Severe Storm Outlook Narrative (AC)
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Issued by NWS
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260 ACUS01 KWNS 171300 SWODY1 SPC AC 171258 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0758 AM CDT Fri May 17 2024 Valid 171300Z - 181200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE SOUTHEAST...AND PARTS OF THE NORTHERN PLAINS... ...SUMMARY... Severe thunderstorms may impact parts of the central into eastern Gulf Coast states and portions of the northern Great Plains this afternoon through tonight. ...Synopsis... In mid/upper levels, a progressive, split-flow pattern will shift eastward across the CONUS. In the northern stream, a complex trough over western Canada and the U.S Pacific Northwest should evolve gradually to a closed cyclone over central parts of AB and SK by the end of the period. A basal perturbation -- evident in moisture- channel imagery over WA -- will move eastward to western ND by 12Z tomorrow. This feature will be preceded by 9-12 hours along a similar path, by a weaker (yet still influential) shortwave trough now over the northern Rockies. This perturbation should reach western ND at or shortly before 00Z. The main pattern split occurs around the northwest side of a separate shortwave trough -- extending initially from the Mid-South west-southwestward across the Red River region of southern OK/north TX, then southwestward over the Big Bend region. This trough should shed its northeastern portion, lose some positive tilt, and reach near a LIT-TXK-ALI line by 12Z. A strong belt of west-southwesterly mid/upper winds will extend from the northwestern Gulf and east TX across much of the Southeast. This flow field will host numerous small-scale vorticity maxima (some convectively induced/magnified). At the surface, 11Z analysis showed a wavy, quasistationary frontal zone extending from just off the SC coast across central FL, to the northeastern Gulf, where it has been overtaken by an MCS and accompanying outflow extending to just off the MS coastline. The front was evident again from a weak low near ARA southwestward over the mid/upper TX coastal waters to between MFE-LRD. The western segment of this boundary should move southeastward through the remainder of deep south TX by mid/late afternoon. Meanwhile, the front-reinforcing outflow boundary from the MCS should move northward to northeastward and inland today, gradually becoming more diffuse amidst a broad, ambient, low-level theta-e advection regime. ...Southeastern CONUS... Ongoing convection capable of isolated severe potential includes: 1. A long-lived MCS/bow echo moving east-southeastward from the northeastern Gulf/Apalachee Bay region across coastal northwestern FL, mainly moving over relatively stable surface air along and north of the front. This complex may penetrate strong to isolated severe gusts to the surface the next 1-2 hours before weakening. See SPC mesoscale discussion 813 for near-term details. 2. Isolated to scattered thunderstorms, elevated atop the cold pool from the northeastern Gulf complex, with potential for isolated severe hail for a few more hours. Thunderstorms in the expanding warm sector, and along the front, should pose a risk for a few tornadoes, along with large hail and severe gusts, this afternoon into tonight. The main uncertainty at this time is coverage, and by which preferred mechanism the greatest lift will occur (related concepts). Warm-frontal passage and diurnal heating will combine to destabilize the airmass inland from southwest-northeast today, with upper 60s to mid 70s F surface dewpoints. This will yield MLCAPE in the 2000-3500 J/kg range, beneath 45-60 kt effective-shear magnitudes, once surface-based parcels are attained. A more-focused area of severe potential may develop within the lengthy corridor outlooked, particularly near the inland-shifting baroclinic zone where low-level shear should be maximized. However, mesoscale uncertainties are still too large to introduce greater unconditional severe probabilities at this time. ...Northern Plains... Scattered, high-based thunderstorms are expected to develop this afternoon over the Bighorns and perhaps farther northward/ northeastward over the Plains of southeastern MT. Activity then should sweep east-northeastward across the northern Great Plains into the eastern Dakotas and perhaps northwestern MN tonight before weakening. Along that swath, one or more clusters of cold-pool- driven convection are possible, offering severe gusts. A deep, well-mixed subcloud layer will support locally intense downdrafts, which may also include downward momentum transfer from strong midlevel winds, and which should be the most common in and near the 15%/"slight" probabilities. Surface dewpoints initially analyzed in the 40s to low 50s across the region should lower to the mid/upper 30s and 40s today, as diurnal heating and mixing reduce boundary-layer moisture content. Nonetheless, steep low/middle- level lapse rates will be fostered by the approaching shortwave perturbation and surface heating. Meanwhile, remaining moisture will support surface-based buoyancy across much of the area, with MLCAPE 200-700 J/kg being common. At least loosely organized cold pools should drive this activity across a broad swath of the Dakotas before weakening tonight over or near the Red River of the North. ...South TX... Ongoing areas of thunderstorms over south TX in the SAT-CRP-LRD triangle, and approaching deep south TX from adjoining MX, are moving generally east-northeastward across this region. Activity will be capable of isolated, episodic large hail through midday, amid favorable deep shear and elevated buoyancy, before shifting east of the area. ..Edwards/Goss.. 05/17/2024 $$