FXUS66 KMTR 211303
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
503 AM PST Sun Dec 21 2025
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 151 AM PST Sun Dec 21 2025
- Flood Watch in effect now through Monday afternoon for the
North Bay; extension in time and coverage likely
- Moderate to heavy rain reaches North Bay this morning; Bay Area
and Central Coast this afternoon
- Moderate to heavy rain continues with increasing flooding
potential through the extended
- Impactful and likely very hazardous winds Tuesday through early
Friday along our entire coastline and higher elevations inland
- Dangerous beach conditions likely late Tuesday through Friday
for all Pacific Coast beaches
&&
.SHORT TERM...
Issued at 151 AM PST Sun Dec 21 2025
(Today and tonight)
Moderate to heavy rain is entering the North Bay with flooding
concerns to increase over the course of the day. Currently, the
heaviest rain is still located to our north in the Eureka WFO's
county warning area. This band is expected to shift southwards
overnight with the heaviest rain reaching Sonoma, Napa, and Marin
Counties by late morning into the early afternoon hours. The main
rainband will shift further south into the rest of the Bay Area and
northern Monterey Bay by late tonight into early Monday morning. The
highest rain totals are still expected to be across the North Bay
where the higher elevations will see between 5-6" of rain, 2-3"
across the valleys, and 1-3" across the rest of the Bay Area. The
Santa Clara Valley will be rainshadowed by the Santa Cruz Mountains
with totals ranging from 0.5-1.0" expected. Up to half an inch is
expected for the Monterey Bay region with between 1-2" across the
Big Sur/Santa Lucia Range.
The surface low pressure associated with this system is expected to
move inland to our north along the California/Oregon border. While
we will still see gustier winds, particularly along the coast, the
strongest winds will be located closer to the low pressure system
across far northern California. Gusts will peak between 30 to 40 mph
along the coastline and higher elevations with gusts between 25 to
35 mph possible within the interior valleys. All outdoor
preparations for both precipitation and wind concerns should be
completed by this time. There is a 15-20% chance of thunder across
the coastal waters and a 10-15% chance of thunder across the Bay
Area this afternoon and evening as the main rain band shifts south.
High resolution guidance suggests stratiform rain will continue
through the remainder of today with MUCAPE values increasing to
around 100 J/kg this evening/overnight. The highest chances for
thunderstorms would be within the Monterey Bay region tonight into
tomorrow. Not going to rule out an isolated thunderstorm or two
tonight for the Monterey Bay region but confidence in thunderstorm
development at this time is low.
The initial Flood Watch remains in effect for the North Bay through
4PM Monday and is expected to be followed by a more widespread Flood
Watch across our CWA. Initial flooding concerns tonight remain low
but will start to increase by late today as light rain gives way to
more moderate to at times heavy rain. For residents of the North
Bay, nuisance flooding is likely to start increasing in frequency by
this evening with sharp rises in flashy (i.e. smaller) creeks and
streams expected. The best example of this would be the Mark West
Creek (forecast to reach action stage Sunday evening) in Sonoma
County which can rise quickly during periods of heavy rain. If you
encounter a flooded roadway, Turn Around, Don't Drown! Over the last
few years, particularly at Mark West Creek, there have been several
fatalities with regards to people driving into and getting swept
away by flood waters. It is not worth it to try and drive through a
flooded roadway, plan alternate routes to reach your destination in
the event that you encounter a flooded roadway. Flooding concerns
will ramp up as we head into next week and the long duration
atmospheric river continues.
&&
.LONG TERM...
Issued at 151 AM PST Sun Dec 21 2025
(Monday through Saturday)
Not too much change to the overall precipitation totals in the Long
Term forecast. Monday looks to be a break between incoming rounds of
moderate to heavy rain as forecast IVT values weaken and generally
light to moderate rain continues for the Bay Area and northern
Monterey Bay. Orthographic lift may sustain locally higher rain
rates/totals across the Santa Cruz Mountains which could see up to
1" of rain on Monday. Tuesday into Wednesday, widespread moderate to
heavy rain returns to the Bay Area and Central Coast as a low
pressure system moves northwards parallel to the California
coastline. A weaker low pressure system will move into northern CA
Thursday into Friday and bring another round of moderate to heavy
rain to the region. Light rain then continues into the weekend but
precipitation totals look to be on the light. Flooding impacts are
expected to increase moving through the week as soils become
saturated from successive rounds of rain. This will result in
increased ponding/flooding on roadways and increased surface runoff
into creeks/streams resulting in sharp rises and potential for
rivers in the North Bay (Russian and Napa) to reach at least action
to minor flood stage. More flood prone creeks such as the San
Lorenzo in Santa Cruz and the Mark West Creek in Sonoma are likely
see rises as heavy rain continues. If you live along the
Russian/Napa Rivers it is highly recommended you stay up to date on
the latest river forecasts in the event that river flooding
potential increases. As mentioned in the short term, the initial
Flood Watch remains in effect through 4PM Monday. This Flood Watch
is expected to be followed by a more widespread Flood Watch that
will last for much of the upcoming week.
