SPC 1200Z Day 1 Outlook
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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