SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0739 AM CDT Thu Oct 29 2020

Valid 291300Z - 301200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM A SMALL
PART OF THE FLORIDA PANHANDLE INTO NORTH CAROLINA AND SOUTHERN
VIRGINIA...

...SUMMARY...
Isolated severe gusts and a tornado or two are possible today from a
small part of the Florida Panhandle into North Carolina and southern
Virginia.

...Synopsis...
In mid/upper levels, a series of northern-stream shortwaves will
traverse a more broadly cyclonic flow field over the Upper Midwest
and Great Lakes, leading to height falls that will shift eastward
across the mid/upper Mississippi then Ohio Valleys.  That will
contribute to the eastward acceleration and diminution of the
initially well-defined, cold-core cyclone centered over the
southeastern OK/northeast TX region.  By 00Z, the associated 500-mb
low should reach the HSV-BNA corridor, with the trough south-
southwestward over western AL and southeastern MS.  By 12Z tomorrow,
the resultant shrunken, open-wave trough should be moving rapidly
offshore from the Tidewater NC/VA region.

In response to these developments, what now is Tropical Storm Zeta
will continue to weaken and race northeastward per the NHC forecast,
while entrained in fast, somewhat confluent mid/upper flow on the
synoptic scale.  The associated surface low -- analyzed on the 11Z
chart over extreme southeastern TN -- should race offshore of
NJ/Delmarva by 00Z, attaching itself to a frontal zone that will
extend southwestward across VA, the Carolinas, GA, and the FL
Panhandle.

...Southeast...
Fast-moving convection trailing southeast and south of the remnants
of Zeta will offer the potential for a tornado or two, as well as
damaging to locally/isolated severe gusts, with the threat shifting
northeastward over the outlook area from now through afternoon.

Relatively backed surface winds and enlarged low-level hodographs
will shift northeastward out of the northeastern Gulf coastal region
this morning, and over a modestly but favorably destabilizing warm
sector in the Carolinas and southern VA.  As this occurs, the
remaining tornado potential will translate likewise.  As Zeta's
surface low accelerates across the southern Appalachians, VA and the
Chesapeake Bay region through the day, flow to its south in the
most-favorable warm sector air will veer, reducing low-level
hodograph curvature and rendering low/midlevel wind vectors more
parallel.

However, diabatic destabilization will remove MLCINH and increase
boundary-layer lapse rates and buoyancy over parts of the Carolinas
and southern VA, amidst still-strong speed shear.  The strengthening
instability may be a counterbalancing factor to the weakening flow
for a few hours.  The tornado threat still appears too conditional
and widely spaced for greater than 2% lines.  This is related to
concerns over weakening of deep-layer lift with time, and related
doubts on attainment of enough of the favorable convective modes,
despite still-large shear values this morning into early afternoon. 
Cells embedded in the still-fast low/middle winds also may transfer
some momentum down in the form of damaging downdrafts.

Severe potential is expected to end by this evening as the low
shifts offshore, low-level flow veers further, and already modest
low-level instability diminishes.

..Edwards/Mosier.. 10/29/2020

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