SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0248 PM CDT Wed Apr 14 2021 Valid 142000Z - 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. ...20Z Update... ...Northwest Gulf coast through early tonight... Storm coverage is expected to continue increasing along the TX Coastal Plain this afternoon. The environment is characterized by strong buoyancy and moderate vertical shear, supporting the potential for strong to severe thunderstorms. Primary severe threat is large hail and damaging wind gusts, and Severe Thunderstorm Watch 95 was recently issued to address these threats. Thunderstorms will likely persist in this area through this evening when the loss of diurnal heating leads to gradually diminishing strength and coverage. Elsewhere across the region (i.e. southern LA), a strong storm or two may be possible over the next few hours. ...Southern VA/northern NC late this afternoon/evening... A few strong to severe thunderstorms are still possible this afternoon and evening across the region amid modest destabilization and moderate vertical shear. Limited buoyancy will likely keep most storms from becoming severe but a few supercells capable of damaging wind gusts and/or hail may develop. ..Mosier.. 04/14/2021 .PREV DISCUSSION... /ISSUED 1121 AM CDT Wed Apr 14 2021/ ...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.
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