In terms of forecast confidence, there continues to be decent
uncertainty as to the system arriving Tuesday-Wednesday with large
differences between the ECMWF and GFS models continuing. The ECMWF
suggests the low will deepen to around 977 hPa as it moves up the CA
coastline before moving inland along the CA/OR border. The GFS keeps
the low less well defined and only has it dropping to around 1000
hPa. Under a situation described by the ECMWF, impacts would be
greater and more widespread across our CWA whereas under the GFS
impacts would be not as impactful. The current forecast falls more
in line with the scenario provided by the ECMWF where stronger
impacts are expected across our CWA. While rain and flooding are two
of the most predominant hazards, strong, hazardous winds are
expected to develop Tuesday-Thursday. The 925 mb ECMWF winds shows a
strong low level jet (peaking between 70 to 80 knots) moving
parallel up the Bay Area and Central Coast coastline. This will
result in the potential for widespread gusts in excess of 40-50 mph
across the CWA and gusts in excess of 60 mph directly along the
coastline, the Salinas Valley, and over the marine environment. At
this time, there is strong consideration of wind hazard products for
the Tuesday-Thursday time period but the exact locations are still
being narrowed in on. As these strong winds will come after several
days of rain, soils are likely to be saturated (or close to
saturated) which may result in trees being toppled more easily and
potentially result in power outages. Conditions along the coastline
will be extremely dangerous for beachgoers (and over the marine
environment for mariners) as stronger winds move in Tuesday to
Thursday. The highest end WRF guidance suggests that isolated
hurricane force gusts (winds in excess of 64 knots/74 mph) may occur
along the Big Sur coastline as the jet moves northwards. Winds
strengthen Tuesday into early Wednesday, briefly diminish Wednesday
morning, then strengthen again Wednesday into Thursday before
diminishing by Friday. Another factor is the potential for
thunderstorms. There is a 15-25% chance of thunderstorms across the
South Bay and Central Coast Tuesday night into Wednesday as this
system moves inland. ECMWF guidance shows several hundred joules of
CAPE moving in with this system so confidence is slightly higher
that thunderstorm activity may develop as this system moves up the
coastline. The next 24 hours will be important to finalize what the
impacts from the Tuesday-Wednesday system will be as that timeframe
starts to get in the range of high resolution models.
Bottom line: moderate to heavy rain and hazardous winds are expected
for much of next week. Residents should be prepared for impacts to
holiday travel (driving, flight delays, etc) and should pay careful
attention to the weather forecast, adjusting holiday travel plans as
needed to avoid the most impactful weather. If the forecast of
hazardous, storm force to isolated hurricane force winds holds true;
the beach and marine environment will be very dangerous this week.
&&
.AVIATION...
(12Z TAFS)
Issued at 503 AM PST Sun Dec 21 2025
Low clouds are spreading across the region as the next rainmaker
builds in the North Bay. Light prefrontal rains continue to spread
south ahead of the front through the day. Strong southerly winds and
gusts along with moderate to heavy rains build into the afternoon in
the North Bay as the main rain band arrives. This band of rain will
slowly push south and east through the day. The band slows as it
passes into the South Bay and Monterey Bay, leading to chances for
prolonged strong winds into the night. The system moves northward
into early into early Monday, but winds and rain rates look to
reduce. Mid-level and high level clouds persist through the
forecast, but low cloud cover becomes spottier into Monday.
Vicinity of SFO...Light showers and low CIGs continue continue to
move through the area into the early evening. Expect southerly winds
to increase into the late afternoon and early evening as the main
rain band approaches. The added mixing from the increased winds
could offer some moments of lifted CIGS, but these look to be short
lived. Expect visibilities to suffer as the rain rates increase
through the evening. Shower chances thin into the late night as the
rain band move farther south, but reform as the front shifts
northward into late Monday morning.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Low clouds are building with light rain
chances through the morning. Southerly winds increase into the
afternoon, helping lower clouds and showers scatter. Winds increase
into the late night as the main rain band approaches. Expect reduced
visibilities from rainfall as the band focuses over the area through
the end of the TAF period.
&&
.MARINE...
(Today through Friday)
Issued at 503 AM PST Sun Dec 21 2025
Rainfall intensity increases through 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 AM PST this morning for Pigeon Pt
to Pt Pinos 0-10 nm-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.
Gale Warning from 9 AM this morning to 9 PM PST this evening for
Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm-Pt Arena to Pt Reyes
0-10 nm-Pt Arena to Pt Reyes 10-60 NM-Pt Pinos to Pt
Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm-SF
Bay N of Bay Bridge.
Small Craft Advisory until 3 AM PST Monday for Pigeon Pt to Pt
Pinos 10-60 NM.
&&
$$
SHORT TERM...Kennedy
LONG TERM....Kennedy
AVIATION...Murdock
MARINE...Murdock
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Prepared by Boulder-Creek.com Weather at: Sun Dec 21 06:30:02 PST 2025
From the National Weather Service
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