Portola Valley, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

057
FXUS66 KMTR 250859
AFDMTR

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

...New SHORT TERM, LONG TERM...

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

 - Cool weather continues through Saturday

 - Temperatures return closer to normal starting Sunday

&&

.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...
(06Z TAFS)
Issued at 958 PM PDT Thu Jul 24 2025

CIGs have filled around the bays and the coast and are moving
farther inland. Winds are reducing across the district, becoming
light to moderate through the night and much of Friday morning. CIGS
continue to spread deeper inland into the late night with Widespread
CIGs expected. HAF, MRY, and STS look to fall to IFR-level CIGS
overnight. Inland sites begin to clear in the late morning with most
TAF sites going VFR in the afternoon. The exception will be HAF,
which will go from IFR to MVFR CIGs, but will not clear with CIGs
falling back to IFR there Friday evening. Expect modest wind
increases into Friday afternoon with most areas peaking around 15 kt
winds and the occasional stray gust. Winds reduce again that evening
with CIGs quickly returning into the bays and becoming widespread
again that night.

Vicinity of SFO...VFR lasts into the evening. Winds reduce slightly
as MVFR CIGs cross over the SF Peninsula and fill over SFO in the
late evening and early night. Cloud cover scatters in the early
afternoon as west winds increase, but re-fills over the terminal
again in the early evening on Friday.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Winds Are reducing with MVFR CIGs in place.
Winds reduce further into the night and MRY falls to IFR CIGs, but
snaps back to MVFR in the mid morning. The terminals clear in the
early afternoon on Friday as winds increase, but MVFR CIGs build
back into MRY again in the late afternoon and at SNS in the early
evening.

&&

.MARINE...
(Tonight through next Wednesday)
Issued at 555 PM PDT Thu Jul 24 2025

Gentle winds will increase to a moderate NW breeze Friday. A
fresh breeze will push through the San Francisco and Monterey Bays
through Friday afternoon, generating occasional whitecaps.
Otherwise low seas will prevail through the weekend. A deep
marine layer will bring consistent low cloud cover and periods of
drizzle.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Coastal Flood Advisory until 2 AM PDT early this morning for
     CAZ006-506-508.

PZ...None.
&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Murdock
MARINE...Canepa

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