SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0700 AM CST Wed Dec 02 2020

Valid 021300Z - 031200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
NEAR-COASTAL SOUTHEAST TEXAS AND SOUTHWESTERN/SOUTH-CENTRAL
LOUISIANA...

...SUMMARY...
Isolated severe thunderstorms are possible over portions of
near-coastal southeast Texas and southwestern/south-central
Louaisana.

...Synopsis...
A highly amplified but very evolutionary upper-air pattern will
dominate much of the CONUS this period.  The deep, large
Northeastern cyclone -- now centered over southern QC -- will move
northward to northern QC through 12Z tomorrow.  A series of
shortwave troughs initially over central Canada will dig
southeastward through the peripheral cyclonic-flow field, across
northern ON and the upper Great Lakes.

Farther southwest, moisture-channel imagery depicts a large cyclonic
gyre over the southern/central Rockies and adjoining Great Plains,
with primary vorticity maxima over the northeastern TX Panhandle and
southeastern WY.  In a loose Fujiwhara interaction, the southern
circulation will remain over the eastern Panhandles/western OK
region through 00Z, while the western one retrogrades southwestward
toward the Grand Canyon.  Overnight, the TX/OK portion will become
the main low and start to pivot/eject eastward over OK, while the
other lobe (then an open-wave/shortwave trough) digs south-
southeastward over AZ and western NM.  The net effect by 12Z should
be a 500-mb low between TUL-FYV, with height trough and vorticity
ribbon extending west-southwestward across the TX South Plains and
southern portions of NM/AZ.

At the surface, 11Z analysis showed a low over northwest TX between
CDS-MWL, with cold front southwestward across the lower Pecos
River/Big Bend region.  The low should move erratically over
southern OK today and occlude, while the cold front surges
southeastward to a 00Z position over east-central TX and the lower-
middle TX Coast.  A new low may develop along the boundary,
overnight, over the Gulf, and move inland over southwestern LA after
09Z.

...Western Gulf Coastal Plain...
A plume of precip, with scattered embedded shower and isolated to
widely scattered thunderstorms, is expected to expand and become
more dense today from the northwestern Gulf northwestward across
east TX and southeastern OK.  This will occur as warm advection and
moisture advection/transport continue in a conveyor related to the
deep-layer cyclone, most of it elevated above a relatively stable
boundary layer.

However, ongoing and future marine airmass modification over the
next several hours should support sufficient theta-e advection into
near-coastal southeast TX for surface-based effective-inflow parcels
to develop inland, perhaps as far north as roughly I-10.  MLCAPE of
500-800 J/kg may develop inland.  As a result, isolated surface-
based supercells may either develop over the Gulf and graze the
immediate coastline, or (somewhat less probably) form inland, before
the flow near/ahead of the front veers substantially and frontal
passage occurs.  45-55-kt effective-shear magnitudes and 150-300
J/kg effective SRH will support supercell and small-bow potential,
though the bulk of strong-severe convection should be over the Gulf
where the inflow-layer air mass is better optimized.  An
effective/marine warm front marking the northern rim of the
modifying/surface-based parcels may move very slightly inland
farther east across southwestern/south-central LA overnight, in
support of marginal severe potential.

..Edwards.. 12/02/2020

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