Palo Alto Post Office, California Weather Forecast Discussion

098
FXUS66 KMTR 290929
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
229 AM PDT Mon Sep 29 2025
...New SHORT TERM, LONG TERM...
.KEY MESSAGES...
Updated at 119 AM PDT Mon Sep 29 2025
- A cold front will bring rain and a slight chance for
   thunderstorms today
- A second cold front will bring renewed rain chances Wednesday
&&
.SHORT TERM...
Issued at 119 AM PDT Mon Sep 29 2025
(Today and tonight)
Fall is here and we have our first wet cold front of the season.
According to the OPC/WPC 06Z surface analysis, the front is
located about 300 miles west of Bodega Bay and will be heading
our way today. We are already seeing widespread southerly wind and
extensive cloud cover ahead of the front. As the cool, dense air
behind the front lifts the warm moist air mass, many of these
clouds will grow dark and bring some rain later today,
particularly for the North Bay. After a recent drying trend in the
forecast, the lastest update has stabilized with around 1/4"
expected in the North Bay and 1/10" along the Peninsula this
afternoon and evening. The rest of the area (East Bay, South Bay,
Central Coast) should only see a few hundredths if anything from
this first cold front. It now looks like the associated surface
low will make landfall around 12Z Tuesday. These landfalling lows
tend to serve as a good trigger for stronger convection if the
environment is unstable. The 06Z HRRR model soundings show surface
CAPE peaking around 200 J/kg this afternoon at Santa Rosa, with
0-6 km shear reaching as high as 50-60 kts ahead of FROPA around
18Z. The ingredients are all there, but they don`t line up
perfectly in time (first high shear, then CAPE, then trigger) and
while the chance for thunderstorms has increased, it remains
slight at around 15% in the North Bay from the late afternoon
through the evening.
&&
.LONG TERM...
Issued at 119 AM PDT Mon Sep 29 2025
(Tuesday through Sunday)
Brief ridging will build in after the first front on Tuesday.
This will bring drier, more stable weather, and perhaps allow the
sun to peak back out. Temperatures will remain suppressed as the
850 mb temperature drops below 10C for just the 2nd time since
June 22 (9.8C 00Z 9/10). This calmer weather won`t last long as a
second cold front arrives Wednesday. This system looks a little
more robust than the Monday system, with the West WRF ensemble
mean IVT peaking around 400 kg/ms and remaining above 250 for
around 30 hours from 30/18Z to 02/00Z. This moisture has the
potential to bring around 1/2" of rain to the North Bay and
between a few hundredths to 1/4" to the rest of the Bay Area.
Again, the Central Coast will be lucky to get more than a few
novelty drops. While the moisture looks pretty robust and the deep
layer shear persists, the instability looks a little more anemic
on Wednesday with 700-500 mb lapse rates around -5 C/km compared
to around -7 C/km on Monday. This should keep the chance for
thunderstorms lower than 10%, but still higher than zero. When all
is said and done, we should be off to a good start for the first
day of the 2026 water year. By Thursday, most of the interesting
weather will be behind us, but the details of the more subtle
pattern become uncertain. The ensemble cluster analysis reveals
significant uncertainty by Saturday with either weak ridging or
weak troughing possible. While there isn`t any real threat of
more rain, the exact temperatures are hard to pin down with this
uncertainty. Our official forecast shows a warming trend, while
the current WPC progs actually bring a dry cold front through on
Saturday.
&&
.AVIATION...
(06Z TAFS)
Issued at 950 PM PDT Sun Sep 28 2025
Extensive mid-level to MVFR cloud cover blankets the Bay Area and
much of the Central Coast and is expected to persist through the
night. An approaching front on Monday will cause ceilings to lift
and southerly winds to increase, with scattered pre-frontal showers
possible at the immediate coast. The main rain band begins to impact
the North Bay towards Monday afternoon and evening, but starts to
break up as it heads southward towards the end of the TAF period.
The coastal regions and the East Bay Hills look to give the best
chances for precipitation late Monday through Tuesday.
Vicinity of SFO... Overcast conditions persist through the night
with MVFR to mid-level ceilings. Breezy southwest winds resume
Monday afternoon with an approaching cold front. Low confidence for
scattered pre-frontal showers at SFO tomorrow afternoon, with
greater confidence in the western side of the San Mateo Peninsula
and at OAK. The main rain band approaches the terminal Monday night
and Tuesday morning, but is expected to break up as it moves down
from the North Bay, limiting confidence in precipitation at the
terminal.
SFO Bridge Approach... Similar to SFO.
Monterey Bay Terminals... Widespread MVFR to mid-level clouds return
this evening and persist through the day Monday as a cold front
approaches. Light winds overnight before the onshore winds turn
towards a more southerly direction on Monday afternoon. Pre-frontal
showers could impact the coast through the day Monday but beyond the
TAF period, the main rain band is not expected to survive long
enough to bring rain to the region.
&&
.MARINE...
(Tonight through next Saturday)
Issued at 950 PM PDT Sun Sep 28 2025
Gentle to moderate southerly winds generally prevail across the
majority of the coastal waters through late this week. An upper
level trough and surface cold front brings rain showers to the
coastal waters Monday into early Tuesday with embedded
thunderstorms possible across the northern waters. A second system
will bring additional rain showers to the waters late Tuesday
into Wednesday. Each system may bring isolated to scattered fresh
to strong wind gusts for the northern waters. Seas become moderate
to rough and build to 10 to 12 feet mid to late week with winds
expected to strengthen again late week into next weekend.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...DialH
MARINE...DialH
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NWS MTR Office Area Forecast Discussion