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Saturday, October 16, 2021

SPC Oct 16, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0752 AM CDT Sat Oct 16 2021 Valid 161300Z - 171200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF THE MID-ATLANTIC AND NEW ENGLAND... ...SUMMARY... Damaging to severe thunderstorm gusts and a marginal tornado threat are possible through the afternoon over portions of the Mid-Atlantic and New England. ...Synopsis... A highly amplified -- yet still progressive -- mid/upper-level pattern will affect the CONUS through the period. A deep trough -- initially located from far northern ON across the upper Great Lakes to AR -- will shift eastward to western QC, the inland Mid-Atlantic, the Carolinas, and GA by the end of the period. As this occurs, the basal shortwave trough -- apparent in moisture-channel imagery from western OH to the Mid-South -- will pivot northeastward and accelerate, reaching Lake Ontario, central PA and central VA by 00Z. This perturbation then will eject northeastward across QC and New England overnight while weakening, and while other, trailing shortwaves move through the base of the larger-scale trough. Synoptic ridging will cross the Rockies, while a sharp trough approaches the Pacific Northwest (but still offshore at 12Z tomorrow). At the surface, 11Z analysis showed a cold front from Lake Ontario across western NY, northwestern PA, WV, southwestern VA, to southern AL and the northwestern Gulf. By 00Z, the front should reach western New England, coastal NJ/Delmarva, the eastern Carolinas, and northern FL. By 12Z, the cold front should be over CONUS land only in ME and central FL, offshore elsewhere. ...Portions of the Mid-Atlantic and New England... A low-CAPE/strong-shear convective event is expected in the Northeast today, with the delineation between threat categories realistically fuzzier than can be depicted by a sharp categorical contour line on a map. A frontal/prefrontal band of thick clouds and precip, with isolated embedded thunderstorms, is ongoing from portions of northern/western NY and Lake Ontario across western PA and WV to northeastern TN. Through the remainder of the morning and into early afternoon, convective organization and coverage each should grow, with a gradual increase of the severe-wind and tornado threat from initially marginal and very isolated nature now. The convective band will encounter a warm sector undergoing gradual destabilization, from both theta-e advection and muted diabatic heating. This will help to reduce already weak MLCINH and offset modest mid/upper-level lapse rates just enough to support peak/ preconvective MLCAPE of 250-750 J/kg. Very long and somewhat curved low-level hodographs -- larger with northward extent -- may lead to 0-1-km and effective SRH each in the 150-250 J/kg range, amidst effective-shear magnitudes ranging from 45-50 kt in southern parts of the outlook to 55-60 kt in the north. This will support some storm-scale rotation -- be it discrete to semi-discrete supercells or line-embedded mesovortices. Overall severe potential should wane this evening as already weak low-level instability diminishes further. ..Edwards/Leitman.. 10/16/2021 Read more LIVE: spc.noaa.gov
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