SPC May 12, 2021 1630 UTC Day 1 Convective Outlook

SPC 1630Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1125 AM CDT Wed May 12 2021 Valid 121630Z - 131200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS TODAY ACROSS PARTS OF FAR SOUTHERN SOUTH CAROLINA...SOUTHEASTERN GEORGIA...NORTHERN FLORIDA AND THE EASTERN FLORIDA PENINSULA... ...SUMMARY... Strong storms may continue to impact parts of the southern Atlantic Seaboard this afternoon, accompanied by at least some risk for hail and localized, potentially damaging wind gusts. ...Synopsis... While blocking remains prominent downstream, and within the large-scale pattern in general, flow over southern Canada and the U.S. is trending a bit more progressive. The remnants of a mid-level closed low, which developed a week or so ago near James Bay, now appear in the process of consolidating with the more prominent mid-level low near/east of the Canadian Maritimes. And an upstream mid-level high, emerging from the Canadian Prairies in preceding days, is now in the process of weakening while shifting east-southeastward across the international border, toward the Upper Midwest/Great Lakes region, while mid-level troughing emerging from the Intermountain West crosses the Rockies into the Great Plains. However, mid-level flow will remain west-northwesterly, and broadly confluent across and east of the Rockies through the Atlantic Seaboard, contributing to the maintenance of expansive cold surface ridging across most areas east of the Rockies. 

A surface frontal zone on the shallow leading edge of this seasonably cold air mass has shifted into/offshore of Texas and southern Mid Atlantic coastal areas. It is forecast to continue advancing southward into northern Florida and offshore of northeastern Gulf coastal areas through this period, with colder/stable surface-based air gradually deepening southward across much of the Gulf and south Atlantic coast states. Seasonably moist air will remain confined to a plume along and south of the frontal zone. However, daytime heating across the Southwest and Great Basin, and beneath seasonably cold mid-level air across the Rockies into Great Plains, as well as across the lower Great Lakes into Northeast, will contribute to weak destabilization associated with steepening lower/mid tropospheric lapse rates. ...

South Atlantic Coast region... Perhaps aided by forcing associated with a weak/subtle perturbation within broadly cyclonic mid-level flow, an organized cluster of storms is ongoing and propagating across southwestern into south central Georgia. This appears near/just to the south of the surface front, with additional storms developing upstream along the trailing outflow across the Florida Panhandle. Convection has generally been weakening the past couple of hours, but modest boundary-layer destabilization (including mixed-layer CAPE increasing to 1000+ J/kg), aided by daytime heating in advance of it, could provide support for renewed intensification as it approaches Georgia/northeastern Florida coastal areas through early afternoon. 

A belt of 30-50 kt westerly winds in the 700-500 mb layer is contributing to modest deep-layer shear and storm motion, but lower/mid tropospheric lapse rates are modest to weak. Coupled with somewhat limited time/area to undergo substantive intensification before spreading offshore, the overall severe weather potential, with locally strong surface gusts the primary hazard, appears marginal. Otherwise, beneath light southerly to westerly lower tropospheric flow, low-level convergence associated with the sea-breezes is expected to become focused near the east coast of the Florida peninsula. Somewhat stronger daytime heating, steepening of low-level lapse rates and CAPE (compared to areas to the north), in the presence of greater deep-layer moisture content, may provide support for a few storms capable of producing locally strong downbursts. ...

Northern Great Plains... Beneath a cold mid-level thermal trough (including 500 mb temps of -22 to -26 C), steepening lower/mid tropospheric lapse rates aided by daytime heating probably will contribute to sufficient destabilization for widely scattered thunderstorm development across the higher plains of eastern Montana and Wyoming into the western Dakotas and Nebraska. Some of the stronger activity may be accompanied by hail and strong wind gusts which could perhaps approach or briefly exceed severe limits. However, due to the marginal nature and expected sparse coverage, severe weather probabilities still appear less than 5 percent. ..Kerr/Moore.. 05/12/2021 Read more LIVE: spc.noaa.gov
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