SPC 2000Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0100 PM CST Sun Nov 15 2020

Valid 152000Z - 161200Z


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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