SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0648 AM CST Wed Jan 06 2021 Valid 061300Z - 071200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF SOUTHEAST TEXAS... ...SUMMARY... Severe thunderstorm winds and a couple tornadoes are possible today into tonight along parts of the western Gulf Coastal Plain. ...Synopsis... In mid/upper levels, a highly amplified, progressive synoptic pattern is dominated by: 1. A broad, slow-moving cyclone over the North Atlantic, centered south of NS, 2. Troughing from the length of MB south-southwestward across the Plains states to Chihuahua, and 3. A strong shortwave trough offshore from the Pacific Northwest and northern CA that will contribute to isolated thunder potential near the coast. In the southern part of the second synoptic trough, a strong, basal vorticity max/PV anomaly and accompanying shortwave trough are located initially over eastern NM, based on moisture-channel imagery and objective analyses. This perturbation is forecast to dig southeastward to central TX by 00Z, then eastward to northern LA by 12Z. This will form a closed 500-mb cyclone in the process, while the accompanying 250-300-mb speed maxima blend with a nearly zonal subtropical jet preexisting across northern MX and south TX. As that occurs, a surface low -- initially analyzed near SPS with cold front across the Edwards Plateau -- will move east- southeastward over northeast TX and northern LA, while occluding. By 00Z, the cold front should arc from the low near TYR across east TX then southwestward over the middle/lower TX Coastal Plain and deep south TX. By 12Z, the cold front should extend from a triple point in the SIL/BIX area across the mouth of the Mississippi River to the west-central Gulf. A diffuse/marine warm front will extend eastward close to the AL coast then southeastward over the eastern Gulf, while a synoptic warm front in cooler/less-modified air should extend across the northern/central FL Peninsula. ...Near-coastal TX/LA... Scattered thunderstorms should develop close to the front -- as early as late morning over central TX -- and build southward with time to the middle TX Coastal Plain. The entire resulting, arching MCS should shift eastward and southeastward across the western outlook area through this evening, offering damaging to locally severe gusts, isolated/marginally severe hail, and some tornado potential. A related but separate area of more conditional, marginal severe potential may develop late tonight into early tomorrow morning over extreme southeastern LA and vicinity, as the convection (which may continue in elevated form across the intervening gap) encounters a surge of sufficiently moist, unstable boundary-layer air. With surface dew points now only in the mid 50s to low 60s F across parts of south-central TX, modified RAOBs and RAP soundings already indicate minimal MLCINH, related to a combination of cooling air aloft and lack of a stronger EML. This partially modified warm sector will shift eastward and destabilize further through the day, from a combination of continued theta-e advection and slow diabatic surface heating (blunted by cloud cover). That should support around 1000 J/kg MLCAPE over western parts of the outlook area, decreasing northeastward and eastward over east-central/southeast TX where deep-layer lapse rates will be weaker overall. Effective- shear magnitudes around 40-50 kt and effective SRH in the 100-200 J/k range will support supercells and line-embedded LEWPs/ mesovortices, locally boosting the wind/tornado threat. As the MCS proceeds eastward through the Golden Triangle region into southwestern LA, it will encounter a progressively more-stable boundary layer. This will be related to substantial, isallobarically forced easterly component to surface winds that emanates from (and helps to maintain) a relatively stable continental boundary layer. With convective inflow accordingly becoming elevated, the potential for severe gusts and tornadoes will diminish over land, though severe weather may still affect offshore interests southeast of GLS and south of the western/central LA coastline. Though the most favorable thermodynamic support will remain over the Gulf, non-negligible wind/tornado threat with the remains of the MCS may extend northward across parts of southeastern LA and coastal MS the last few hours of the period. ..Edwards/Leitman.. 01/06/2021
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