SPC 1730Z Day 2 Outlook

Day 2 Outlook Image

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1149 AM CST Wed Dec 23 2020

Valid 241200Z - 251200Z


Severe thunderstorms are expected across the Carolinas and Mid
Atlantic region Thursday afternoon and evening. A few severe
thunderstorms are also possible across portions of southeast
Alabama, southwest Georgia, and Florida Thursday.

A strong upper trough with an expansive area of surrounding/cyclonic
flow will shift steadily eastward into the eastern half of the U.S.
Thursday, with a closed low gradually evolving across the
Midwest/Ohio Valley.  Very strong flow aloft will accompany this
system, including mid-level south-southwesterly flow in excess of
100 kt which will spread across the Appalachians into the East Coast
States during the period and an 80 to 90 kt southerly low-level jet
from the Carolinas into New England.

At the surface, a deep low initially near Lake Superior is progged
to fill gradually as it moves northeast across Ontario, to be
replaced by a deepening frontal low which is forecast to develop
over the Carolinas during the afternoon and shift quickly northward
to the Lower Great Lakes region by the end of the period.  A
strong/trailing cold front will sweep eastward across the
Appalachians during the afternoon, and then off the middle and
southern Atlantic Coast overnight.

...The Carolinas to the Mid Atlantic region...
As a strong cold front crosses the mountains during the afternoon,
pre-frontal low-level warm/moist advection will occur east of the
Appalachian crest.  However, weak lapse rates and very limited
potential for any heating will hinder CAPE development across the
warm sector.  Still, sufficient destabilization for thunderstorm
development is expected, and thus expect an increase in convective
coverage and intensity across the region through the afternoon and
into the evening.  

CAM runs continue to indicate that pre-frontal storms will affect
the eastern North Carolina and southeastern Virginia vicinity, in
response to strong low-level warm advection.  Particularly during
the late afternoon, as the upper system advances, evolution of a few
of these storms into supercells appears likely.  Given
intense/veering flow with height spreading across the area, shear
will be quite favorable for both mid-level and low-level rotation. 
As such, potential for tornadoes with this pre-frontal convection
appears sufficient to upgrade to ENH risk/10% tornado probability.

Later, a well-defined band of frontal convection will spread across
the eastern seaboard.  Given the very strong flow through the lower
troposphere, locally damaging winds are expected along the
convective line, along with the possibility for brief QLCS-type
tornadoes.  Risk will likely continue through the evening, with the
convective band expected to eventually clear the coast near or
shortly after midnight.

...Southeastern AL/southern Ga/FL vicinity...
Strong thunderstorms will likely be ongoing at the start of the
period, along an advancing cold front.  Modest instability/weak
lapse rates ahead of the boundary will likely limit storm intensity,
as storms progress eastward.  Still, strong/unidirectional
southwesterly flow aloft is expected across the region, on the
southeastern periphery of the strong upper system.  This suggests
potential for local updraft enhancement -- and associated risk for
locally damaging wind gusts, and possibly a brief tornado.  Storms
will spread across central and eastern Georgia and into the Florida
Peninsula through late afternoon/early evening, with any severe risk
likely to diminish after dark.

..Goss.. 12/23/2020

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