FXUS66 KMTR 181800
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1000 AM PST Thu Dec 18 2025

...New AVIATION...

.KEY MESSAGES...
Updated at 143 AM PST Thu Dec 18 2025

 - Dense fog through this morning

 - Widespread beneficial rainfall returns Friday

 - Impactful rain and wind return Saturday, persisting at least
   through Wednesday

&&

.UPDATE...
Issued at 913 AM PST Thu Dec 18 2025

No update to the forecast this morning. Fog conditions are
improving but pockets of dense fog persist in many areas so will
keep the Dense Fog Advisory in place until 11AM. Light rain is
expected to arrive in the North Bay late tonight which will begin
the the extended period of rainfall we've been messaging through
this weekend and all of next week.

&&

.SHORT TERM...
Issued at 143 AM PST Thu Dec 18 2025
(Today and tonight)

Today will start out foggy for some coastal and valley locations. If 
driving, slow down, use your low-beam headlights, and leave plenty 
of distance ahead of you. If commuting, allow extra drive time to 
reach your destination safely. Coastal/higher terrain drizzle is 
possible for places with fog/stratus and a few isolated showers are 
possible for the North Bay this afternoon, but generally speaking 
today will be dry. The same cannot be said for the rest of the 
forecast period. Today is your last guaranteed dry day for the next 
seven days. All preparations to your home should be made by tonight 
including but not limited to cleaning out your gutters and storm 
drains, securing loose outdoor items (like Christmas inflatables), 
raking and properly disposing of leaves, and trimming tree branches 
away from your home and power lines. 

&&

.LONG TERM...
Issued at 143 AM PST Thu Dec 18 2025
(Friday through Wednesday)

The first of a series of systems arrives tomorrow. Surface high 
pressure off the California/Mexico border and surface low pressure 
in the Gulf of Alaska/British Columbia will allow for a conveyor 
belt of subtropical moisture to stream into the state through 
Saturday. A cold front will sweep across the region Friday bringing 
widespread beneficial rainfall except for the Interior Central Coast 
which may stay dry outside of some higher terrain drizzle. Winds 
will increase with the cold frontal passage with gusts up to 30 mph 
expected along the coast, across ridgelines, and through gaps and 
passes. It seems as if there may be a brief break in the rainfall 
Saturday morning before it increases in coverage and intensity 
Saturday afternoon. While there is no trigger for this, the reason 
seems to be warm air advection from the aforementioned surface high 
pressure, a persistent influx of IVT in excess of 250 kg/ms, and the 
aforementioned cold front providing a convergence zone for all of 
these to materialize. Saturday will likely be the day where things 
begin to transition from beneficial to impactful. Through Saturday, 
the North Bay is expected to receive the brunt of the rainfall. As 
such, if you live near or know that you will be travelling through 
the area, please be aware of problematic flashy areas of Mark West 
Creek and do not drive around barricades. We are not quite within 
the range of hi-res models to look at hourly precipitation rates for 
all of Saturday yet, but like above, residents or travelers near the 
Pickett Fire burn scar need to be aware of and prepare for the 
incoming weather.

The second system arrives Sunday by way of a surface low moving into 
Northern California. There is still uncertainty in exact location, 
strength, and timing of the low, but confidence is increasing that 
there will be one. Nonetheless, the proximity to it will bring more 
impacts than the ones that have remained in the Pacific Northwest. 
The surface low will get its moisture from the same source region in 
the subtropics. The difference this time around is that instead of a 
quick peak in IVT, values will remain near 500 kg/ms for near 24 
hours based on the GEFS and ECMWF 80 member ensemble mean. Moderate 
to heavy rainfall is expected across most of the region on Sunday. 
Rainfall totals will be locally higher in southwest facing terrain 
such as Coastal North Bay, Santa Cruz Mountains, and Big Sur Coast. 
In addition to the rain, wind will be another hazard as it increases 
Saturday night. Probabilities are increasing for impactful wind 
(gusts of 45+ mph) during this time frame. Any wind impacts will be 
exacerbated by the rain and vice versa. The combination of wind and 
moist soils will increase the risk for downed trees. Additionally, 
any leaves that are left on trees will pose the risk of 
blocking/clogging storm drains if/when they fall, potentially 
leading to more flooding. The accompanying cold front will pass 
through on Monday, bringing yet another day of rain and wind. As of 
now Monday does not look as impactful as Sunday, but there is still 
uncertainty in rainfall totals and the cumulative impacts that four 
days of rain will have on the region at this point. 

The third system arrives Tuesday and will linger into Wednesday 
(Christmas Eve) as a surface low develops off the coast. There is a 
great deal of uncertainty in location, strength, and timing. Even 
so, it does look like it will be impactful for our area in terms of 
rain and wind with IVT rebounding and a low-level jet nosing into 
the region. A potential silver lining in terms of rainfall is that 
this system will spread from south to north, bringing rainfall to 
places that will not have seen much up until this point such as the 
Interior Central Coast. 

All of this to say that there is still a great deal of uncertainty 
in location, strength, and timing of things and subsequently their 
impacts. There is certainty that it is going to rain for the next 
seven days and likely into the Christmas holiday and beyond. Impacts 
will not only be felt with each individual system, but also the 
cumulative effect of the ones before it. All preparations for rain 
and wind should be done today. If you are travelling, please account 
for the weather and plan accordingly. Minor/nuisance flooding of 
urban, low-lying, and poor drainage areas as well as flashy 
creeks/streams will likely happen at some point, potentially as soon 
as Saturday.

&&

.AVIATION...
(18Z TAFS)
Issued at 952 AM PST Thu Dec 18 2025

Patchy dense FG is improving this morning, but IFR stratus will
linger across North Bay and Bay Area terminals into midday. By
this evening, shower chances increase beginning in the North Bay,
spreading into the Bay Area overnight with the best chances being
Friday mid-morning.

Vicinity of SFO...IFR stratus likely to linger until around the 
20Z hour with skies mostly clearing afterward into the afternoon.
Slight chance for shower activity late this evening. Better chance
for -SHRA beginning midday Friday as the initial front of a
multi-day atmospheric river system moves into place.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Dense FG improving rapidly across the
region with some IFR/LIFR stratus linger into the 19Z hour.
Otherwise skies clearing to VFR through midday and return of IFR
stratus this evening. Rain chances for Monterey Bay region hold
off until after the current TAF period.

&&

.MARINE...
(Today through Tuesday)
Issued at 913 AM PST Thu Dec 18 2025

A dense fog advisory across the Monterey Bay is in effect until
11 am today. Light to moderate onshore breezes will prevail with
moderate seas through late week. Periodic rain chances continue
through the week as well with additional rain developing over the
weekend. More substantial rainfall and strong storm system
developing around Christmas day.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Dense Fog Advisory until 11 AM PST this morning for CAZ506-510-
     516-528>530.

PZ...Dense Fog Advisory until 11 AM PST this morning for Mry Bay.

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

&&

$$

SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...Behringer
MARINE...Behringer

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Prepared by Boulder-Creek.com Weather at: Thu Dec 18 10:30:02 PST 2025