SPC 1200Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1104 PM CST Thu Dec 03 2020

Valid 041200Z - 051200Z


Strong thunderstorms may impact coastal areas of the Florida
Panhandle today, and parts of the North Carolina coastal plain
tonight, accompanied by at least some risk for severe weather.

Much of North America will remain under the influence of relatively
complex, amplified branching flow, downstream of a strong zonal jet
across the southern mid-latitude western and central Pacific.  

Within this regime, positively tilted large-scale mid-level ridging
likely will remain prominent (though with an embedded area of lower
heights), across the northwestern U.S. and western Canada. 
Positively tilted troughing will also linger to its southeast,
across the southern Great Plains into the Southwest and Mexican
Plateau, although the center of a broadening cyclonic circulation
approaching the Texas Big Bend may become its most significant

More prominently, one or two vigorous short wave perturbations are
forecast to continue digging into and through the base of
larger-scale mid-level troughing across the Great Lakes into Ohio
Valley region.  As this occurs, models indicate that an elongated
mid-level circulation emerging from the southern Great Plains will
pivot eastward across the Mid South, before becoming increasingly in
phase with the larger-scale troughing as it continues across the
southern Appalachians into the southern Mid Atlantic coast.  It
appears that strong large-scale forcing for ascent associated with
these developments may contribute to rapid, significant surface
cyclogenesis, from the lee of the southern Appalachians by late this
evening into areas off the Mid Atlantic coast by daybreak Saturday. 

This will be accompanied by considerable strengthening of deep-layer
wind fields, contributing to sufficiently strong shear to
potentially contribute to a risk for severe thunderstorms across a
sizable portion of the Southeast and Mid Atlantic, given sufficient
low-level moistening and destabilization.  However, conditions
across much of this region remain generally cool/dry and stable,
with the air mass yet to begin to substantively destabilize off the
south Atlantic coast, and still in the process of modifying over the
north central into northeastern Gulf of Mexico.  As a result, it
appears that severe weather potential will remain relatively low and
confined mostly to coastal areas of the Florida Panhandle and North
Carolina, if not the offshore waters.

...Florida Panhandle...
NAM/Rapid Refresh forecast soundings suggest that lower/mid 60s
surface dew points may advect into immediate coastal areas early
today.  Despite generally weak lapse rates, it appears that this
could contribute to mixed-layer CAPE on the order of 500+ J/kg, in
the presence of strengthening westerly mid/upper flow.  While
deep-layer shear likely will become strong, it appears that
low-level wind fields will remain modest in strength, and contribute
to generally unidirectional shear profiles.  It is possible that
scattered thunderstorm development initiating offshore during the
day could spread inland and pose a marginally severe hail and wind
threat before weakening.

...North Carolina coastal plain vicinity...
Considerable spread is evident among the various model output
concerning the timing of more rapid surface cyclone deepening.  In
addition to the potential impacts on possible low-level hodograph
structure, this contributes to additional uncertainty concerning 
inland warm sector boundary-layer moistening and destabilization.  A
"worst case" scenario for surface cyclone evolution and associated
severe weather potential could include a supercell tornado threat
across eastern North Carolina and adjacent portions of South
Carolina/southeast Virginia.  However, based on the bulk of model
output, including the convection allowing guidance, appreciable
severe weather potential with vigorous thunderstorm development will
probably evolve east of the North Carolina coast, but could perhaps
impact the Outer and Inner Banks vicinity of North Carolina during
the 05/03-09Z time frame.

..Kerr/Lyons.. 12/04/2020

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