Saint Helena, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

738
FXUS66 KMTR 122348
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
448 PM PDT Tue Aug 12 2025

...New AVIATION, MARINE...

.KEY MESSAGES...
Updated at 320 AM PDT Tue Aug 12 2025

 - Moderate HeatRisk today for portions of interior Monterey and San
Benito counties.

 - Elevated fire weather concerns persist through Wednesday for the
interior Central Coast.

- Pattern change today, with more widespread cooling and coastal
clouds and drizzle starting Wednesday.

&&

.SHORT TERM...
Issued at 153 PM PDT Tue Aug 12 2025
(This evening through Wednesday)

Highlights through the short term haven`t changed that much from
the previous forecast: pattern change is occuring, cooldown on the
way, night/morning marine layer remains, eyeing moisture off the
CA moisture.

Let`s look into the details. The longwave upper ridge that
brought heat is fading quick as an upper trough is settling yet
again over the west. The details are very nuanced, especially
when it comes to eyeing the moisture tracking up the coast. That
is the bust portion of the forecast. The upper level moisture
lurking off SoCal is the leftovers of TS Ivo. The leading edge of
this moisture is already being seen on the satellite in the way of
high based CU. Radars are evening picking up a few returns high
up this afternoon. The key here is the moisture is pretty high
up. While one can see it on model data from 700-500mb layer it`s
better seen slightly higher at 500-300mb. Meaning, from a high
based tstorm ingredient it`s just a tad higher than ideal. How
about forcing? Lapse rates are pretty solid tonight and Wednesday
with pockets of MUCAPE. These lapse rates are also being aided by
a weak shortwave trough riding the western edge of the departing
high. Additionally, the 1.5 PVU pressure surface also shows a
ripple aloft. FWIW PV surfaces, it`s a great way to see subtle
dynamic tropopause lifting/ascent features aloft. What about
overall flow? That`s the real kicker. While we have some moisture
(not ideal) and upper level instability, it`s the flow aloft that
will likely be the show stopper for thunderstorms. Flow is more
west-east (zonal) and not equator-pole (meridional). While
moisture aloft does advect north, the flow quickly turns zonal
ahead of the moisture influx almost blocking its northward push.
Therefore, still feel comfortable not mentioning thunderstorms in
the forecast. A passing shower or some virga seems more
reasonable. A slightly deviation in the zonal flow timing or
strength will change the outcome. Finally, probabilistic guidance
and calibrated lightning forecasts both show no convection
either. Regardless it will be interesting watch the evolution.

&&

.LONG TERM...
Issued at 240 PM PDT Tue Aug 12 2025
(Wednesday night through next Monday)

The cooldown noted from Wednesday will continue Thursday and then
linger into the upcoming weekend. One of the bigger weather
impacts for the long term will be increasing onshore flow and
subsequent winds on Friday into Saturday. Winds initially ramp up
over the waters Friday and then spread inland Friday evening and
early Saturday, especially inland gaps/passes.

&&

.AVIATION...
(00Z TAFS)
Issued at 433 PM PDT Tue Aug 12 2025

High confidence continues for the return of low MVFR/IFR/LIFR
marine stratus this evening first over the terminals near the
coastline, then later into the interior, with gusty onshore flow
easing somewhat after sunset.

Vicinity of SFO...VFR conditions are expected for the remainder of
the afternoon and early evening with the edge of the thicker low
stratus inching toward KSFO already, less coverage in the vicinity
of KOAK attm. However high confidence that clouds will begin to
develop across the inner bay around sunset and persist through the
remainder of the overnight into mid/late Wednesday morning.

SFO Bridge Approach...Similar as SFO.

Monterey Bay Terminals...MVFR/IFR stratus ceilings are expected to
develop early this evening with the support of current satellite
trends. LIFR ceilings are expected later tonight, likely to
persist through the overnight hours and clear out late Wednesday
morning/early afternoon, similar to past couple of days. Patchy
dense fod and drizzle may limit visibility at times too.

&&

.MARINE...
(Tonight through next Sunday)
Issued at 433 PM PDT Tue Aug 12 2025

A gentle to moderate northwesterly breeze will prevail tonight,
increasing to become fresh to strong Wednesday with widespread
strong gusts by Thursday. Moderate seas will build to become rough
for inner waters and outer waters by Thursday. Northwesterly winds
will increase even more so by Friday with near gale force gusts.

&&

.BEACHES...
Issued at 248 AM PDT Tue Aug 12 2025

A long period (17 seconds) southwesterly swell will result in an
increased risk for sneaker waves and rip currents through this
evening. Greatest risk will be along southwest facing beaches,
including but not limited to Stinson Beach, Santa Cruz Boardwalk
Beach, and Twin Lakes Beach. Sneaker waves can unexpectedly run
significantly farther up the beach than normal, including over
rocks and jetties. Remain out of the water to avoid hazardous
swimming conditions and never turn your back on the ocean!

Sarment

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement until 9 PM PDT this evening for CAZ505-
     529.

PZ...Small Craft Advisory until 9 PM PDT this evening for SF Bay N of
     Bay Bridge.

     Small Craft Advisory from 1 PM to 9 PM PDT Wednesday for SF Bay
     N of Bay Bridge.

     Small Craft Advisory until 3 AM PDT Thursday for Pt Pinos to Pt
     Piedras Blancas 0-10 nm.

     Small Craft Advisory from 9 PM Wednesday to 3 AM PDT Thursday
     for Pt Arena to Pt Reyes 10-60 NM.

&&

$$

SHORT TERM...MM
LONG TERM....MM
AVIATION...JM
MARINE...RGass

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