SPC 0830Z Day 3 Outlook
Day 3 Convective Outlook NWS Storm Prediction Center Norman OK 0230 AM CST Thu Dec 09 2021 Valid 111200Z - 121200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE OHIO/TENNESSEE VALLEYS INTO THE SOUTHEAST AND MID-ATLANTIC... ...SUMMARY... Isolated strong to severe thunderstorms may occur Saturday from parts of the Ohio/Tennessee Valleys into the Southeast and Mid-Atlantic. Damaging wind gusts should be the main threat. ...Ohio/Tennessee Valleys into the Southeast... The northern portion of a large-scale upper trough should continue to move east-northeastward across the Midwest, OH Valley, Mid-Atlantic, and Northeast on Saturday. Very strong flow at low/mid-levels will accompany this upper trough over much of these regions. These enhanced southwesterly winds should gradually weaken with southward extent across the Southeast. The primary surface low associated with this cyclone should be located over the Great Lakes Saturday morning. This low is forecast to develop northeastward into Ontario/Quebec by Saturday evening, with a trailing cold front expected to sweep east-southeastward over much of the eastern CONUS. An extensive, broken line of thunderstorms will likely be ongoing at the start of the period extending from portions of the OH Valley across the Southeast and into coastal TX. Both low-level and deep-layer shear should remain strong through at least Saturday morning across these regions, which should support continued thunderstorm organization. The mainly linear nature of this convection suggests that isolated strong to damaging winds should be the main threat as it develops east-southeastward along or just ahead of the cold front. Still, given the strength of the low-level flow, a brief embedded QLCS tornado or two may also occur. Across the OH/TN Valleys into the central Appalachians, the low-level moisture will likely become pinched off against the higher terrain, which may lessen the severe threat to some degree. Greater low-level moisture will be present across the Southeast, but this region will remain displaced to the north of the better ascent associated with the departing upper trough. In addition, the low/mid-level flow is expected to become increasingly parallel to the surface front over the Southeast, especially by Saturday afternoon. This should act to weaken low-level convergence along the front, and the overall severe threat should gradually diminish with southeastward extent. ...Mid-Atlantic... It is not out of the question that a low-topped convective line may form with the eastward-moving cold front along/east of the Blue Ridge Mountains by late Saturday afternoon. Latest guidance suggests that surface dewpoints may reach into the mid 50s to perhaps lower 60s across the warm sector. With mid-level lapse rates expected to remain quite poor, it remains uncertain whether sufficient boundary-layer instability will exist to encourage the strong low-level flow accompanying the upper trough to reach the surface through convective downdraft processes. Regardless, have included low severe probabilities for damaging winds if this scenario unfolds. ..Gleason.. 12/09/2021 Read more LIVE: spc.noaa.gov
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