SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0651 AM CST Wed Feb 17 2021 Valid 171300Z - 181200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS NEAR THE CENTRAL GULF COAST... ...SUMMARY... Damaging thunderstorm gusts, a few tornadoes, and perhaps some hail are possible this evening and overnight near the central Gulf Coast. ...Synopsis... In mid/upper levels, a longwave trough will be maintained over the CONUS as a series of subsynoptic perturbations pass across of near its axis over the Plains States. In the southern part, vorticity maxima now over OK and northwest TX will eject northeastward across the mid Mississippi and Ohio Valley regions and weaken. Meanwhile, somewhat separate perturbations over NM will move more slowly eastward through the period, reaching from the Ozarks to north- central TX by 12Z tomorrow. A shortwave trough -- apparent in moisture-channel imagery over the inland Pacific Northwest -- will dig southeastward to the large-scale trough position over extreme northern MX, south of west TX, by 12Z. At the surface, a decelerating cold front was analyzed across parts of south FL, arching westward as a quasistationary front over the east-central/central Gulf, then a warm front west-northwestward to a frontal-wave low. The low was drawn offshore from the lower TX Coast between BRO and rig station KHHV. The low is expected to move northeastward today to just south of the south-central LA coastline, then across the Mississippi River Mouth area between 00-03Z tonight. By 12Z, the low should reach the west-central FL Panhandle or adjoining extreme southeastern AL. The warm front will arc a short distance eastward, then southeastward from the low across the northeastern Gulf, potentially reaching the east-central FL Panhandle/AAF area around the end of the period. A cold front will trail southwestward from the low, moving eastward across the northwestern/north-central Gulf. ...Central Gulf Coast... Thunderstorms should organize by this afternoon over the Gulf near the surface low, trailing cold front, and along an elevated low- level convergence zone extending northward from the low. Activity near the low and cold front will post a threat for damaging gusts, a few tornadoes, and isolated marginally severe hail, as it crosses parts of extreme southeastern LA and coastal areas from MS eastward tonight. Little direct upper support is expected in terms of substantial shots of DCVA or jet-level large-scale forcing, south of the main 250-300-mb southwesterly jet axis, and with the shortwave train remaining well west and north of the area. As such, midlevel lapse rates will remain modest. However, subtle height falls and tightening of height gradients aloft will contribute to favorable deep/speed shear, while persistent low-level warm advection and return flow destabilizes the air mass north of the warm front (in support of elevated convection). From the low eastward then southeastward along the warm front, low-level winds will be most backed for surface-based effective- inflow parcels, relatively maximizing low-level shear/SRH/vorticity. Effective SRH in the 150-250 J/kg range is possible along and just north of the warm front, where forecast soundings also depict 50-60 kt effective-shear magnitudes. Any supercells or well-organized main-line segments interacting with the warm front will pose the greatest tornado potential. The main concern remains instability, where 60s F surface dew points south of the front may contribute to 500-1000 J/kg MLCAPE in the warm sector. However, extent of warm-sector/warm-frontal penetration inland tonight is uncertain, with enough variability around a consensus near the present northern bound of the categorical "slight" probabilities to maintain the general shape of the outlook lines. Two minor shifts are: 1. Eastward on the western end in southeastern LA, where airmass recovery is more in doubt, especially with such intense static stability and offshore flow in place and potentially more southeastward low track, and 2. The eastern portions shifted slightly east, to allow more room for destabilization and associated severe potential in the last few hours of the period (09-12Z). ..Edwards/Smith.. 02/17/2021
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