SPC AC 071242

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0742 AM CDT Wed Oct 07 2020

   Valid 071300Z - 081200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS TODAY OVER
   PORTIONS OF NEW YORK AND NEW ENGLAND...

   ...SUMMARY...
   Isolated marginally severe gusts are possible with thunderstorms
   later today from New York into New England.

   ...Synopsis...
   In mid/upper levels, a large, well-defined cyclone will meander
   around the northern fringes of Hudson Bay, anchoring a broad
   cyclonic-flow regime that covers the north-central to northeastern
   CONUS.  Embedded within that flow, a strengthening shortwave trough
   was apparent in moisture-channel imagery the last few hours over
   southern ON and Lake Huron.  This perturbation should amplify
   further and pivot rapidly eastward across western/central/northern
   NY and much of New England through 00Z, then move offshore before
   06Z.

   At the surface, 11Z analysis showed a wavy frontal zone from
   offshore the Carolinas across northern FL and the northern Gulf
   shelf waters.  This boundary may continue to drift northward in
   segments through the period.  A well-defined low -- related to the
   northern-stream shortwave trough -- was drawn over the ON/QC border
   northeast of Lake Huron, with cold front across western Lower MI,
   central Lake Michigan, southeastern MN, and southeastern ND,
   becoming quasistationary from there to near the northeastern corner
   of MT.  The low should deepen and move eastward over southern QC to
   near the northern border of ME by 00Z, then across parts of the Gulf
   of St. Lawrence overnight.  The cold front will proceed
   southeastward and eastward over the Mid-Atlantic region and New
   England through the period.

   Meanwhile, NHC forecasts depict that Hurricane Delta and associated
   wind fields supporting any tornado potential will remain well south
   of the west-central Gulf Coast through this period.

   ...NY/New England...
   Widely scattered thunderstorms are possible during the daylight
   hours, sweeping roughly eastward across the outlook area.  Isolated
   severe/50-kt+ gusts are possible, as well as sporadic gusts that are
   subsevere but still locally damaging.

   Large-scale ascent is forecast to increase across the outlook area
   today ahead of the mid/upper trough, both as warm advection and
   DCVA, while diurnal heating occurs erratically amidst cloud breaks. 
   Resulting deep-layer destabilization (increasing lapse rates in the
   midlevel and boundary layer) will combine with meager but sufficient
   low-level moisture to yield surface-based buoyancy and minimize
   MLCINH ahead of the main UVV plume.  A swath of 200-600 J/kg MLCAPE
   (decreasing southward with weaker midlevel lapse rates) should
   develop across northern NY and shift eastward, in the presence of
   strong deep-tropospheric wind fields beginning just above the
   surface.  The nearly unidirectional nature of vertical wind profiles
   will limit bulk shear somewhat; however, sufficient speed shear
   still will develop as the trough approaches to yield 35-50-kt
   effective-shear magnitudes.

   The main change to the outlook for this cycle is to add some more of
   ME to the severe probabilities, given forecast soundings and planar
   fields reasonably showing enough diurnal destabilization to permit
   the preconvective field of around 500 J/kg MLCAPE to extend over
   this area briefly.  Some consideration was given to a 15% area,
   given the strength of ambient flow.  However, lack of greater
   low-level instability/moisture, and associated potential for
   convection to be rather shallow and skeletal, still indicates
   organized wind potential is rather conditional at this stage.

   ..Edwards/Goss.. 10/07/2020

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