Belvedere Tiburon, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

049
FXUS66 KMTR 251845
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1145 AM PDT Fri Jul 25 2025

...New AVIATION...

.KEY MESSAGES...
Updated at 1211 AM PDT Fri Jul 25 2025

 - Cool weather continues through Saturday

 - Temperatures return closer to normal starting Sunday

&&

.UPDATE...
Issued at 907 AM PDT Fri Jul 25 2025

Widespread coastal drizzle and favored west-east facing upslope
drizzle this morning with most sites picking up generally less than
0.04" of rainfall. This is as the marine layer has deepened to above
3,000 feet in depth per the Fort Ord Profiler. With the marine layer
this deep, low clouds are expected to scatter out and/or dissipate
across much of the region by this afternoon. However, expecting low
clouds to return tonight with another round of coastal drizzle.
Please see the Short Term discussion below for additional details.

&&

.SHORT TERM...
Issued at 1211 AM PDT Fri Jul 25 2025
(Today and tonight)

The most interesting feature tonight is a cluster of thunderstorms
in the Sacramento Valley drifting towards NE Napa County. The
cells are moving into a less favorable environment with CAPE less
than 20 J/kg according to the RAP. While there is still a slight
chance for a thunderstorm to develop in the North Bay through the
morning, the radar presentation seems to be waning recently and
the already slim chance is decreasing. As a plume of PWAT over
1.0" drifts south towards the North Bay through the day, there is
another slight chance for high-based convection this afternoon
into Saturday morning across the North Bay. Despite the higher
moisture and good 700-500 mb lapse rates near 8 C/km, low-level
moisture is actually pretty low outside of the marine layer. The
CAPE does increase, but so does the CIN. As such, the probability
remains less than 5% for thunderstorms from Friday afternoon
through Saturday morning.

Otherwise the short term forecast continues to be pretty
steady. The cut-off low is centered directly over the Bay Area at
500 mb and won`t budge much in the short term. That means another
day with a deep marine layer, some patchy drizzle in the morning,
and afternoon temperatures 5-15 degrees below normal. Now that the
low is directly over the Bay Area, the 850 temp is bottoming out
around 15C (bottom 10th percentile), and temperatures will be a
couple degrees cooler than previous days. In fact San Rafael is
only expected to reach 72F today. That would tie the lowest max
temperature for July 25 on record (record set 1993, records go
back to 1894). Several other cities have a similar shot at this
record today. With a robust stratus blanket tonight, we don`t
have any chance to set record low minimum temperatures.

&&

.LONG TERM...
Issued at 1211 AM PDT Fri Jul 25 2025
(Saturday through Thursday)

The long wave pattern is mounting a rescue mission for the cut-off
low this weekend. By Sunday an approaching trough over the East
Pacific will gradually re-absorb the low. The pattern change might
not be obvious at the surface as the trough will then linger near
the coast, providing a similar cooling effect as the cut-off low.
At the same time high pressure will build over the desert SW,
bringing triple digit heat to our friends in southern Arizona.
The battle between these two features will cause temperatures to
rise Sunday, but still remain slightly below seasonal normals. The
loss of the cut-off low will cause the marine layer to thin a bit,
but don`t expect to see a sunrise anytime soon. This new pattern
looks very stable, with temperatures remaining near or just below
seasonal normal through next week. That`s still warmer than it`s
been, however.

So will a summer heat wave ever hit? There is some indication in
the ECMWF ensemble mean that the SW US high and subtropical high
over the central pacific will merge over northern California
sometime around August 6. The GEFS mean shows similar timing.
We`re talking nearly 2 weeks out, so take this with a grain of
salt. In fact we had similar indications a month ago about July.
Not only did that heat wave not happen, July 2025 may end up in
the coldest 10% of Julys on record.

&&

.AVIATION...
(18Z TAFS)
Issued at 1145 AM PDT Fri Jul 25 2025

The marine layer got close to 2500-3000ft this morning, which lead
to an impressive image via satellite earlier this morning. Stratus
is expected to return to all TAF sites, with MVFR to IFR cigs
expected during the evening and overnight hours. Pockets of fog and
drizzle are expected as well, with clearing expected to begin mid to
late morning and VFR skies expected for interior locations.

Vicinity of SFO...The status of the stratus will be the continued
forecast challenge for SFO today. A lot of guidance has indicated
that SFO should be clearing out by now, but if you take a peak at
satellite and the KSFO observation that is clearly not the case.
Westerly flow keeps feeding stratus into San Francisco and San Mateo
counties, which should leave us with MVFR cigs for the afternoon. An
uptick in westerly winds is expected as well later this afternoon
and into the early evening hours where gusts to 20 to 25kt may be
possible. As the evening approaches, the marine layer should deepen
again which should leave SFO with IFR cigs overnight. Skies should
gradually begin to clear by mid to late tomorrow morning. Looking at
winds, there is a chance we could see a slightly stronger
opportunity for westerly winds and associated gusts tomorrow
afternoon and evening as various models show an opportunity for the
SFO to SAC gradient potentially peaking at 4-5mb.

SFO Bridge Approach...Similar to SFO; however, the stratus has
eroded over the southern half of the Bay. There could be some
additional clearing over the Bay, perhaps with either some SCT MVFR
cigs or ever perhaps a brief period of VFR. This should be short
lived as stratus will return this evening.

Monterey Bay Terminals...An otter eddy has formed which helped clear
out the south half of Monterey Bay and those areas along the
immediate coast, while keeping the northern half and a good portion
of Santa Cruz county socked in. Expect clearing to continue for KMRY
and KSNS, with a period of VFR conditions expected later this
afternoon. For those to the north, the otter eddy may lead to
cloudier skies and MVFR cigs through the afternoon if mixing isn`t
able to occur. Nonetheless, the stratus will return this evening,
and bring another night of MVFR to IFR cigs, patchy fog, and
drizzle.

&&

.MARINE...
(Today through Wednesday)
Issued at 907 AM PDT Fri Jul 25 2025

A moderate northwesterly breeze and moderate seas will prevail
through Tuesday. Low clouds with drizzle can be expected through
Saturday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Sarment
MARINE...KR

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