FXUS66 KMTR 301241
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
441 AM PST Tue Dec 30 2025
...New SHORT TERM, LONG TERM, AVIATION, MARINE...
.KEY MESSAGES...
Updated at 432 AM PST Tue Dec 30 2025
- Cold morning conditions for interior and valley locations again
this morning.
- Rain returns Wednesday, becomes widespread for New Years Eve,
and additional rains expected over the weekend and into the
next work week.
- Coastal flooding from King Tides is expected Wednesday through
Sunday.
&&
.SHORT TERM...
Issued at 432 AM PST Tue Dec 30 2025
(Today and tonight)
Another mostly quiet night so far, with the cooler and dry airmass
allowing for temperatures to fall into the mid 30s for the more
interior areas. Cold Weather Advisories are in effect for these
areas, and Dense Fog Advisories are also in effect for the North and
East Bay valleys. The overnight forecast package had a focus on the
cool overnight lows and the intrusion of fog from the Sacramento
Valley. Additional adjustments were made to winds as well to better
capture some of the increases in gusts during the offshore flow as
well as the mid week pattern change.
Today's weather continues the trend of a cold morning followed by
mostly clear conditions into the afternoon and highs peaking again
in the 50s and 60s. Though, the late afternoon will show signs of
the upcoming pattern change, as high clouds build from the south.
These clouds will fill over the region as the large cut-off low to
the southwest re-enters the jet stream and makes its way toward the
California coast.
The orientation of this low relative to us will allow warmer and
more moist air to work its way up from the south. This will have
most areas seeing overnight temps plateauing in the 40s and prevent
another round of Cold Weather Advisories. Then the first wave of
rain arrives to the southern counties early Wednesday.
&&
.LONG TERM...
Issued at 432 AM PST Tue Dec 30 2025
(Wednesday through Monday)
Models have been locked in on this cut-off low providing the
California coast with another round of rain for a while, but timing
and overall rain amounts had been harder to narrow down. The more
recent updates and the inclusion of shorter-term, higher resolution
models have lead to a better consensus. However, there still seems
to be some struggles in model guidance on the finer details,
possibly from how uncommon of a set up our next rainmaker will be.
It's interesting to watch, as it is already in motion: Our current
ridging pattern and trough to the west has begun transporting a cut-
off low northward from around the 30th parallel. This southerly pull
of moisture wouldn't be out of place in months like August, but it's
fairly rare mid-winter. What will this mean? Quite a bit of rain for
SoCal, while light to moderate rain builds into the Central Coast
early on the 31st and becoming CWA-wide as we welcome in the New
Year.
The low will push inland through the second day of the new year as
it gets folded into the building trough ahead of the next, more
classic, winter rain system arriving into the end of the week. The
time of arrival for the this system could still be delayed, but the
current forecast shows a cold front arriving along the coast this
Friday with it's parent low building in close behind into that
weekend. This looks to offer higher rain rates and much stronger
winds than the cut-off low, but does look to move through the area
at a good pace, preventing chances for elevated rain rates from
staying over one area for too long.
Model agreements splinter through the late weekend and into the next
work week, but the ECMWF and GFS outputs do call for a wet January.
The big differences between the two are more down to the timing and
the paths these systems take, but they are both calling for the jet
stream to become a conveyer belt of cold fronts and low pressures
moving through the area. This won't leave a lot of time between rain
systems and could lead to more of a marathon of rain chances. This
will be something worth checking back in on as the forecast evolves.
&&
.AVIATION...
(12Z TAFS)
Issued at 432 AM PST Tue Dec 30 2025
Tule fog has seeped into portions of the east bay and is
trying to spread through some of the valleys. This has dropped
visibility around Byron and Concord but has yet to infiltrate LVK
yet, though it is flirting with APC. Opted to trend slightly more
pessimistic with APC and hinted at the possibility of reduced cig
and vis for LVK. To the north of there, STS has been dealing with
its own visibility issues and will likely do so through the mid
morning. Once the fog burns off, VFR conditions prevail. Rain may
need to be introduced if showers are able to hold together and make
it past the drier air a loft today. Otherwise, rain should hold off
until late tonight or Wednesday morning and it will be coming in
from the south.
Vicinity of SFO...Offshore winds and VFR conditions should prevail
today. However, CAMs within the HREF suggest the possibility of
showers trying to push in from the south from 17-23Z today.
Confidence is low given current radar returns over San Luis Obispo
look like, but it will be something to watch. If this does pan out,
rain chances may need to be added to the TAF after 12Z Wed as it
would favor the faster HREF/CAM solution, rather than the slower NBM
PoP and timing. Medium confidence for the forecast.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Quiet conditions prevail with offshore
winds holding and high clouds starting to stream in from the south.
Some of the CAMs show showers trying to push into the Central Coast
from the south later today, with rain chances increasing late
Tuesday night into Wednesday morning. Will need to keep an eye on
the TAFs and adjust the forecast if the trend continues to the rain
arrival earlier.
&&
.MARINE...
(Today through Sunday)
Issued at 432 AM PST Tue Dec 30 2025
Moderate to fresh east to northeast winds winds and moderate seas
will prevail through Wednesday. Fresh to strong southerly breezes
return Thursday into Friday with gale force gusts likely. Seas
build to become rough Thursday into Friday. Rain returns Wednesday
and lasts through the weekend.
&&
.BEACHES...
Issued at 925 PM PST Mon Dec 29 2025
Perigean spring tides (king tides) return Wednesday - 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 onshore winds, generating some storm surge. The
combination of these factors will bring minor coastal flooding to
the low-lying areas near shorelines and tidal waterways during
high tide Wednesday through Sunday. At the San Francisco tidal
gauge, high tide is expected to be 1.5 ft above normal at 7:51 AM
Wednesday, 1.7 ft above normal at 8:41 AM Thursday, 1.8 ft above
normal at 9:34 AM Friday, 1.8 ft above normal at 10:26 AM
Saturday, and 1.6 ft above normal at 11:18 AM Sunday. These
predictions include 0.5 feet 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 from 5 AM Wednesday to 2 PM PST Sunday
for CAZ006-505-506-508-509-529-530.
Cold Weather Advisory until 9 AM PST this morning for CAZ504-506-
510-514>518.
Dense Fog Advisory until 10 AM PST this morning for CAZ506-510.
PZ...None.
&&
$$
SHORT TERM...Murdock
LONG TERM....Murdock
AVIATION...KR
MARINE...KR
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Prepared by Boulder-Creek.com Weather at: Tue Dec 30 08:30:02 PST 2025
From the National Weather Service
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