FXUS66 KMTR 311802
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1002 AM PST Wed Dec 31 2025

...New AVIATION...

.KEY MESSAGES...
Updated at 859 AM PST Wed Dec 31 2025

 - High astronomical tides combined with storm surge will bring
   minor to moderate coastal flooding around high tide through 
   Sunday.

 - A prolonged rainy period starts today.

 - Strong southerly winds are likely Friday and into Saturday
   morning.

&&

.UPDATE...
Issued at 859 AM PST Wed Dec 31 2025

Light rain is occurring across the coastal ranges, especially across 
the Santa Lucia Range and Santa Cruz Mountains with generally less 
than 0.05" of rainfall in the past 6 hours. Rain will increase in 
coverage throughout the day, yet amount are forecast to generally 
remain light. The ongoing forecast remains on track at this time 
with no updates anticipated this morning. The San Francisco tidal 
gauge reached 1.47 ft above normal (7.31 ft MLLW) at around 7:51 AM 
PST and is gradually lowering. 

RGass

&&

.SHORT TERM...
Issued at 1229 AM PST Wed Dec 31 2025
(Today and tonight)

We switched KMUX to precip mode as rain and/or virga moving in
from the south is coming in range. Most of this activity is over 
the coastal waters, and the lowest radar scan can only see 7,000 
feet at this range. Unfortunately there aren't any buoys or ships
capable or willing to report if anything is reaching the surface.
Based on the 00Z sounding, most of these rain drops will 
evaporate in the well defined dry layer below 800 mb where the 
average relative humidity is less than 15%. Of course the 
evaporation itself raises the humidity and makes it easier for 
subsequent drops to make it further down. The moistening process 
will continue through the morning, priming the lower atmosphere 
for more efficient rain as the day progresses.

The overall pattern is driven by a vertically stacked cut-off low
roughly 500 miles west of the Channel Islands. The eastern
periphery of this feature is pumping a large area of southerly
winds and associated moisture from the deep tropics. This low will
catch the next ride NE on an the upstream portion of an approaching
long wave trough moving across the Eastern Pacific. This upper
level pattern will also provide divergence aloft, removing weight
from above the surface low, thus decreasing the surface pressure
and providing some moderate cyclogenesis as the feature moves
towards the Bay Area. We should pick up between 0.5-1.0" of rain
over the next 24 hours as a warm up for the rest of the week. Will
it rain for NYE celebrations? Probably, yes. In the Bay Area a 
rain jacket or umbrella should do just fine in the steady light 
rain and gentle wind expected up till midnight. If you're 
celebrating on the Central Coast, it may be a little heavier in 
the last hour or two of the year.

&&

.LONG TERM...
Issued at 1229 AM PST Wed Dec 31 2025
(Thursday through Tuesday)

The warm, moist southerly winds with an approaching, deepening low
pressure system to our west means we should get a somewhat 
uncommon warm frontal passage Thursday. Antecedent warm moist air
with increasing kinematics support as the front approaches should
support some stronger convection and possibly trigger some 
thunderstorms as the front passes. This threat will be greatest if
the triple point (where the occlusion, cold front, and warm front
meet) moves over land. The Central Coast has the best chance for 
thunderstorms. The thermodynamics don't look great until midday 
on Thursday, but if the timing lines up with frontal passage, it 
could get interesting. A NAM point sounding at Salinas shows the 
FROPA timing will be very close to the instability window opening 
around noon. Otherwise the afternoon of New Years Day should just
bring scattered showers as opposed to the more widespread rain on
NYE.

After the low makes landfall Thursday, a more typical atmospheric
river set-up will takes shape for Friday. A deep trough over the 
Eastern Pacific will bring strong SW flow to the California, with 
a ribbon of moisture pointed somewhere between the North Bay and 
Southern California. The high moisture anomaly near Baja never 
really clears out, and will push north again, reinforcing this 
round of rain. This should be the wettest and windiest day of the
week, with bigger impacts bleeding into Saturday morning.
Southerly winds will likely gust into the 50 mph range along the
coast and in higher terrain just ahead of the cold frontal 
passage. Conditions will improve behind the front, but scattered
showers will continue through the weekend, with 0.5-1.0" expected
each day from Saturday through Monday. A new low pressure system
and associated cold front is possible Monday or Tuesday. The
ensemble clusters have differing opinions on the strength and
timing of this feature, but they all show a trough off the coast,
which is enough to keep rain chances around through the entire 
7-day forecast period. KMUX will stay in precip mode for the 
foreseeable future.

