SPC 1630Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1030 AM CST Wed Dec 30 2020

Valid 301630Z - 311200Z


Isolated severe hail and damaging winds, along with a brief tornado
will be possible through tonight across parts of east-central to
Deep South Texas.

...East-central to Deep South TX...
No appreciable change appears needed to the cat 1/MRGL risk area,
although an adjustment has been made to expand the underlying
wind/tornado probabilities for this afternoon's convective threat in
east-central/southeast TX.

A shortwave trough over western Chihuahua will gradually progress
east-southeast towards the Coahuila/Durango border area through 12Z
tomorrow. In the wake of a northern stream shortwave trough, a
surface cold front will move southeast across eastern to south TX
through the period. The movement of the front will slow later
tonight in response to weak cyclogenesis near the lower TX, in the
zone of ascent downstream from the northern Mexico trough. This
pattern will also maintain a broad southerly low-level jet across
the TX coast that should diurnally weaken through sunset before
strengthening again late tonight, with a continued inland transport
of mid to upper 60s boundary-layer dewpoints.

The severe threat will probably occur within two phases, one this
afternoon across a portion of east-central/southeast TX and the
other overnight towards the middle TX coast. A pre-frontal
confluence arc of convection is ongoing just ahead of the impinging
cold front across central TX. A few cloud breaks ahead of this arc
should yield thin MLCAPE increasing to between 500-1000 J/kg. While
guidance generally suggests low-level flow will subside somewhat
this afternoon (with the RAP appearing more aggressive than the
NAM/ECMWF), modest hodograph curvature might prove sufficient for a
lower-end supercell or two capable of producing all hazards. The
undercutting nature of the front will further limit longevity of any
surface-based supercell. After a potential lull in the severe threat
this evening, the risk for a severe storm or two should slowly
increase after 06Z tonight as deep-layer and low-level shear
gradually strengthen in the confined warm sector near the lower to
middle TX coast. The bulk of a greater tornado threat should still
await until after 12Z in the D2 period.

..Grams/Dean.. 12/30/2020

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