FXUS66 KMTR 210457
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
857 PM PST Sat Dec 20 2025
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 346 PM PST Sat Dec 20 2025
- Flood Watch in effect now through Monday afternoon across the
North Bay
- Moderate to heavy rainfall tonight into early Sunday across the
North Bay
- Light to moderate rain shifts south once again to Bay Area and
Central Coast during the day Sunday
- Impactful and hazardous wind Tuesday through Friday along our
entire coastline, and higher elevations inland
- Hazardous beach conditions likely Wednesday through Friday for
all Pacific Coast beaches
&&
.UPDATE...
Issued at 545 PM PST Sat Dec 20 2025
We're seeing a slight lull in the rain this evening, but don't let
that fool you. The short term forecast remains confident and very
wet through Sunday night. The 00Z balloon measured the
precipitable water at 1.46". Not only is that a daily record, it's
in the top 10 of all December observations going back to 1948. So
the atmosphere is primed and ready for the next round of rain.
There won't be much lost to evaporation on the way down. Both the
GEFS and ECMWF ensembles show a 95%+ chance of at least AR3
conditions, with around a 40-50% chance of reaching AR4 conditions
on the CW3E Atmospheric River Scale. This first push will end
sharply Monday morning. We're still expecting some rain Monday,
but much less than Sunday. The next pulse arrives Tuesday, but the
rain totals are looking less impressive as JM mentioned in the
long term section. Using the same AR scale, there is only a 30-40%
chance of reaching AR3 conditions from Tuesday through Thursday.
Bottom line, there is an increasing chance that the wettest day
will be tomorrow for most of the Bay Area, and especially in the
North Bay. The same is not true for the Central Coast, which will
get progressively wetter as the week goes on.
&&
.SHORT TERM...
Issued at 346 PM PST Sat Dec 20 2025
(This evening through Sunday)
The atmospheric river moisture plume continues to oscillate this
afternoon, the focus has been on the Bay Area since mid-morning and
will then gradually move back toward the North Bay tonight into
early tomorrow morning. After sunrise Sunday, we'll see a notable
shift south toward the Central Coast as the upper level trough
associated with the strengthening system in the Gulf of Alaska
deepens. The twenty-four hour rainfall totals in the North Bay are
ahead of pace, with the Flood Watch beginning this afternoon and
periods of moderate to heavy rainfall expected overnight within the
area covered by the watch.
Satellite imagery this afternoon shows a large cloud shield
extending from Monterey Bay well north into southern Oregon. The
combination of a surface trough shifting south from NorCal to the
Santa Cruz mountains by late Sunday, a fully saturated boundary
layer up to 850mb and continuous shortwave energy embedded in the
500mb flow will result in rainfall totals across the North Bay
around 2-3" in the valleys and other low-lying areas, with higher
elevations potentially seeing up to 6" or more locally. The Mark
West Creek Near Mirabel Heights is forecast to reach action stage
by late Sunday evening, travelers should be cautious driving
through through the North Bay for the remainder of the extended
forecast.
&&
.LONG TERM...
Issued at 346 PM PST Sat Dec 20 2025
(Sunday night through next Friday)
Monday offers a bit of a reprieve for the North Bay as the
atmospheric river moisture plume is once again pushed south toward
the Santa Cruz Mountains and the remainder of the Central Coast. The
deepening and expanding upper low moving south out of the Gulf of
Alaska begins to assert its influence over the synoptic pattern at
our latitude. As the atmosphere resets to the west over open
water we'll still see scattered light rain across most of our area
during the day Monday with minimal impacts.
By Tuesday, some uncertainty is introduced into the forecast. The
most recent guidance suggest a less dynamic surface response to
the deepening trough off the coast, especially early on. Previous
guidance indicated rapid surface pressure falls during the day
Tuesday, with the strong area of low pressure taking aim at the
Bay Area/North Bay by late Tuesday night. Tuesday into Wednesday
is now in the later time frame of higher resolution data as well,
with the NAM12 indicating a less organized response at the surface
over that same time frame as well. Deterministic guidance depicts
a more organized surface response later in the forecast period
and farther north off the coast of NorCal by late Wednesday night.
The differences are somewhat subtle and confidence for impactful
rainfall across the North Bay between now and the end of the week
remains very high.
For the most part all the ensemble members have begun to pick up on
the subtle changes in the surface response and pattern. While some
of the inland rainfall totals may come down somewhat as we move
through the week, the pattern remains wet throughout our entire
forecast area and travel impacts will persist, especially the
farther north you are. Storm force to near hurricane force winds
at times up and down our coast Tuesday through early Friday will
continue to produce exceptionally dangerous conditions along our
coastal beaches and coastal waters.
&&
.AVIATION...
(06Z TAFS)
Issued at 857 PM PST Sat Dec 20 2025
Current radar showing scattered light rain showers mostly confined
to the North Bay. This placement will continue while the intensity
of the precip will trend upwards through tonight. Plume of moisture
and associated moderate rainfall will migrate southward Sunday
afternoon around the 00Z hour, beginning to impact Bay Area
terminals through Sunday night. Gusty S winds will also accompany
the front as it slowly moves south through the region. Outside of
rainfall concerns, MVFR/IFR cigs will prevail through the TAF period.
Vicinity of SFO...Mostly dry overnight and early Sunday as the main
moisture plume stays north of the SF Bay. This begins to change
Sunday afternoon as the plume drifts southward, taking aim at the
Bay Area around the 00Z hour. Preceding the precip, gusty S winds
will develop overnight tonight as the storm system strengthens.
These gusty S winds will continue through much of the period until
the front passes late Sunday night.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Moisture begins to increase tonight ahead
of heavier rainfall later in the period. Gusty S winds anticipated
by mid-morning Sunday, lasting through the rest of the period.
Pretty much keeping VCSH through the period due to terrain
enhancement of isolated showers, but heavier, more persistent rain
will likely hold off until just after the current TAF period
(Sunday night).
&&
.MARINE...
(Tonight through next Friday)
Issued at 857 PM PST Sat Dec 20 2025
Unsettled weather prevails through the next several days. Rainfall
intensity increasing tonight and Sunday across the northern waters
with a strong cold front sagging southward to near Pigeon Point by
Sunday evening. Gusty southerly winds increase along the front
through Sunday before weakening into Monday. A more substantial
period of unsettled weather enters the picture Tuesday with a gale
force low forecasted to enter the coastal waters. This system
appears likely to bring near storm force and isolated hurricane
force gusts to the waters south of Pigeon Point and near Point Sur
Tuesday night into Wednesday morning. Seas build through the week
and will be very hazardous to mariners with the potential for wave
heights in excess of 20 feet.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Flood Watch through Monday afternoon for CAZ502>506.
PZ...Small Craft Advisory until 9 PM PST Sunday for Pt Arena to Pt
Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM-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 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...JM
LONG TERM....JM
AVIATION...Behringer
MARINE...Behringer
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Prepared by Boulder-Creek.com Weather at: Sat Dec 20 22:30:02 PST 2025
From the National Weather Service
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