SPC 1200Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1127 PM CST Tue Dec 29 2020

Valid 301200Z - 311200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF THE LOWER AND MIDDLE TEXAS COASTAL PLAIN VICINITY THIS AFTERNOON
INTO TONIGHT...

...SUMMARY...
Thunderstorms -- and limited severe potential -- will affect
portions of the Texas Coastal Plain and Hill Country this afternoon
into the overnight hours.

...Synopsis...

A progressive but split flow regime is in place over the CONUS
today. An upper shortwave trough over the Upper Midwest will shift
eastward across the Great Lakes to the Northeast. Meanwhile, in the
southern stream, an upper shortwave trough over northwest Mexico and
the southern Rockies will develop an upper low over northern Mexico,
and pivot eastward toward the Rio Grande in TX by Thursday morning.
Another shortwave trough will move over the Pacific Northwest
overnight, and could bring isolated thunderstorms to the WA/OR
coast. 

...Middle and Lower TX Coastal Plain vicinity...

A southeastward-advancing cold front will extend from northwest AR
into southeast OK and towards the Edwards Plateau of TX. Low level
southerly flow will continue to transport Gulf moisture northward
ahead of the front. Instability will be limited however, due to
ongoing showers and thunderstorms and general cloudiness precluding
stronger insolation ahead of the front. Additionally, a warm layer
is evident around 700 mb in most forecast guidance and point
forecast soundings. This warm layer is also evident in 00z RAOBs.
While cooling aloft will occur, it likely will be ill-timed with
frontal passage. As a result, this should act to limit updraft
intensity. Nevertheless, strong shear will be in place, an a couple
of storms capable of marginally severe hail are possible across
parts of the Edwards Plateau to the middle and lower TX coast. 

During the evening and overnight hours, a surface low will develop
east/northeast along the lower and middle TX coast. As this occurs,
low level winds will become more southeasterly, resulting in
favorably curved low-level hodographs supporting rotating storms.
This threat should remain confined closer to the coast and just
ahead of the cold front. While instability will remain limited,
favorable low-level and bulk shear could result in a couple of
storms capable of producing a tornado or locally damaging wind
gusts.

..Leitman.. 12/30/2020

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