SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0650 AM CST Sat Jan 23 2021

Valid 231300Z - 241200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
A few non-severe thunderstorms may develop across parts of southern
California near the coast today, and late overnight over northern
Louisiana.

...Synopsis/Discussion...
A complex, mid/upper-level cyclone -- now covering parts of the
Great Basin westward to coastal north-central CA -- is expected to
consolidate in favor of its western lobe through the period.  This
will occur as an eastern vorticity max and original cyclone center
-- now apparent on moisture-channel imagery over central NV --
ejects northeastward to the central Rockies, and weakens in
confluent flow.  Meanwhile, the western portion of the gyre contains
vorticity maxima over inland northern CA and southwest of MRY. 
Those will phase enough to form a new, dominant cyclone moving
southeastward down the southern CA coast through this evening.  The
new cyclone and attendant trough should pivot eastward across
southern CA to near the lower Colorado River Valley and northern
Baja between 06-12Z.  

At the surface, mostly cool to cold and stable conditions prevail
across the interior CONUS from the Rockies to the East Coast.  A
wavy/quasistationary frontal zone was drawn at 11Z from central FL
across the northern Gulf, bending southwestward/southward off the
lower TX Coast. 

...Southern CA to southern AZ...
Midlevel cooling and related steepening of low/middle-level lapse
rates will occur atop a moist marine layer, as the cyclone develops
and approaches.  This should result in sufficiently deep buoyancy to
extend at least locally/briefly through a rising inversion and into
icing layers suitable for lightning production near the coast. 
Forecast soundings suggest 100-300 J/kg MUCAPE, with similar MLCAPE
over water and immediate coastal areas as the cyclonic cold core
moves nearby.  Farther east, an area of widely scattered to
scattered showers may develop over northwestern MX and the northern
Gulf of CA tonight, moving/spreading north-northeastward over parts
of southern AZ.  Although forecast soundings suggest CAPE will
remain too shallow for an areal thunderstorm threat north of the
international border, very isolated lightning cannot be ruled out. 

...Northern LA and vicinity late tonight...
A few thunderstorms are possible within the last few hours of the
period -- roughly 09-12Z -- embedded within a growing area of precip
rooted in an elevated inflow layer.  A broad area of low-level warm
advection and isentropic lift will continue to expand and strengthen
through the period over the western Gulf States, beneath broadly
anticyclonic mid/upper flow and north of the surface baroclinic zone
over the Gulf.  Though forcing aloft will be minimal, gradual
moistening within the warm advection plume is expected above a
relatively stable boundary layer.  The increase in theta-e from
about 800-900 mb results in MUCAPE 300-800 J/kg off the east rim of
the EML after about 09Z, with isentropic lift to an LFC.  Modest
lapse rates and lack of buoyancy should preclude a severe threat.

..Edwards.. 01/23/2021

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