modest low-level lapse
rates inland, and less-than-ideal phasing with the ejecting
mid-level trough and surface cyclone glancing the northwest side of
the warm sector. 12Z HREF and recent HRRR guidance remain subdued
with the intensity and coverage of rotation potential within
simulated cells, lowering confidence for identifying a corridor of
10 percent or greater probability of significant tornadoes.

..Grams/Cook.. 12/31/2020

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