SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0100 PM CST Sun Nov 15 2020 Valid 152000Z - 161200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE MID-ATLANTIC AND SOUTHERN NEW ENGLAND... ...SUMMARY... Severe/damaging wind gusts should continue with low-topped showers and thunderstorms across parts of the Lower Great Lakes to the Northeast and Mid-Atlantic States through early tonight. ...Early Afternoon Update... A low-topped squall line has produced numerous measured severe wind gusts of 60-75+ mph across northern/central OH over the past couple of hours. Current expectations are for this line to continue moving rapidly east-northeastward across northeastern OH into northwestern PA and western NY through the rest of the afternoon. Very strong low-level flow (50-70+ kt) noted in various VWPs will remain present across this region. The main uncertainty is whether sufficient destabilization will occur ahead of the ongoing line given rather widespread cloudiness and showers/rain occurring ahead of it. Still, there appears to be enough clearing occurring just ahead of the line to support robust downward momentum transfer of the very strong flow in low levels to the surface. Have therefore adjusted the Slight Risk for severe/damaging wind gusts to include areas ahead of the low-topped convective line from far eastern OH into northwestern PA and western NY. For more information on the near-term severe threat across this area, see Mesoscale Discussion 1800 and recently issued Severe Thunderstorm Watch 510. A small westward expansion has been made to the separate Slight Risk extending from parts of the Mid-Atlantic into southern New England based on latest observational trends and short-term guidance. Earlier thoughts regarding the development of another low-topped line capable of producing severe/damaging wind gusts later this afternoon/evening across this region remain unchanged. ..Gleason.. 11/15/2020 .PREV DISCUSSION... /ISSUED 1029 AM CST Sun Nov 15 2020/ ...Lower Great Lakes to Mid-Atlantic/Northeast... Scattered thunderstorms are ongoing across southeast Lower MI into western OH with a history of widespread strong to isolated severe wind gusts. This initial activity may struggle to further intensify owing to boundary-layer dew points in the lower 50s within a narrow spatial corridor ahead of this convection. It should eventually outpace this narrow wedge of surface-based instability as it spreads east/north of Lake Erie later this afternoon. A more pronounced squall line is expected to develop around 21-23Z from central NY to central VA and sweep east-northeast across the Northeast and New England coast through early tonight within an intense shear/minimally buoyant environment. This squall should develop as the robust cold front impinges on low-level moisture advecting north from the Carolinas/South Atlantic Coast. Between 00-03Z, mid 50s boundary-layer dew points should reach as far north as eastern PA into southern New England along the track of secondary cyclogenesis. While low-level lapse rates will probably remain weaker relative to the Lower Great Lakes region, the comparatively richer low-level moisture should compensate. It is plausible that lightning generation may be confined to areas farther north amid scant elevated buoyancy and greater large-scale ascent. Nevertheless, the presence of a low-topped squall line amid 50-60 kt 925-mb winds suggest strong to severe wind gusts are possible even in the absence of lightning. As such, this region has been upgraded to Slight Risk.
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