SPC Dec 22, 2023 1300 UTC Day 1 Convective Outlook

[NorthGeorgiaWX]

SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0653 AM CST Fri Dec 22 2023

Valid 221300Z - 231200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
SOUTHERN ARIZONA...

...SUMMARY...
Isolated large hail and strong-severe gusts are possible from midday
into evening across parts of southern Arizona.

...Synopsis...
A closed, mid/upper-level cyclone was apparent in moisture-channel
imagery offshore from southern CA and northern Baja -- centered
about 200-225 nm west-southwest of SAN.  Separately, a strong,
northern-stream shortwave trough was evident just offshore from the
Pacific Northwest.  The northern-stream trough will move inland by
around 18Z today, then reach the northern Rockies by the end of the
period.  It will be increasingly phased with the increasingly
progressive, southern-stream cyclone/trough, whose circulation
center should cross northernmost Baja around 00Z, then shift
northeastward toward the Grand Canyon/FLG region by 12Z tomorrow.  

...Southern AZ...
Isolated severe hail and/or gusts are possible today over portions
of southern AZ. 

Ahead of the mid/upper cyclone, a baroclinic-leaf pattern apparent
in satellite imagery -- with precip and embedded thunderstorms that
should remain elevated and nonsevere for several more hours -- will
continue to spread eastward to northeastward over the region through
the day.  Meanwhile, at the surface, and in low levels, warm
advection and moisture transport are forecast to increase today over
much of the Desert Southwest, beneath a zone of strong difluence
aloft preceding the mid upper-level cyclone.  So will large-scale
DCVA-related lift closer to the cold-core region.  predominantly
separate areas of strong/isolated severe-thunderstorm potential will
develop in each regime from midday into early evening, but with some
spatial overlap in the outlook area due to the progressive nature of
the system as a whole.  

Diurnal destabilization should be gradual, given the presence of
cloud cover to limit the pace and strength of diabatic surface
heating.  However, in combination with boundary-layer theta-e
advection and cooling aloft, low/middle-level lapse rates should
steepen, with that and increasing low-level moisture contributing to
lessening MLCINH through the afternoon.  Activity developing in MX,
or near the international border, near or off the southern rim of
the early cloud/precip shield, will move into an increasingly
buoyant environment, with each regime yielding peak MLCAPE in the
300-800 J/kg range.  Strengthening deep shear should aid in storm
organization, though potentially messy mode and lack of greater
instability may be counterbalancing factors.  Forecast soundings
suggest strong deep/speed shear, with long but not especially large
hodographs by the time surface-based effective-inflow parcels are
common.  45-55-kt effective-shear magnitudes are expected area-wide.
 Decreasing instability should diminish severe potential with time
overnight.

..Edwards/Broyles.. 12/22/2023

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