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Friday, October 8, 2021

SPC Oct 8, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0747 AM CDT Fri Oct 08 2021 Valid 081300Z - 091200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM SOUTHEAST GEORGIA INTO NORTHEAST FLORIDA...AS WELL AS PARTS OF THE NORTHERN GREAT BASIN... ...SUMMARY... A few storms capable of damaging wind gusts and marginally severe hail are possible today from southeast Georgia into northeast Florida, as well as parts of the northern Great Basin. ...Synopsis... The upper-air pattern over the CONUS will be dominated by progressive synoptic troughing from the upper Great Lakes to the southern Atlantic Coast States, and western mean trough, each involving several embedded shortwaves. A broad cyclone -- initially centered over northern IL -- is forecast to move slowly northeastward to Lower MI through the period, while devolving to an open-wave trough. As that occurs, the southern part of the broader trough, including a few mesoscale vorticity lobes, will move eastward to the southern Appalachians, GA and northwestern FL by 00Z, then portions of the Carolinas, southeastern GA and northern FL by 12Z tomorrow. In the West, a sharply defined shortwave trough -- apparent in moisture-channel imagery just offshore from the Pacific Northwest and Vancouver Island -- will dig south-southeastward. This feature should move ashore over northern CA tonight, then possibly developing a closed 500-mb low by 12Z over the Sacramento Valley or nearby northern Sierra. As that process occurs, a strong southern- stream shortwave trough -- now just offshore from the CA coastline -- will eject northeastward across central/southern CA and NV, reaching western UT and southeastern NV by 00Z. This trough should proceed northeastward to WY and southeastern MT by the end of the period. At the surface, 11Z analysis showed a wavy, quasistationary frontal zone from near coastal southeastern NC across south-central GA, through a frontal wave near TLH, becoming a weak cold front southwestward across the north-central/west-central Gulf. This boundary will move southeastward across the northeastern/central gulf today then decelerate, while a low persists offshore from the Carolinas. ...Northern Great Basin... Widely scattered thunderstorms, moving northeastward across the area from midday through afternoon, may produce sporadic strong-severe gusts and isolated large hail. The approaching/ejecting southern- stream perturbation will increase both winds aloft and low-level mass response across much of the region. The outlook area will reside beneath the left-front segment of the 500-250-mb jet, and in a zone of both low-level warm advection and strengthening midlevel DCVA/cooling. The large-scale ascent, along with weak but supportive low-level moisture, and modest diabatic/diurnal heating near the north rim of the jet-related cloud plume aloft, will result in favorable instability for thunderstorms this afternoon, with MLCAPE in the 200-700 J/kg range. Mid/upper-level buoyancy will be greater northward to northwestward over the outlook area, into colder air and steeper lapse rates aloft. Meanwhile, increasing vertical shear is expected, with effective shear commonly 35-45 kt amidst strong veering of flow with height. Locally enlarged hodographs also may support some supercell structures amidst multicell clusters and small bows. Diminishing of both low-level instability and large-scale lift this evening should weaken the convection. ...Southern Atlantic Coast... Clusters of thunderstorms should cross the outlook area today along/ahead of the front, beginning with ongoing convection over south-central/southeastern GA, with additional development expected farther south and southwest. Potential exists for isolated, marginally severe hail and/or damaging gusts from water-loaded downdrafts/wet microbursts. A very moist boundary layer over the region will support the convection, in tandem with surface heating and weak large-scale ascent ahead of the mid/upper trough. Surface dew points commonly in the low 70s, PW 1.75-2.25 inches, and mean mixing ratios around 16 g/kg will contribute to enough theta-e to support peak MLCAPE around 2000 J/kg. Low/middle-level flow will remain modest, keeping hodographs small and effective-shear in the 20-30-kt range, though stronger upper/anvil-level flow may aid somewhat in storm organization. Modes should be clustered and mostly multicellular. ...Elsewhere... Isolated to widely scattered thunderstorms are possible over parts of Lower MI ahead of the approaching/weakening mid/upper trough, and the northwestern MN/northeastern ND area ahead of a weak northern- stream perturbation. In both cases, isolated strong gusts or hail approaching severe limits may occur. However, the forecast lack of greater amounts of both buoyancy and low/middle-level flow/shear keeps organized severe potential too conditional for a categorical outlook area at this time. ..Edwards/Gleason.. 10/08/2021 Read more LIVE: spc.noaa.gov
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