SPC Feb 19, 2024 1300 UTC Day 1 Convective Outlook
SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0648 AM CST Mon Feb 19 2024

Valid 191300Z - 201200Z


A tornado or two may occur in the Sacramento Valley region of
California today.

The large-scale pattern will continue to be characterized by a
longwave trough just off the West Coast, ridging over the Rockies
and High Plains, and a trough over parts of the eastern CONUS and
eastern Gulf. Progressive, well-developed shortwave troughs -- now
apparent in moisture-channel imagery over GA and the central High
Plains -- will move through the broader-scale cyclonic-flow field of
the eastern longwave trough. Conditions should remain too stable in
low levels to support a thunder threat over land from the
Intermountain West eastward to the Atlantic Coast.

Upstream, a large cyclone off northern CA and the Pacific Northwest
will weaken gradually and shift northward through the period,
devolving into an open wave synoptic trough. However, the broader-
scale troughing will be maintained, as a strong shortwave
perturbation now in the cyclone's northwestern quadrant digs
southward then southeastward, approaching 130W west of southern CA
and northern Baja by 12Z. These processes will maintain a fetch of
southwest flow aloft across CA. A weaker shortwave trough is moving
inland over north-central CA, and should cross northwestern NV today
before reaching parts of ID around 00Z.

Scattered showers and isolated to widely scattered thunderstorms are
expected this afternoon into early evening. Isolated supercells are
possible, with a threat for a tornado or two. Mostly subsevere hail
and convective winds are expected, through the largest hail and
gusts may briefly reach severe limits.

As noted above, the initial shortwave trough is crossing the area,
preceded by a precip plume with isolated embedded thunderstorms over
the central/northern Sierra and San Joaquin Valley. In the weak
subsidence following that perturbation, a few hours of relatively
tranquil conditions are expected. Thereafter, upper-level forcing
will be nearly neutral, but with diurnal/diabatic heating
contributing to low-level destabilization over the Sacramento
Valley/Delta region and perhaps northernmost parts of the San
Joaquin Valley, beneath favorable low/middle-level lapse rates and
weakening of MLCINH. Episodic, isolated to scattered convection and
a few thunderstorms are possible again this afternoon into evening,
in an environment characterized by favorably veering/increasing
winds with height.

Orographically forced backing of surface flow in the valley likely
will be slightly greater than most models indicate as well, further
enlarging already favorable hodographs progged for supercells. Any
quasi-linear modes also may contain LEWP/bow features with embedded
mesocirculations. Effective SRH of 150-300 J/kg and 40-55-kt
effective-shear magnitudes are possible. With surface dewpoints
holding generally in the low/mid 50s F, the preconvective/inflow
sector may attain 250-700 J/kg peak MLCAPE, based on modifications
to the 12Z OAK RAOB and model soundings. A combination of nocturnal
surface cooling/stabilization and outflow air should reduce severe
potential after dark. Isolated storm-scale rotation also is
possible with cells mainly over the Pacific marine layer off the
northwestern CA coastline, with waterspout/tornado potential nonzero
but too low/conditional over land for an outlook area at this time.

..Edwards/Mosier.. 02/19/2024

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