SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0650 AM CST Sun Nov 15 2020

Valid 151300Z - 161200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE NORTH-CENTRAL APPALACHIANS AND MID-ATLANTIC...

...SUMMARY...
Thunderstorms with damaging to marginally severe wind gusts will be
possible this afternoon and evening across parts of the north-
central Appalachians and Mid-Atlantic.

...Synopsis...
The main mid/upper-level feature for this forecast is a strong
trough, now apparent in moisture-channel imagery over the mid/upper
Mississippi Valley region.  This perturbation will move quickly
east-northeastward, reaching the lower Great Lakes, upper Ohio
Valley and eastern KY by 00Z.  By 12Z tomorrow, the trough should
reach New England and the coastal Mid-Atlantic region.

The 11Z surface chart showed a strong/986-mb low over the central
U.P. of MI, with trailing occluded/cold front arching across
central/southern Lake Michigan, extreme western portions of KY/TN,
southeastern AR, northwestern LA, and south-central TX to the Big
Bend region and northern Chihuahua.  The surface cyclone will eject
northeastward across the James Bay region to northern QC by the end
of the period.  By 00Z the front should reach eastern portions of
NY/PA/VA, central NC, southwestern GA, and the western/central FL
Panhandle.  By 12Z the front should be offshore all the Atlantic
Coast north of central FL.

...Mid-Atlantic region...
A relatively shallow, strongly forced band of convection is expected
to develop close to the front this afternoon, near the western rim
of the outlook area.  This low-CAPE/high-shear regime -- much of
which may produce no thunder given the truncation of buoyancy below
ideal thermal layer for lightning generation -- should sweep
eastward toward the Mid-Atlantic coast and southwestern New England,
offering strong, locally damaging gusts.  Isolated severe/50+ kt
gusts are possible as well.

This should occur as the zone of strong frontal forcing and cooling
aloft (related to the mid/upper trough) impinges on a marginally
modified Atlantic-origin boundary layer.  Surface dew points should
increase to the mid 40s to low 50s north, and low 60s near the
southern edge of the outlook. Because of the incomplete airmass
modification, and presence of cloud cover, low-level theta-e and
lapse rates each will be modest, barely supporting surface-based
convection.  Forecast soundings generally depict MLCAPE in the
50-250 J/kg range, dropping to near zero north of the outlook area
where elevated MUCAPE of similar values is possible.

Still, strong large-scale and frontal forcings are expected across
the current outlook area (each weakening to the south).  Severe
deep-tropospheric gradient winds should extend down to levels not
far above surface (e.g., 50-70 kt common in the 700-900-mb layer). 
As such, downdraft momentum transfer is possible, with surface gusts
potentially reaching severe limits on at least an isolated basis.
For this outlook cycle, the "marginal" area has been expanded
northward.  This intense wind field conditionally may even support a
rogue strong/damaging gust even from elevated cells farther north,
penetrating a stable boundary layer.  Evidence of any greater degree
of destabilization and/or moistening than currently progged may
prompt a subset area of larger unconditional probabilities in
succeeding outlooks.

..Edwards/Leitman.. 11/15/2020

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