SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0646 AM CST Sat Nov 28 2020

Valid 281300Z - 291200Z


Severe thunderstorms appear unlikely across the contiguous United
States today and tonight.

The dominant upper-air feature for this forecast will be a closed
(but not quite cutoff) cyclone -- evident in moisture-channel
imagery over NM.  A separate, northern-stream trough -- initially
over the Canadian Rockies -- is progged to move east-southeast and
amplify, reaching the Dakotas and eastern WY by the end of the
period.  As that occurs, the southern-stream 500-mb low will move in
an erratically eastward path, reaching the Red River region of
southern OK or north-central TX by 12Z tomorrow. 

At the surface, 11Z analysis depicted a weakening cold front from
Lake Ontario across WV, middle TN, and central LA, losing definition
as it penetrates deeper into a poorly modified, continental/polar
air mass left behind the slower-moving frontal zone analyzed near
the FL/GA line, to near the mouth of the Mississippi River, just
offshore from the western LA and upper TX Coasts, and across deep
south TX.  A weak frontal-wave low may develop by 00Z near the lower
TX Coast, then ripple northeastward along the boundary, near or
barely offshore from the middle/upper TX Coast overnight.  The
remainder of the southern front should linger quasistationary near
the Gulf Coast from southern LA to northwestern FL, eastward across
the northern peninsula. 

...Southern States...
Ahead of the NM cyclone, precursory DCVA is spreading across the
southern High Plains.  The related plume of ascent/cooling aloft is
yielding increasing convective coverage and forming a baroclinic-
leaf configuration on IR imagery.  As the UVV field translates
eastward over progressively greater (though still modest) moisture
in an elevated, low/middle level convective inflow region,
associated general thunderstorm potential will spread over most of
TX to the east, as well as parts of the Red River Valley, southern
OK, and the Arklatex region.  Isolated subsevere hail cannot be
ruled out, though forecast buoyancy and inflow-layer moisture
content each appear too scant to support an unconditional/organized
severe threat.

Over south TX, convection should be mainly on the cool side of the
front, with frontal forcing aiding weak large-scale ascent and
overcoming minimal CINH to support convection.  Isolated
thunderstorms cannot be ruled out along and south of the front in a
conditionally favorable CAPE/shear parameter space that ordinarily
would cause some severe concern.  However, weak lift will temper
convective organization.  Farther downshear, the broad fetch of
elevated, low-level warm advection and moisture transport, along and
north of the front, will support isolated to widely scattered
thunderstorms near the central/eastern Gulf Coast and across
southern GA/northern FL.  Sufficient near-surface destabilization to
support overland severe potential should delay until at least early
day-2, when the frontal-wave low is forecast to move inland over LA
and MS, and deepen.  See the SPC day-2 outlook for more details on
the severe threat after 29/12Z.

..Edwards/Broyles.. 11/28/2020

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