ACUS01 KWNS 141253
SWODY1
SPC AC 141251

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0651 AM CST Sat Dec 14 2024

Valid 141300Z - 151200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE BAY
AREA AND NEARBY COASTAL CALIFORNIA...

...SUMMARY...
Isolated strong to marginally severe, low-topped thunderstorms are
possible for a few more hours across the Bay Area and nearby coastal
California.

...Synopsis...
A progressive mid/upper-level pattern over the CONUS features two
primary troughs promoting convective potential this period, from
east to west:
1.  A strong shortwave trough was apparent in moisture-channel
imagery over the central Plains from west-central NE to western OK,
with a 500-mb low apparent near HYS.  The low is expected to move
eastward along I-70 into near COU by 00Z, with trough northward to
southern IA and southward to southern AR.  By the end of the period,
the low should reach northeastern IL around IKK, along a trough
aligned roughly from MKE-BNA.
2.  A synoptic-scale trough from the Gulf of Alaska to offshore from
the CA Coast, including a small cyclone off western WA and a basal
shortwave now approaching coastal central/northern CA.

At the surface, 11Z analysis showed a low over KS between ICT-FRI,
with cold front southwestward across southwestern OK and portions of
west-central/southwest TX.  By 00Z, the low should become nearly
stacked under the 500-mb low over north-central MO, with cold front
extending over northwestern AR, and northeast TX, to near AUS and
between DRT-LRD.  Substantial weakening of the front should be
underway by then, especially south of the Ozarks, with surface winds
less than 10 kt on both sides of the front over east and south TX.
The TX portion of the front should nearly dissipate overnight.

...Bay Area and vicinity...
Isolated strong-severe gusts and hail near severe limits are
possible this morning near the coast, in about a 150-nm-long
corridor centered just south of SFO.

A low-level frontal band precedes the basal shortwave trough over
CA, with associated precip forecast to continue spreading obliquely
eastward/southeastward down the Sierra and -- to a lesser extent --
over central/southern CA. Behind that, strong cooling aloft  --
related to DCVA immediately preceding the mid/upper trough -- is
supporting favorable instability through a deep-enough layer for
thunderstorms, with areas of 200-500 J/kg MLCAPE apparent over the
Pacific marine layer between SFO and the OR border.  That plume of
buoyancy will expand southeastward past the MRY area over the next
few hours, combining with favorable deep shear (40-50 kt
effective-shear magnitudes) to support slight onshore penetration of
strong/isolated severe thunderstorms, before activity weakens in
lower inland theta-e. Convection should move over the outlook area
through midmorning local time, before the trough passes.

...East to southeast TX...
Large-scale ascent preceding the eastern mid/upper trough -- in the
form of DCVA over northern parts of the thunder outlook area and a
broad plume of low-level WAA/moisture transport -- will continue to
support scattered, predominantly elevated thunderstorm potential
from the Arklatex to the lower Missouri and mid Mississippi Valleys.
Farther south across east TX to near the upper TX Coast, isolated to
widely scattered thunderstorms are possible through this afternoon.
This activity also will be tied mainly to the WAA plume, and related
isentropic ascent to LFC.  Forecast soundings show potential for
inflow-layer parcels to become surface-based amid continuing warming
from both advective and diabatic processes.  Wind profiles will veer
with height, though lower midlevel (roughly 600-700 mb) weaknesses
and lack of greater winds even higher in altitude will limit bulk
shear, with effective-shear magnitudes generally remaining below 40
kt.  Counterbalancing effects precluding unconditional severe threat
include rising heights throughout the day, stable layers and other
areas of weak lapse rates in midlevels that will keep MLCAPE from
getting much greater than 1000 J/kg, and weakening of both frontal
and large-scale lift with time.

..Edwards/Goss.. 12/14/2024

$$
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