Sonoma, California Weather Forecast Discussion

979
FXUS66 KMTR 271156
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
456 AM PDT Sat Sep 27 2025
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 222 PM PDT Fri Sep 26 2025
- Rain and thunderstorms return Monday
- Wet weather Monday through Wednesday
&&
.SHORT TERM...
Issued at 227 AM PDT Sat Sep 27 2025
(Today and tonight)
Overnight satellite fog product shows quite the difference N to S
across the forecast area - either you have marine layer clouds or
you don`t and the break point is Santa Cruz. Santa Cruz coast,
Monterey Bay, and Salinas Valley all have marine layer clouds.
Elsewhere is clear. A battle of pressure gradients is a key
player in this setup. The stronger N-S gradient from SFO- ACV
initially helped to keep marine layer clouds out of the area. That
gradient relaxed and we saw a S-N gradient from SFO-SMX increase
bringing clouds northward along Big Sur and Santa Cruz. Their
northward track is halted around Monterey Bay due to drier air
northward and weak offshore flow.
The bigger story for today and tonight, the 5 to 15 degree cool
down across the region. The shortwave ridging that brought the hot
weather Friday is quickly being replaced by an approaching trough
dropping 500mb heights. The airmass aloft also has a 2-3 degree C
drop. Despite some afternoon sunshine temperatures today will be
60s to 70s coast and 70s to mid 80s inland. The marine layer will
have a better shot at reforming tonight with the falling heights
and cooler airmass.
&&
.LONG TERM...
Issued at 327 AM PDT Sat Sep 27 2025
(Sunday through Friday)
To round out the weekend temperatures will drop another 5 to 10
degrees with further falling 500mb heights as the upstream trough
begins to really assert itself over the West Coast. Sunday will
be dry, but highs will be 60s and 70s most areas, which is 5 to 10
degrees below normal.
Sunday night into Monday is the transition period back to wet
weather. While the upstream trough knocks on our door the surface
reflection shows warm sector intrusion from the low pressure
parked off the PacNW coast. Taking a peek at isentropic surfaces
295-300k show decent isentropic lift / upglide over NorCal,
including the North Bay. As a result, moisture will increase
leading to increasing clouds and steady stratiform precip.
Speaking of moisture, still a solid feed back the HI with PWATs of
1-1.5" or 150% PON. This scenario will persist into the Monday AM
commute with precip most likely along the coast and north of San
Jose. By Monday afternoon the cold front approaches from the
north and makes its way S and E through early Tuesday. The fropa
will have a lot more dynamics to work with as the jet streak
rounds the base of the upper trough, cyclonic curvature, providing
larger ascent. A few convective parameters are still being
highlighted with the fropa - higher lapse rates, MUCAPE, and
modified Total Totals. Longer range models are showing some hints
of a NCFR on Monday too. Still far out to dial in those features
just yet. Additionally, while convective parameters are still
being shown they are trending less and farther north. One part of
the forecast that hasn`t really changed with the Monday fropa is
that it will weaken by the time it reaches the Central Coast. Post
frontal showers will develop Monday night and Tuesday.
Post frontal showers will be the brief "break" before system two
arrives later Tuesday into Wednesday. Just like system one this
system will have some tropical moisture too, but just a different
flavor. The source region of this moisture will be the remnants of
TS Neoguri currently in the WPac. Showers will increase in
coverage and intensity by Tuesday evening/night over the N Bay
before spreading S and E Wednesday. It too will fade over the
Central Coast. One interesting trend that may be developing with
system two is a stalling moisture plume. Upper level steering flow
becomes parallel to the sfc boundary. Atmospheric River guidance
shows a solid feed of higher IVT wavering over the N Bay and SF
Bay area. That would be a much longer period of precip. Current
forecast doesn`t quite reflect this scenario just yet, but we`ll
need to watch it closely. As mentioned on previous discussions
system two looks less convective too.
Interesting to note that system two will arrive just in time for
the upcoming water year, which begins Oct 1.
Storm total QPF with both system remains in the 1-1.5" North Bay,
but definitely has trended drier Golden Gate southward. In fact,
interior Central Coast will be struggling to get a 0.01".
Drier weather return by Thursday.
&&
.AVIATION...
(12Z TAFS)
Issued at 455 AM PDT Sat Sep 27 2025
IFR-MVFR along the coastline with low confidence that stratus will
expand inland. The SFO-ACV gradient continues to weaken with an
almost negligible (-0.1 mb) northerly gradient observed as of 11Z.
The SFO-ACV shifts weakly positive (southerly gradient) by later
this morning with the SFO-SMX and SFO-SBA gradients southerly
gradients peaking around 2 to 3 mb this afternoon. The transition
back to southerly flow is allowing stratus to spread northward from
southern CA into the Central Coast and up the coastline. This is
reminiscent of a southerly surge but confidence is low that stratus
will spread into the interior/beyond the coastline. Patchy stratus
may affect the North Bay Valleys and HAF with persistent stratus
over MRY and SNS. Went slightly more pessimistic for MRY and SNS
TAFs by reducing the amount of time clearing is expected at both
sites today. Stratus looks to be more widespread tonight and returns
to most airports Saturday evening.
Vicinity of SFO...VFR becoming MVFR late tonight. Winds stay onshore
through this evening before gradually shifting southerly overnight.
Gusts to around 23 knots are expected during the afternoon/evening
hours. Stratus is not expected to reach SFO this morning but it is
likely to return Saturday night and continue through the end of the
TAF period.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...IFR-MVFR through remainder of the morning.
Adjusted the TAFs to be more pessimistic as to stratus clearing
times today. Thinking is that increased southerly flow will lead to
a stronger feed of stratus into MRY and SNS through the rest of
today (satellite looking reminiscent of a southerly surge). This
will keep MRY overcast through the entire TAF period while there are
a few hours of VFR conditions at SNS this afternoon. Moderate to
high confidence in MVFR CIGs late afternoon through the end of the
TAF period for both sites.
&&
.MARINE...
(Today through Thursday)
Issued at 455 AM PDT Sat Sep 27 2025
Near gale force winds and elevated seas between 10 to 12 feet
continue through early this morning across the northern outer
waters. Winds diminish, becoming moderate to fresh, and seas
subside by late Saturday. An approaching upper level trough and
surface cold front will bring rain, isolated embedded
thunderstorms, and building seas to the coastal waters Monday into
early Tuesday. A second system will arrive Tuesday into Wednesday
and continue rain chances across the coastal waters.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 1 PM this afternoon to 9 PM PDT this
     evening for SF Bay N of Bay Bridge.
Small Craft Advisory until 9 AM PDT this morning for Pt Arena to
     Pt Reyes 10-60 NM.
&&
$$
SHORT TERM...MM
LONG TERM....MM
AVIATION...Kennedy
MARINE...Kennedy
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NWS MTR Office Area Forecast Discussion