Los Trancos Woods, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

958
FXUS66 KMTR 201219
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
419 AM PST Sat Dec 20 2025

...New AVIATION...

.KEY MESSAGES...
Updated at 238 AM PST Sat Dec 20 2025

 - Mostly beneficial, light rainfall through the morning for the
   Bay Area

 - Moderate to heavy rainfall returns this afternoon for the Bay
   Area and Monterey Bay

 - Moderate to heavy rainfall will persist through Thursday for
   the Bay Area and Central Coast

 - Impactful and potentially hazardous wind returns Tuesday
   through Thursday

 - Hazardous beach conditions return Thursday for all Pacific
   Coast beaches

&&

.SHORT TERM...
Issued at 238 AM PST Sat Dec 20 2025
(Today through Monday)

Some light rain showers are streaming in from the west along a
stationary front with a subtropical moisture tap. These will
increase in coverage and intensity through the morning and into the
afternoon, relatively speaking. Rainfall during this timeframe is
expected to be beneficial; however, it will act to prime the pump
for the succession of systems that are slated to impact our region
through the forecast.

A Flood Watch is in effect from 4 PM today through 4 PM Monday for
the North Bay as flooding caused by excessive rainfall is possible.
Impacts include but are not limited to: excessive runoff may result
in flooding of rivers, creeks, streams, and other low-lying and
flood-prone locations, creeks and streams may rise out of their
banks, low water crossings may become flooded, and standing water is
likely in urban areas. People living or travelling through the North
Bay should monitor later forecasts and be alert for possible Flood
Warnings. Those living in areas prone to flooding should be prepared
to take action should flooding develop. Confidence is high that even
though the mainstem rivers (namely the Napa and Russian) will rise,
they will not have any problems during this timeframe with ensemble
forecasts showing a 0% probability of them reaching action/monitor
stage. This is not to say that their flashy creeks/streams will not
have problems, like Mark West Creek. If you encounter flooded roads,
do not drive around barricades and turn around, don`t drown!

Moderate to heavy rainfall arrives this evening for the North Bay as
IVT rebounds above 500 kg/ms for 24 hours and gets picked up by a
developing surface low off the California Coast. The firehose will
be focused on the North Bay through Sunday morning before it slowly
begins to slide south Sunday afternoon to include the rest of the
Bay Area and Northern Monterey Bay. As it does so, moderate to heavy
rainfall will spread across the aforementioned areas. Southern
Monterey Bay will remain on the periphery, so periods of moderate to
heavy rainfall can be expected there as well. While the Central
Coast and the rest of the Bay Area are not in the Flood Watch, I
would still expect ponding on roadways and at least some flooding in
low-lying, poor drainage, and urban areas including connectors,
off/onramps, and underpasses of freeways. Strong southerly winds
will also arrive tonight and persist through Sunday as the attendant
cold front approaches the area. Gusts up to 40 mph can be expected
along the coast, across ridgelines, and through gaps and passes. The
cold front will slowly move in Sunday into Monday, allowing moderate
to heavy rainfall to persist across the same areas. Likely after we
get through this period will we issue a region wide Flood Watch
beginning sometime in the Monday evening/Tuesday timeframe.

&&

.LONG TERM...
Issued at 238 AM PST Sat Dec 20 2025
(Tuesday through Friday)

By Tuesday, a developing gale force low off the California Coast
will tap into that same stream of subtropical moisture.
Unfortunately, there still remains a lot of uncertainty between the
deterministic ECMWF and GFS as well as their ensembles, most notably
when it comes to moisture content and strength. These differences
also get propagated downstream into location and timing.
Nonetheless, this system is expected to bring the most impacts up to
this point. Why? Well, one of the reasons is that it is forecast to
track parallel to our coastline, not only allowing for more impacts
based on proximity but also geography. An efficient wind field will
develop along coastal jet regions and through northwest/southeast
oriented valleys. The other reason is that the region will have seen
three days of rain at this point with at least some minor/nuisance
flooding expected by this point. This is where the differences
matter. The ECMWF is offering a bomb cyclone as a solution while the
GFS is not as strong. I understand bomb cyclone can be a scary word
for those not in the atmospheric sciences world. All it means is
that an extratropical cyclone decreases in central minimum pressure
by 24 mb in 24 hours using a reference latitude of 60 degrees North.
Without boring you with the math, since we are lower in latitude (36
to 38 degrees North), we don`t need the full 24 mb. Similar to
atmospheric river, the term bomb cyclone does not matter, but its
impacts do. Continuing along with the theme that the ECMWF is
stronger than the GFS, the former is nosing a 70 knot low-level jet
into the region. Some meteorological rule of thumb is that in a
perfect atmosphere these speeds (80 mph) can be realized at higher
elevations and potentially along the coast. More tree impacts may be
possible due to the opposite of normal wind directions and loose,
saturated soils increasing the risk for downed trees and thus downed
power lines. Thunderstorm chances increase to become slight (15%)
during this timeframe as well, but once again this will be heavily
dependent on the track of the low. While there is a lot of
uncertainty, it is certain that there will be more rain and wind
Tuesday into Wednesday as a surface low and its attendant cold front
make landfall somewhere in Northern California. This is also the
timeframe where we can start to see probabilities of exceedance
starting to creep up on ensemble forecasts for mainstem rivers.

