SPC 1630Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1121 AM CDT Wed Apr 14 2021 Valid 141630Z - 151200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON AND EVENING ALONG THE COASTAL PLAIN OF SOUTHWEST LA AND THE MIDDLE-UPPER TX COAST... ...SUMMARY... Large hail and isolated damaging winds will be possible this afternoon/evening across the coastal plain of southwest Louisiana and the middle-upper Texas coast. ...Northwest Gulf coast through early tonight... Widespread elevated convection has been ongoing over LA through the morning, to the northeast of a rain-reinforced outflow/effective front just inland from the LA coast. The convection is linked to low-level warm advection and a subtle midlevel perturbation now approaching the Sabine River. The 12z LCH sounding revealed MUCAPE greater than 2500 J/kg, though buoyancy decreases with northeastward extent away from the boundary, and as a result of substantial convective overturning. The greater threat for a few severe storms should be focused along the surface boundary (near the southwest/south central LA coast), where another cluster of storms is expected, which will likely keep the boundary near the coast. Occasional large hail and/or damaging gusts will be possible with the stronger embedded storms/supercells through the day. Farther west along the TX coastal plain, 12z soundings reveal that low-level moisture depth has increased and cap strength has decreased over the past 12-24 hours. Lapse rates above the 700 mb level remain quite steep (> 8 C/km), and boundary-layer dewpoints of 72-74 F are driving MLCAPE of 2500-3000 J/kg. Given the weakened cap and the approaching of another subtle midlevel perturbation (now south of the Big Bend), the chance for surface-based storms has increased along a slow-moving front this afternoon/evening. Assuming a few storms form along the middle and upper TX coastal plain, the large CAPE and effective bulk shear of 30-40 kt will support a mix of organized clusters and supercells capable of producing large hail and damaging winds. ...Southern VA/northern NC late this afternoon/evening... A subtle shortwave trough now over western KY/TN will progress eastward toward VA/NC by this evening, around the southeastern periphery of the midlevel low over the upper Great Lakes. A weak surface cyclone is expected to form across the Piedmont in advance of the midlevel trough, and then move eastward through this evening. Modest low-level moisture return across NC into southern VA (boundary-layer dewpoints now in the mid 50s) and afternoon temperatures of 75-80 F will contribute to some destabilization this afternoon. However, there is uncertainty regarding how much vertical mixing will offset moisture advection and limit buoyancy. Even with just a few hundred J/kg of MLCAPE, long hodographs with some low-level curvature will support a threat for somewhat high-based/low-topped supercells capable of producing a few damaging gusts and marginally severe hail for a few hours late this afternoon/evening. ..Thompson/Kerr.. 04/14/2021
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