SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

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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