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Wednesday, October 27, 2021

SPC Oct 27, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0258 PM CDT Wed Oct 27 2021 Valid 272000Z - 281200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF SOUTHERN LOUISIANA AND MISSISSIPPI... ...SUMMARY... Scattered severe thunderstorms capable of producing damaging winds and tornadoes, some of which could be strong this afternoon, will continue spreading eastward across Louisiana to southern Mississippi through this evening, and across the northeast Gulf Coast overnight. ...20Z Update... Primary change with this update was to remove severe probabilities behind a line of storms moving eastward across southern LA. The environment ahead of this activity over southern/eastern LA into southern MS will remain favorable for organized severe convection, including supercells across the open warm sector. Based on recent radar and observational trends, the Enhanced Risk for wind has been expanded slightly northward to include a little more of southern MS. Both damaging winds and tornadoes will continue to be a threat given the strong low-level shear that is present. The best chance for a couple of strong tornadoes this afternoon should exist with any supercells that can form ahead of the line across mainly southeastern LA and coastal MS. This severe threat is still expected to shift eastward this evening and tonight over southern AL and the FL Panhandle. Additional development of a low-topped line of storms along an occluding cold front still appears possible across parts of eastern OK into AR this afternoon. Isolated gusty winds would be the main threat if these storms develop, but weak instability given widespread cloud cover will likely limit the overall severe risk. The only other change to the outlook was to expand the Marginal Risk for strong winds and perhaps a brief tornado to include more of the immediate Gulf Coast of the FL Peninsula for the last few hours in the Day 1 period (around 08-12Z early Thursday morning). Recent guidance suggests upper 60s to low 70s surface dewpoints should move onshore in this time frame, supporting weak boundary-layer instability in the presence of sufficient low-level and deep-layer shear. ..Gleason.. 10/27/2021 .PREV DISCUSSION... /ISSUED 1121 AM CDT Wed Oct 27 2021/ ...Northern Gulf coast through tonight... A midlevel trough over the southern Plains this morning will move eastward toward the lower MS Valley by early Thursday, while evolving into a closed low. An associated surface cyclone will likewise move slowly eastward while occluding across AR tonight, as a cold front translates eastward from east TX across LA to MS. To the east, a warm front will move northward across southern LA/MS today and southern AL/FL Panhandle by tonight. A maritime tropical air mass is present south of the warm front, with boundary-layer dewpoints in the low-mid 70s, which is contributing to moderate buoyancy (MLCAPE of 1000-2000 J/kg this morning across the northwest Gulf coast. The stronger inland destabilization is expected today across southern LA as the tropical air mass spreads inland beneath the east fringe of the steeper midlevel lapse rate plume through the afternoon as the warm sector spreads northward across southern LA/MS. Wind profiles are favorable for tornadic supercells along the north edge of the warm sector, with effective bulk shear of 40-50 kt and effective SRH of 200-300 m2/s2, per recent soundings/VWPs and short-term model forecasts. The potential for occasional discrete supercells in the warm sector will spread eastward across southern LA, with an attendant threat for a couple of strong tornadoes near the warm front. Within the pre-frontal squall line, damaging winds can be expected with bowing segments, and a tornado threat will also persist with embedded mesovortices. A similar regime will spread eastward to southern MS/AL and the FL Panhandle this evening into tonight, though with slightly weaker buoyancy after the diurnal heating cycle. Read more LIVE: spc.noaa.gov
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