&&

.AVIATION...
(18Z TAFS)
Issued at 1002 AM PST Wed Dec 31 2025

Light rain has returned to much of the Bay Area and the Central 
Coast. Current ceilings are VFR but guidance shows them dropping to 
MVFR by late this afternoon/early this evening. Confidence on timing 
of ceilings dropping is low to moderate with moderate confidence and 
models in agreement that CIGs will drop to around 2000-2500 ft for 
much of this TAF period. Models do show CIGs dropping to IFR 
overnight but confidence is currently low for that scenario. Winds 
stay out of the south to southeast and remain breezy through the 
forecast period. Gusts to around 20 to 25 knots are expected at 
breezier sites (SFO, OAK, and SNS with some potential for gusts at 
HAF and MRY) Thursday morning. Maintained visibilities dropping 
slightly as rain moves in. However, I followed a similar trend as 
the previous forecaster and kept drops in visibility limited to 
around 4SM to 5SM given light rainfall expected. Models show 
visibilities dropping down to 2-3SM but confidence is low that will 
occur unless locally moderate to heavy rain is able to develop.

Vicinity of SFO...Light showers are located offshore of SFO with 
scattered showers to move in throughout the remainder of the 
morning. Moderate confidence in MVFR CIGs developing by late this 
afternoon/early this evening with low confidence that CIGs will 
become IFR overnight. Winds generally remain southerly and are 
expected to strengthen by tomorrow morning. Currently kept gusts to 
around 18 knots at SFO tomorrow but this may need to be adjusted 
upwards to between 20-25 knots.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Light rain is moving over the Monterey 
Peninsula with VFR conditions to continue. Ceilings lower to MVFR by 
late this afternoon/early this evening as showers become more 
scattered in nature. Winds remain southerly through the TAF period 
with winds strengthening after 06Z. Gusts will strengthen at SNS 
overnight with highest confidence in gusts to 26 knots developing 
but there may be occasional gusts up to 30 knots tomorrow morning.

&&

.MARINE...
(Today through Monday)
Issued at 859 AM PST Wed Dec 31 2025

Moderate to fresh winds and moderate seas continue into Thursday.
Widespread light rain and a low potential for thunderstorms across
the southern waters continues into Thursday. Winds rapidly
strengthen from strong to gale force by early Friday as a second,
stronger, system arrives. Periods of moderate to heavy rain and a
slight potential for thunderstorms are expected Friday into
Saturday. Winds diminish early Sunday but breezy conditions and
rain showers continue into next week.

&&

.BEACHES...
Issued at 859 AM PST Wed Dec 31 2025

Perigean spring tides (King Tides) will impact the region through 
Sunday. This is due to the combination of the full moon on 1/3, 
lunar perigee on 1/1, and perihelion on 1/3. In other words all 
three planetary bodies are lined up and close to each other. In 
addition to the high astronomical tide, the incoming low pressure 
system will bring strong southerly winds, generating some storm 
surge. The combination of these factors will bring minor to 
moderate coastal flooding to the low-lying areas near shorelines 
and tidal waterways during high tide through Sunday. At the San 
Francisco tidal gauge, high tide is expected to be 2.0 ft above 
normal at 8:41 AM Thursday, 2.2 ft above normal at 9:34 AM Friday,
2.2 ft above normal at 10:26 AM Saturday, and 1.8 ft above normal
at 11:18 AM Sunday. These predictions include up to 1 foot of 
storm surge that will enhance the astronomical tide and flooding 
threat. High tide varies up to 90 minutes earlier or later along 
the Pacific Coast and through the San Francisco Bay, respectively.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Coastal Flood Advisory until 2 PM PST Sunday for CAZ006-505-506-
     508-509-529-530.

PZ...Small Craft Advisory from 9 PM Thursday to 3 AM PST Friday for 
     Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM.

     Gale Warning from 3 AM to 9 AM PST Friday for Pt Arena to Pt 
     Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM-Pt Pinos to Pt 
     Piedras Blancas 0-10 nm.

     Small Craft Advisory from 3 AM to 3 PM PST Thursday for Pigeon 
     Pt to Pt Pinos 0-10 nm.

     Gale Warning from 9 PM this evening to 3 PM PST Thursday for Pt 
     Pinos to Pt Piedras Blancas 0-10 nm.

     Small Craft Advisory from 9 PM this evening to 3 PM PST Thursday 
     for Pigeon Pt to Pt Pinos 10-60 NM.

     Small Craft Advisory from 3 AM to 9 AM PST Friday for Pigeon Pt 
     to Pt Pinos 10-60 NM.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Kennedy
MARINE...Kennedy

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Prepared by Boulder-Creek.com Weather at: Wed Dec 31 10:30:02 PST 2025