SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0646 AM CST Wed Dec 30 2020 Valid 301300Z - 311200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON THROUGH TONIGHT FOR PARTS OF SOUTH AND SOUTHEAST TX... ...SUMMARY... Isolated large hail, damaging winds, and a tornado or two will be possible this afternoon through tonight across parts of south and southeast Texas. ...South TX later today into tonight... A midlevel shortwave trough over northwestern Mexico will move eastward over northern Mexico and evolve into a closed low south of the Big Bend by Thursday morning, as an upstream trough digs southeastward over the Pacific coast. In the wake of a northern stream shortwave trough, a surface cold front will move southeastward across central TX today, per observed pressure rises this morning from OK into west TX. The movement of the front will slow tonight in response to cyclogenesis near the middle and lower TX coast, in the zone of ascent downstream from the northern Mexico closed low. This pattern will also maintain a 30-40 kt southerly low-level jet over the TX coast, and a continued inland transport of mid-upper 60s boundary-layer dewpoints. A few thunderstorms are expected today along the cold front from central into northeast TX, on the north edge of the richer low-level moisture and at least weak buoyancy. Gradual moistening and weakening of a lingering midlevel cap will allow for more widespread thunderstorm development tonight along the front/near the developing surface low, and along a commonly observed pre-frontal confluence zone from the northwest Gulf of Mexico into the middle TX coastal plain. The threat for supercells and a tornado or two will increase near the end of the period in the confluence zone near the coast, as deep-layer and low-level shear increase in the warm sector. Otherwise, a band of convection will likely form across south TX along the surging cold front overnight, with the potential for isolated damaging gusts and marginally severe hail. ..Thompson/Gleason.. 12/30/2020
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