SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0650 AM CST Thu Nov 12 2020

Valid 121300Z - 131200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
CENTRAL/EASTERN PARTS OF THE CAROLINAS...

...SUMMARY...
A few strong to severe storms are possible across central/eastern
parts of the Carolinas.

...Synopsis...
The mid/upper pattern initially will be characterized by broadly
cyclonic flow cross much of the CONUS, becoming more zonal through
the period, except near substantial shortwave perturbations.  One
such perturbation is located over the central Rockies.  This feature
is forecast to move eastward across the central Plains today and
this evening, phasing with an initially separate trough now over the
Dakotas.  The combined trough should reach eastern Lake Superior,
western Lower MI and eastern IN by 12Z tomorrow.  A more-subtle
perturbation -- evident in moisture-channel imagery over portions of
GA and the western Carolinas -- is expected to move slowly eastward
today, then offshore from the Carolinas around 00-03Z.

As heights fall over the central/southern Appalachians in advance of
that mid/upper trough, T.S. Eta is forecast by NHC to continue
weakening across northern FL today, then accelerate northeastward
over the adjoining Atlantic this evening.  Some re-strengthening is
possible as a post-tropical/extratropical cyclone offshore NC into
day-2.  Late in this period, the cyclone may link with an outflow
boundary or outflow-reinforced iteration of the synoptic surface
baroclinic zone shifting offshore from NC.  See latest NHC
advisories for forecast track/intensity guidance on Eta.

At the surface, 11Z analysis showed a synoptic cold front from
southern New England across north-central/west-central NC,
southwestern GA, and the north-central Gulf.  This front is progged
to proceed eastward to a 00Z position offshore from New England and
the Mid-Atlantic, extending from there over southeastern NC, eastern
SC, and Apalachee Bay.

...Eastern Carolinas...
Numerous showers and scattered thunderstorms are possible across the
outlook area today, generally moving northeastward as the regime
shifts eastward over the area.  The most intense/sustained cells may
produce damaging winds approaching severe limits, and a tornado
cannot be ruled out.

Most of the closed circulation envelope of Eta is forecast to remain
offshore, with some of the northern periphery brushing the coast. 
Its effects on severe potential over this region are largely
indirect -- mainly in terms of:
1.  Gradient flow helping to maintain a pre-existing, warm-sector
fetch of rich low-level moisture/theta-e over the outlook area,
2.  Related strengthening of a low-level convergence zone now
evident to its northeast, from southern SC south-southeastward over
the Atlantic, which will move north-northeastward and intersect a
separate, pre-existing convective regime today over the SC part of
the outlook area, and
3.  Subtly increasing low-level shear over coastal NC/northeastern
SC in particular prior to its passage abeam of the coastline, and

In a very moist/low-LCL setting, hodographs and low-level shear
vectors are forecast to enlarge to levels marginally supporting
storm-scale rotation over the coastal plain.  Time series of
forecast soundings indicate effective SRH in the 100-200 J/kg range,
locally/briefly higher, before low-level flow veers from northwest-
southeast this afternoon and evening as the mid/upper trough passes.
Cloud breaks may foster patches of localized heating sufficient to
support 1000-1500 J/kg peak MLCAPE, despite modest, nearly moist-
adiabatic low/middle-level lapse rates.  Abundant cloud cover
related to both Eta and separate convection over the Piedmont may
keep the most-favorable diabatic heating patchy and short-lived in
nature.  The lack of greater low-level lapse rates, and potential
for messy/outflow-driven convective modes, renders severe potential
isolated and conditional with this regime.  After 00Z, supercells
may be enhanced where the outer northeastern/downshear rim of Eta's
circulation interacts with any outflow/baroclinic boundary, which is
currently shown to be offshore in most higher-res guidance.

..Edwards/Kerr.. 11/12/2020

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