SPC AC 151500

   Day 1 Convective Outlook AMEND 1
   NWS Storm Prediction Center Norman OK
   0900 AM CST Sun Nov 15 2020

   Valid 151500Z - 161200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS LOWER GREAT
   LAKES...NORTH-CENTRAL APPALACHIANS...AND MID-ATLANTIC...

   AMENDED FOR EXPANSION OF MARGINAL RISK ACROSS THE LOWER GREAT LAKES

   ...SUMMARY...
   Thunderstorms with damaging to marginally severe wind gusts will be
   possible through early tonight from the Lower Great Lakes across
   parts of the north-central Appalachians and Mid-Atlantic.

   ...Lower Great Lakes...
   Have expanded Marginal Risk to reflect the expectation of strong to
   locally severe wind gusts farther west with scattered thunderstorms
   along a powerful cold front sweeping east from the Midwest across
   the Lower Great Lakes through this afternoon. See MCD 1797 for
   further short-term discussion. 

   ...Possible 1630Z upgrade...
   Full outlook update expected by 1630Z with consideration of a Slight
   Risk across the Mid-Atlantic/Northeast with convection rapidly
   progressing across this region during the evening to early tonight.

   ...Previous Discussion issued at 1250Z...

   ...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.

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

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