Technical NOAA Weather Discussion

FXUS66 KMTR 290532
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
932 PM PST Sun Dec 28 2025

...New AVIATION...

.KEY MESSAGES...
Updated at 128 PM PST Sun Dec 28 2025

 - Cold overnight conditions for interior and valley locations

 - Rain returns Wednesday and lasts through the weekend

 - Coastal flooding from King Tides is expected Wednesday through
   Sunday

&&

.UPDATE...
Issued at 854 PM PST Sun Dec 28 2025

No update to the forecast this evening, with colder than normal
temperatures across much of our area. The dry and cool airmass
that has replaced the unsettled weather over the holidays will
persist for the first half of the upcoming week, then another
stretch of potentially impactful rainfall for the second half of
the week into next weekend.

&&

.SHORT TERM...
Issued at 128 PM PST Sun Dec 28 2025
(This evening through Tuesday)

High pressure building into the Intermountain West and a coastal 
trough off the West Coast will dominate our weather through Tuesday. 
This is a textbook setup for offshore flow. Fortunately, we do not 
need to worry about fire weather concerns as we normally would. 
Instead, clear and dry conditions will yield cold overnight 
conditions. A Cold Weather Advisory has been issued for tonight for 
interior and valley locations. This goes for everyone but especially 
for those of you who are still without power: do not use generators 
or grills inside as carbon monoxide poisoning can result! Whether 
you are in the Cold Weather Advisory or not, it is going to be a 
cold night and a cold start to the week. Some grief was had over the 
minimum temperature forecast due to the aforementioned offshore 
flow. Why? Well, offshore flow can bring compressional/downslope 
warming. This is why the North Bay Interior Mountains were left out 
of the Cold Weather Advisory. 925mb flow (~2500 ft) from the ECMWF 
and GFS show 30 knots through there overnight. This will mix down 
warmer and drier air aloft which should help keep the area above 36 
degrees. There is even a bust scenario where it mixes all the way 
down to the North Bay Interior Valleys, but that is not as likely. 
Another bust scenario is that the Northern Salinas Valley remains 
above 36 degrees with the help of moderate drainage winds. 
Nonetheless, the warming and drying trend will continue through 
Tuesday with cold overnight conditions.

&&

.LONG TERM...
Issued at 128 PM PST Sun Dec 28 2025
(Wednesday through next Saturday)

Coastal flooding from King Tides returns Wednesday and lasts into 
Sunday. This will impact the Monterey Bay Shoreline, San Francisco 
Bay Shoreline, San Pablo Bay Shoreline, and the Pacific Coast. If 
travel is required, allow extra time as some roads may be closed. Do 
not drive around barricades or through water of unknown depth. Take 
the necessary actions to protect flood-prone property.

A pattern change is on tap for Wednesday as a surface low pressure 
system near the California/Mexico border picks up tropical moisture 
on its journey northward. ECMWF and GEFS ensemble IVT forecast shows 
their 80 member ensemble mean breaching 250 kg/ms during this time. 
It's too soon to say with certainty, but minor flooding in low-
lying, poor drainage, and urban areas as well as flashy 
creeks/streams can be expected as early as Wednesday. Tree debris 
from this past week that hasn't been disposed of may act to clog 
storm drains and gutters, resulting in more flooding. Wind will 
increase during this time, but it is not expected to be impactful by 
itself. However, the combination of wind and moist soils will 
increase the risk for downed trees, especially where trees have 
recently been weakened. If you are looking for a sign to put away 
your outdoor holiday decorations, this is it. All outdoor 
preparations such as securing loose outdoor items, trimming tree 
branches, and cleaning out gutters should be finished by Tuesday. 
There's also a low chance (<15%) for thunderstorms Wednesday into 
Thursday, but that will be heavily dependent on the exact trajectory 
of the low. The more impactful of the two systems looks to be Friday 
as a surface low pressure system and its attendant cold front come 
into Northern California. Both the deterministic ECMWF and GFS 
depict a 50+ knot 925mb jet accompanying it which may equate to a 
Wind Advisory for portions of the area (mainly looking at the coast 
and the higher terrain). The same rain impacts of minor flooding in 
low-lying, poor drainage, and urban areas as well as flashy 
creeks/streams can be expected during this timeframe. Deterministic 
and ensemble river forecasts do not show any mainstem river points 
reaching minor flood stage; however, this could be a result of QPF 
only recently trending up this morning after the river forecasts 
were already completed. HEFS does show a 10% chance of river flow 
exceedance for the notoriously flashy spots in Sonoma County of 
Russian River at Geyserville (GEYC1), Laguna de Santa Rosa at Stony 
Point Road NR Cotati (CTIC1), and Mark West Creek near Mirabel 
Heights (MWEC1) which finally dropped below action stage just as I 
was typing this.

&&

.AVIATION...
(06Z TAFS)
Issued at 932 PM PST Sun Dec 28 2025

Offshore winds and VFR will continue except tule fog /VLIFR-IFR/ 
from the Central Valley is forecast to reach into eastern Contra 
Costa county late tonight and Monday. Patchy valley fog /VLIFR-
IFR/ also developing due to radiative cooling tonight and Monday 
morning.

Vicinity of SFO...VFR. Northeast wind 5 to 10 knots tonight and 
Monday becoming northwest near 5 knots Monday afternoon then light 
northeast Monday night.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR. Winds east to southeast 5 to 12 knots
tonight and Monday morning. Winds becoming light and variable to 
locally onshore late Monday afternoon then light east to southeast
Monday night.

&&

.MARINE...
(Tonight through next Saturday)
Issued at 854 PM PST Sun Dec 28 2025

A gentle northerly breeze will gradually shift to easterly over
the next 24 hours. Seas will remain moderate through Thursday.
Overall marine conditions will remain favorable before the next
low pressure system brings strong southerly winds and rough seas
by Friday.

&&

.BEACHES...
Issued at 945 AM PST Sun Dec 28 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 from 1 AM to 9 AM PST Monday for CAZ506-
     510-513>518-528.

PZ...None.
&&

$$

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

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Prepared by Boulder-Creek.com Weather at: Sun Dec 28 22:30:03 PST 2025
From the National Weather Service