SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0635 AM CST Wed Mar 03 2021

Valid 031300Z - 041200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
At least a few thunderstorms are possible today over parts of
Florida and near-coastal southern California, and tonight over
northern Arizona.

...Synopsis...
A progressive upper-air pattern with a somewhat active, regionally
splitting southern stream is forecast this period.  The first
perturbation in that stream -- a strong shortwave trough initially
located over portions of GA/SC -- will move offshore by 18Z.  Behind
that feature, generally zonal flow across the south-central/
southeastern CONUS will give way to height rises over the southern
Plains.

Those height rises will occur in advance of a marginally synoptic-
scale (roughly 1000 km width of outer closed isohypse) cyclone now
apparent in moisture-channel imagery west of southern CA and
northern Baja.  This vortex will move inland across southern CA
today, with its 500-mb trough assuming negative tilt, and a
cyclonically curved, 120-140-kt 250-mb jet streak over its southern
semicircle.  By 12Z tomorrow, the cyclone should reach northern AZ
and central/southern UT, with trough back near neutral tilt,
southward to northern Sonora.

...Southern CA and AZ...
Widely scattered thunderstorms are possible in the associated
frontal/convergence arc immediately preceding the cyclone core,
where isolated, shallow thunderstorms also may develop and move
ashore.  The most probable area for thunder appears to be over
southern CA from portions of the L.A. area through SAN.  Isolated
thunder also may be noted tonight over parts of northern AZ.  Strong
large-scale lift (with DCVA under left-exit region of aforementioned
jet max) and accompanying cooling aloft will steepen lapse rates. 
Despite cool surface conditions, sufficient low-level moistening
should occur with those greater lapse rates to yield MUCAPE of
50-150 J/kg, extending into icing layers favorable for sporadic
lightning.

...FL...
11Z surface analysis showed a low just offshore from the GA Coast
with a cold front southwestward across northern FL to the central
Gulf.  This front should move southeastward and off the peninsula
around 03Z.  A prefrontal low-level convergence zone and
accompanying band of convection -- initially located across parts of
central FL -- should move eastward with some backbuilding possible
through early/mid afternoon.  With the mid/upper perturbation now
north of the area forecast to move away quickly, mid/upper support
and low-level mass convergence should weaken with time, albeit with
some offsetting influence from diurnal destabilization and weakening
MLCINH of a moist air mass inland.  Still, with weak lapse rates
aloft and lack of stronger lift, thunderstorm coverage should remain
isolated to widely scattered before diminishing late this afternoon
and evening.

..Edwards/Jewell.. 03/03/2021

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