FXUS66 KMTR 161143
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
343 AM PST Tue Dec 16 2025
...New AVIATION...
.KEY MESSAGES...
Updated at 253 AM PST Tue Dec 16 2025
- Patchy dense fog through the morning
- Wet weather returns today and persists through the week
- Confidence is increasing that impactful weather will arrive for
next week's travel
&&
.SHORT TERM...
Issued at 253 AM PST Tue Dec 16 2025
(Today and Wednesday)
Tule fog in the Central Valley has seeped into the Delta and
subsequently into the North Bay Interior Valleys and East Bay
Interior Valleys. Given the slower arrival of the beneficial, light
rain and its accompanying cloud shield, expect conditions to
deteriorate through the night, although they will not be as
encompassing as last night. 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. Now onto
the rain. As mentioned it is arriving behind schedule with no
measurable precipitation falling yet. In fact, you will have to look
upstream at Eureka's radar (KBHX) to see anything at all. There's
also been a small cluster of lightning activity about 200 miles off
the Mendocino County Coast just to the north of our area. There's a
slight (up to 15% chance) for thunderstorms today for far northern
areas of the North Bay and adjacent coastal waters. The critical
ingredients for thunderstorms of lift, instability, and moisture are
there, but barely. Frontolysis of the cold front and conditionally
unstable lapse rates of 6 degrees C/km producing low CAPE of 50 J/kg
are not ideal for the first two ingredients. Moisture is how we even
have a slight chance. The 00Z OAK sounding observed a PWAT value of
0.84 inches which is near the 90th percentile (0.89 inches) for the
date and time. The moisture tap goes back to the subtropics and is
essentially on a conveyor belt between a storm force low in the Gulf
of Alaska and surface high pressure off the California/Mexico
border. PWATs and IVT values will continue to increase through the
day. Still, only light and beneficial rain is expected as our region
remains on the periphery of the main event to the north. More
beneficial light rain is expected into Wednesday as a second cold
front wrings out the leftover moisture from the past two days.
Rainfall totals through Wednesday: Coastal North Bay up to 0.50",
rest of Bay Area less than 0.10", Central Coast coastal/higher
terrain drizzle if not dry.
&&
.LONG TERM...
Issued at 253 AM PST Tue Dec 16 2025
(Thursday through Monday)
Some more beneficial, light rain is possible Thursday as the cold
front becomes a stationary front across the state of California. By
Thursday, global ensemble clusters are in agreement that upper-level
high pressure near the California/Mexico border will begin to weaken
and exit east thanks to an anomalously deep longwave trough
developing over the Northeast Pacific Ocean. This switch to
meridional flow from zonal flow will allow the storm door to open.
The next system is a little complicated with the surface feature
separated from the moisture source. We'll start with the gale force
low that is currently north of Hawaii. This will move very slowly to
the northeast, ultimately becoming stationary. Its moisture will get
pulled into a gale force low in British Columbia. This trajectory
will allow for the firehose of moisture to get directed at the
Pacific Northwest, at least initially. The plume of moisture will
slink down the West Coast into California on Friday with a cold
front following suit on Saturday. This will bring widespread
rainfall from north to south Friday into Saturday. It is important
to note that everything as of now looks beneficial up until this
point and rainfall totals have trended down. Yes, it may rain every
day between now and next Tuesday, but we have been dry for the last
three weeks or so. The aforementioned plume of moisture that is tied
to the stationary front north of Hawaii is expected to gain steam
with the development of a new low and linger into at least Monday
based on ECMWF and GFS IVT forecasts. GEFS and ECMWF ensemble IVT
forecasts generally show the ensemble means and +/- 1 standard
deviation generally in the 250-500 kg/ms range. Rainfall totals
Thursday through Monday: Coastal North Bay up to 5.00", rest of
North Bay: 1.50"-4.00", rest of Bay Area 0.50"-1.50", Central Coast:
0.10"-1.50". Another potential impact could be wind Sunday into
next week. It is important to know that the details of the long
term forecast will be dependent on what happens between now and
then as it will be a case of cumulative impacts. It is also
important to note that any details beyond seven days are beyond
the reaches of the official forecast and are incredibly difficult
to predict with any kind of accuracy. It can be continued to be
said though that we are expecting the wet pattern to continue at
least through Christmas Day (60% confidence) with the potential
(20-40% confidence) for it to linger through December 29th.
Whether you are travelling or staying home for the holiday, now is
the time to begin preparing by cleaning out your gutters and
ensuring that loose outdoor items are secured (like your Christmas
inflatables). If you are travelling out of the area by car, please
take the weather into account and check the forecast of the
National Weather Service for that area and the areas in between
here and there.
&&
.AVIATION...
(12Z TAFS)
Issued at 343 AM PST Tue Dec 16 2025
Conditions vary from LIFR to VFR. The Central Valley tule fog and
low stratus source remains intact with light offshore winds
continuing due to the current 1.4 mb SAC-SFO pressure gradient.
Higher cloud layers are arriving from the west helping to slow
radiative cooling, helping to limit dense fog to patchy coverage.
A few areas of LIFR-IFR remaining by late morning and early
afternoon otherwise MVFR-VFR today. Areas IFR later today, tonight
and Wednesday morning with light rain mainly across the North Bay,
the East Bay and the San Francisco Peninsula.
Vicinity of SFO...VFR-MVFR. Light northeast to east wind becoming
light west to southwest late today and tonight. Best chance of light
rain timing is by late evening to Wednesday morning.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR with light east to southeast winds
to late morning, winds becoming northwest 5 to 10 knots in the
afternoon. IFR due to low clouds developing tonight and Wednesday
morning.
&&
.MARINE...
(Today through Sunday)
Issued at 253 AM PST Tue Dec 16 2025
A plume of humid air from the Pacific will arrive today and
tonight ahead of a cool front moving southeast across the waters
and bays Wednesday. The interaction between the plume of humid
air, chilly air entrenched across the bays and cool front arriving
Wednesday will result in periodic light rain, mainly over the
northern waters and bays later today into Wednesday. Moderate seas
will prevail during the week.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 PM PST this evening for Pt Pinos to
Pt Piedras Blancas 0-10 nm.
&&
$$
SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...Canepa
MARINE...Canepa
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Prepared by Boulder-Creek.com Weather at: Tue Dec 16 04:30:03 PST 2025
From the National Weather Service
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