Global ensemble clusters are in agreement in an anomalously deep
upper-level trough digging off the West Coast into Thursday
(Christmas Day). There is discrepancy on whether it will be an open
wave or a closed low as well as its tilt. Nonetheless, it will pick
up yet another surface low from the Eastern Pacific Ocean and bring
it into California Thursday into Friday. Thus, more rain and wind
can be expected. This will also bring the return of hazardous beach
conditions to all Pacific Coast beaches. Locals and visitors alike
should remain away from the Pacific Ocean during this time.

Over the next seven days expect impacts to worsen as soils become
saturated. 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 and thus downed power lines. 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. When all is said and done, locations can
generally expect a December`s worth of rain or a quarter of their
annual averages. If you are travelling by car for the Christmas
holiday or otherwise, I urge you to take the weather into account.
If you would like a localized forecast, like many of you do, visit
weather.gov/forecastpoints for a zip code specific forecast.

&&

.AVIATION...
(12Z TAFS)
Issued at 418 AM PST Sat Dec 20 2025

An approaching low pressure system is bringing reduced visibilities,
light rain, and fluctuating ceilings to airports across the region.
Confidence is highest in MVFR conditions persisting but temporary
drops to IFR and LIFR CIGs are possible through late this morning.
Guidance favors IFR CIGs but CIGs have been consistently coming in
as MVFR or above 3000 ft. Current thinking is that CIGs will stay
MVFR or higher due to the approaching system and incoming showers.
Similarly, guidance is showing more significant drops in visibility
while obs show it remaining largely stable so far. Thinking is that
visibility will initially remain more stable but decrease as more
persistent moderate showers arrive this afternoon/tonight. Patchy
fog has developed in the North Bay - impacting STS - but this is not
expected to be widespread. Winds strengthen towards the end of the
TAF period with gusts in excess of 20 to 25 knots possible along the
coast, North Bay, and SF Bay Shoreline.

Vicinity of SFO...Light showers are moving through the Bay Area with
ceiling heights fluctuating between MVFR and VFR. Moderate
confidence that VFR CIGs will generally prevail through the
afternoon with a tempo for MVFR CIGs through 15Z. Leaned on LAMP
guidance for the return of MVFR CIGs and brought in MVFR conditions
around 02Z when chances increased and more widespread light to
moderate rain returns to SFO. Winds strengthen by mid to late
tomorrow morning with gusts between 25 to 30 knots likely.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...CIGs remain on the IFR-MVFR border with
light offshore winds continuing. Showers are developing in the
vicinity of MRY and SNS over the Monterey Bay but they are not
expected to cause any major impacts this morning. Rain showers
become more widespread tonight into tomorrow morning with light to
moderate rain reaching both terminals by 08/09Z tomorrow. Generally
expecting overcast conditions to persist for much of today but may
see some clearing at SNS during the afternoon.&&

.MARINE...
(Today through Thursday)
Issued at 238 AM PST Sat Dec 20 2025

Light to moderate rain continues through Tuesday with shower
coverage increasing today. Rain intensity increases over the
northern waters by Sunday. Winds remain light and southerly
through  tonight when winds strengthen to fresh to locally strong
ahead of  cold frontal passage Sunday into Monday. Winds diminish
early Monday  before restrengthening late Tuesday as a more
substantial, stronger  system arrives Tuesday into Wednesday. This
system will bring  widespread gale force to near gale force winds
and the potential for  isolated hurricane force gusts across the
interior coastal waters  from Point Sur to Point Pinos. Seas build
through the second half of  next week, becoming very hazardous for
mariners with the potential  for wave heights in excess of 20
feet.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Flood Watch from 4 PM PST this afternoon through Monday
     afternoon for CAZ502>506.

PZ...Small Craft Advisory from 9 PM this evening to 9 PM PST Sunday
     for Pt Reyes to Pigeon Pt 0-10 nm-SF Bay N of Bay Bridge.

     Small Craft Advisory from 9 AM to 9 PM PST Sunday for Mry Bay-Pt
     Pinos to Pt Piedras Blancas 0-10 nm.

     Small Craft Advisory from 9 PM this evening to 3 PM PST Sunday
     for Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60
     NM.

     Small Craft Advisory from 3 AM to 9 PM PST Sunday for Pigeon Pt
     to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM.

&&

$$

SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...Kennedy
MARINE...Kennedy

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