SPC 1200Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1155 PM CDT Sat Oct 31 2020

Valid 011200Z - 021200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
AND EVENING ACROSS THE NORTH CAROLINA COAST AND OUTER BANKS
VICINITY...AS WELL AS PORTIONS OF EAST CENTRAL FLORIDA...

...SUMMARY...
Strong storms may impact parts of the North Carolina coast and Outer
Banks vicinity, as well as parts of east central Florida, this
afternoon and evening, accompanied by at least some risk for severe
weather.

...Discussion...
Within converging belts of westerlies emanating from the
mid-latitude Pacific, broad, amplified mid/upper ridging is forecast
to continue to gradually shift eastward across much of western and
central North America.  With some further building of this ridging
possible across and inland of the Pacific coast, significant
downstream troughing may also amplify further while progressing into
and across the Atlantic Seaboard today through tonight.  As it does,
an associated 100 kt mid-level jet streak initially digging to the
west of the positively tilted trough axis is forecast to shift to
the east of the trough and intensify further, with the trough axis
taking on more of a neutral to negative tilt across the Atlantic
coast by 12Z Monday.  This will be accompanied by a significant cold
front, and strong surface cyclogenesis from coastal Maine toward the
Canadian Maritimes late tonight into early Monday.

Preceding the cold front, a cold surface high is now shifting east
of the Atlantic coast.  Ridging trailing to southwest lingers to the
east of much of the Appalachians, but is forecast to weaken early
today, with a warm frontal zone advancing inland across the southern
Mid Atlantic coast region.  Coinciding with low-level moisture
return off the Atlantic, and increasing lift ahead of the
approaching trough, models suggest that this will be accompanied by
an expanding area of stratiform precipitation across the Carolina
Piedmont toward the northern Mid Atlantic coast through the day. 
However, east of deepening lee surface troughing, a narrow plume of
low-level warming, aided by daytime heating, may overspread the
Carolina coastal plain, before shifting offshore during the 18-22Z
time frame.  At some point, this appears likely to coincide with
substantive boundary-layer moistening, but it remains unclear due to
model spread whether this will occur inland, near coastal areas, or
mostly offshore.

At the same time, after initially shifting northward into parts of
northern Florida and the northern Florida Peninsula, the lead front
is forecast to return southward, before being overtaken by the
stronger trailing cold front by tonight, preceded by a narrow plume
of seasonably moist air.

...Southern Mid Atlantic...
Marginal severe probabilities will be maintained near coastal areas
of North Carolina and southeastern Virginia.  Uncertainty associated
with continuing model spread precludes increasing severe
probabilities, but destabilization depicted by the NAM and Rapid
Refresh suggests potential for this across the Outer Banks vicinity
of North Carolina.  These models suggest that the return of mid 60s
surface dew points across this region will contribute to mixed-layer
CAPE up to around 1000 J/kg by 20-21Z, despite modest to weak lapse
rates.  This may coincide with considerable strengthening of
lower/mid tropospheric wind fields (40-70+ kt) and shear (including
sizable, clockwise-curved low-level hodographs), and large-scale
ascent, aided by increasing divergence aloft and low-level warm
advection.  The environment may become conducive to the development
of supercells posing a risk for severe hail, locally strong surface
gusts and  perhaps a tornado, before spreading offshore late this
afternoon.

...East central Florida...
Models suggest that locally enhanced convergence, near the
intersection of the southward advancing lead front and surface
troughing over the interior Peninsula, may become a focus for
isolated strong storm development by early this evening.  In the
presence of a seasonably moist environment with CAPE in excess of
1000 J/kg, deep-layer shear on the southern-most fringe of the
westerlies may become sufficient to support supercell structures. 
This activity could pose a risk for severe hail and wind around the
Orlando area into the Space Coast, before spreading offshore later
this evening.

..Kerr/Elliott.. 11/01/2020

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