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Tuesday, August 17, 2021

SPC Aug 17, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0745 AM CDT Tue Aug 17 2021 Valid 171300Z - 181200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF NORTHEASTERN GEORGIA INTO THE WESTERN CAROLINAS AND SOUTHWESTERN VIRGINIA... ...SUMMARY... A few tornadoes may occur today from parts of northeastern Georgia into the western Carolinas and southwestern Virginia. ...Synopsis... In mid/upper levels, a strong trough was evident in moisture-channel imagery from southern BC across western WA, then offshore from the OR coastline. This feature is expected to intensify further as it moves east-southeastward and further inland across the interior Northwest today, with a closed 500-mb low forming over WA. By 12Z tomorrow, that low should be located near the WA/OR/ID border confluence, with trough northeastward to southern AB and southwestward across the central/northern Sierra. Other than the cyclonic flow around this system, the main belt of midlatitude westerlies will remain north of the CONUS through the period. As heights fall ahead of the Northwest trough, a weaker perturbation -- initially over NV -- will pivot eastward then northeastward across UT and southern ID, with some elongation/weakening of the vorticity field into late tonight. A positively tilted mid/upper ridge and accompanying anticyclone -- initially just offshore from the Carolinas/Mid-Atlantic and southwestward over northern FL -- will strengthen/expand through the period. As this occurs, the remnants of T.D. Fred will eject northeastward across the southern to central Appalachians region. At 11Z, the surface map depicted a wavy frontal zone from the northern Mid-Atlantic region across Lake Erie and southwestward down the Ohio Valley to the Ozarks region, becoming diffuse northward from there. A separate frontal zone was drawn (along with several attached/weak lows) from northeastern ND across east-central/ southeastern MT to a frontal-wave low in northwestern MT. A cold front extended from there southwestward across northeastern OR and northwestern CA. Cyclogenesis will occur today over central/eastern MT, with the resulting frontal-wave low becoming well-developed over south-central/east-central MT by 00Z. By then, the cold front will extend from the low across southwestern MT, southeastern ID, to central NV. By 12Z tomorrow, the low should reach western ND, with cold front across northern portions of WY and UT. ...GA-VA: Fred... Several supercells and likely tornadoes have occurred the past 6 hours in GA as Fred has weakened from T.S. to T.D., with a few tornado-detection signatures apparent in WSR-88D imagery. Favorable conditions -- both in terms of parameter space and mode (supercells within/ahead of the main spiral band) -- are undergoing net northward shift over central/northern GA. See SPC tornado watch 443 and related mesoscale discussion for the latest near-term info on the GA portion of the tornado threat this morning. Pockets of sustained surface heating under cloud breaks will combined with continued rich low-level moisture transport from the Atlantic and theta-e advection into the western Carolinas to destabilize the boundary layer through afternoon, perhaps reaching southwestern VA into this evening. MLCAPE around 500-1500 J/kg is expected despite weak midlevel lapse rates. This will occur: 1. Conterminous with an increase in low-level shear/hodograph size from southwest-northeast over the outlook area, related to the approach of the downshear/northeast sector of Fred, and 2. Despite weakening in surface winds and rising pressures around Fred's center, with the winds above the surface taking longer to "spin down". However, the overall weakening of the wind fields, combined with nocturnal stabilization, should make the threat more isolated and conditional with northeastward extent this evening and overnight. ...Great Basin/AZ... Isolated to widely scattered thunderstorms should develop this afternoon and evening across higher terrain from the Mogollon Rim northward to the southern/eastern Great Basin and southwestern Colorado Plateau. Isolated severe gusts and marginal hail will be possible. Activity near the Rim will be influenced by the decaying remnants of the Southwestern upper ridge for one more day, with some southwestward/southward and outflow-driven propagation possible to nearby lower deserts. Convection farther north from the Grand Canyon region into UT, ahead of the cold front, will come under progressively greater influence of westerlies and southwesterlies aloft, as heights fall and difluent mid/upper winds spread over the region in advance of the progressive/deepening Northwest trough. Monsoonal moisture in the warm sector and areas of strong diabatic surface heating will minimize MLCINH this afternoon in support of convective growth, atop well-mixed subcloud layers favorable for maintaining hail and accelerating gusts to the surface. ...MT... Isolated to widely scattered, high-based convection (including some thunderstorms) should form over parts of southwestern/south-central MT this afternoon, on either side of the front. The main concern will be for isolated strong/briefly severe gusts. Diurnal heating will steepen low/midlevel lapse rates considerably through the day and generate a deeply mixed boundary layer. A good deal of the existing moisture over the region will mix out, but may still remain in barely sufficient quantities to support MLCAPE near 500 J/kg atop the deep boundary layer. That, along with strengthening southwesterlies aloft east of the Pacific Northwest mid/upper trough, will support marginal severe-gust potential. ..Edwards/Kerr.. 08/17/2021 Read more LIVE: spc.noaa.gov
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