SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0649 AM CST Thu Dec 24 2020

Valid 241300Z - 251200Z



The main threat for severe thunderstorms will be across portions of
the Carolinas and Mid-Atlantic region this afternoon and evening,
with tornadoes and damaging gusts possible.

In mid/upper levels, broadly cyclonic flow will shift eastward from
the central to eastern CONUS through the period, in step with a
high-amplitude synoptic trough.  As one associated shortwave eject
northeastward across the upper Great Lakes, an upstream perturbation
-- now evident in moisture-channel imagery over the MB/ON border --
will dig southeastward.  Another perturbation initially over central
TX will pivot eastward to the southern Appalachians.  As this
occurs, a closed 500-mb cyclone should develop over the lower OH
Valley region during the 00-06Z time frame, its center moving
eastward past SDF before 12Z tomorrow. 

At the surface, the primary, occluded synoptic low over the eastern
Lake Superior region will fill and eject northeastward across
northern ON.  The associated cold front -- initially drawn across
parts of OH, eastern KY/TN, AL, to near the Mississippi River mouth,
will move eastward across the southern/central Appalachians through
this evening.  As the mid/upper cyclone closes to the west, a weak,
closed area of low pressure over the eastern AL/northwestern GA
region should move up the front to the eastern WV/southwestern PA
corridor by 00Z, becoming the primary low over NY tonight and
reaching southern QC by 12Z.  By then, the trailing cold front will
reach eastern PA, then offshore from the remainder of the CONUS
Atlantic Coast, having crossed south FL around 06Z. 

...Eastern/central Carolinas and vicinity...
Widely scattered to scattered thunderstorms are expected to cross
this region today into this evening in two primary regimes:
1.  Ahead of the cold front, moving northward to north-northeastward
from a favorably moistening/destabilizing marine boundary-layer
region over the Atlantic.  This convection will cross lesser but
still sufficient/surface-based buoyancy across the eastern
Carolinas, as the warm sector modifies and becomes more unstable,
amidst strengthening deep shear.  Warm-sector dew points in the 60s
will offset modest midlevel lapse rates enough to contribute to
MLCAPE generally in the 500-1000 J/kg range from eastern NC across
the southern sounds/Outer Banks of NC, locally higher near the
southern NC coastline.  Discrete supercells are possible, offering
tornadoes and isolated damaging wind.  Then...
2.  A near-frontal band of thunderstorms will sweep eastward to
east-northeastward over the region, presenting a more-dominant wind
threat, but with some line-embedded/QLCS tornado threat as well.  

Height falls and tightening height/thermal gradients aloft are
expected to spread over the region near and just ahead of the front
throughout the day, as the synoptic trough deepens to the west.  A
65-75 kt LLJ will develop and shift across this region, leading to
uncommonly similar, 40-50-kt magnitudes of shear vectors in the 0-1,
0-3, and effective layers. Despite the nearly unidirectional,
southerly low/middle-level flow, even very slight hodograph
curvature contributes to 200-350 J/kg effective SRH in forecast

The air mass will become still more stable with northward extent
past a diffuse warm front that will move northward over the
Delmarva/Mid-Atlantic this evening and tonight.  However, at least a
marginal wind and tornado threat appears to be justified, given the
strength of the ambient deep-layer flow and at least weak
instability near the surface.

See the remainder of tornado watch 517, mesoscale-discussion updates
for that, and separate mesoscale discussion 1372 for more details on
the severe threat with ongoing convection in the near-frontal squall
line and warm sector over the FL Panhandle region.
The main belt of convection will sweep eastward across the remainder
of the outlook area through this evening, presenting a threat for
sporadic damaging gusts and a tornado or two.  Moisture and buoyancy
will continue to be greatest over open Gulf waters near the Loop
Current, where marine thermal/moisture fluxes optimize boundary-
layer theta-e.  Still, at least marginally favorable thermodynamic
conditions will exist in a northward-narrowing corridor ahead of the
main convective line, as partly modified parcels advect from the
Gulf.  This air mass, characterized by modest lapse rates but also
minimal MLCINH, also may support maturation of cells developing in
the warm sector before they cross into more-stable air.

..Edwards/Smith.. 12/24/2020

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