FXUS66 KMTR 221928
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1128 AM PST Mon Dec 22 2025

...New AVIATION...

.KEY MESSAGES...
Updated at 336 AM PST Mon Dec 22 2025

 - Flood Watch remains in effect for the North Bay through Friday
   evening; Flood Watch for the rest of Bay Area and Central Coast
   begins Tuesday morning

 - Light to moderate rain continues through Monday with minor 
   nuisance flooding and rises in rivers/streams across North Bay 
   possible. Turn Around, Don't Drown!

 - Increasing chances for urban flooding and extensive travel
   delays late Tuesday into early Friday across entire Bay Area
   and Central Coast as widespread heavy rain returns

 - Very hazardous, impactful winds Tuesday through early Friday
   along our entire coastline and higher elevations inland with
   power outages likely

 - Dangerous beach conditions anticipated late Tuesday through
   Friday for all Pacific Coast Beaches

&&

.SHORT TERM...
Issued at 242 AM PST Mon Dec 22 2025
(Today and tonight)

A Flood Watch remains in effect through 4 PM PST for the East Bay 
with the overall plan of expanding the Flood Watch in areal coverage 
and time on track. The Russian River at Geyserville reached minor 
flood stage at 11:30 PM PST 12/21 (39.74 ft) with a Flood Advisory 
in effect through 7:15 AM this morning. Minor flooding is likely to 
continue in the vicinity of the Russian River between Geyserville 
and Jimtown in Sonoma and in the vicinity of Mark West Creek. If you 
are driving through the North Bay, be prepared in case you encounter 
flooded roadways. Roadway flooding may be hard to see in the dark so 
stay aware of your surroundings and if you encounter flooding - Turn 
Around, Don't Drown. Flooding impacts will be exacerbated by the 
additional rounds of moderate to heavy rain expected Tuesday through 
Friday. Wind impacts will also increase Tuesday with a High Wind 
Watch in effect from 7PM PST Tuesday to 4AM PST Wednesday.

Today is a transition day between the first big round of rain 
yesterday and the next big round of rain coming in tomorrow. Light 
rain will continue across the Bay Area through late tonight as 
stationary front lingers over the Bay Area. Rainfall amounts are 
expected to decrease with the IVT plume weakening from around 500 
kg/ms this morning to around 250 kg/ms by this evening. The main 
rain band will remain locked in over the Bay Area today, with minor 
oscillations towards the North and South Bays, rain will be more 
stratiform within the main rain band with scattered showers on the 
periphery of it. This will result in an additional 0.5" of rain 
across the interior today. For the coastal mountain ranges (Santa 
Cruz Mountains, Mt. Tamalpais), the orientation of this moisture 
plume is perpendicular with the coastline which supports increased 
orographic uplift. The coastal mountains will see an additional 1-
1.5" today (primarily in the early morning hours when the IVT plume 
is the strongest) with locally higher totals possible depending on 
where the most orographic uplift is able to occur. Locally breezy 
winds peaking around 30 mph are will continue through the late 
morning along the coastline, higher elevations, and SF Bay Shoreline 
through late morning. Winds will then generally decrease throughout 
the afternoon and evening as the surface low progresses eastward. If 
you did not finish your outdoor preparations over the weekend or 
need to make additional preparations, today is your last chance to 
do so before two stronger systems arrive Tuesday into Wednesday and 
Wednesday into Thursday. Thunderstorm chances are low today (5-10%). 
The ECMWF and HRRR models show around 100 J/kg of MUCAPE across the 
South Bay this morning but wind shear in the lowest levels does not 
look too supportive of thunderstorm development.

&&

.LONG TERM...
Issued at 242 AM PST Mon Dec 22 2025
(Tuesday through Sunday)

To best address the upcoming weather and associated hazards, the 
Long Term discussion will be broken into three parts: Flooding, High 
Winds, and Thunderstorms.

Flooding: Tuesday morning, light rain will reintensify to heavy rain 
first over the North Bay before extending southwards into the rest 
of the Bay Area and Central Coast Tuesday afternoon into Tuesday 
evening. The ECMWF continues to show a rapidly intensifying cyclone 
meeting bomb cyclone criteria (a drop of 24 millibars or more in 24 
hours with some adjustments required depending on the latitude of 
the cyclone) moving parallel to the California coastline on Tuesday. 
This low pressure system will deepen from around 1000 hPa early 
Tuesday morning to 974-975 hPa as it moves up the coastline by early 
Wednesday morning. The GFS and Canadian models are starting to catch 
up to the ECMWF and are also showing a deepening, strong low 
pressure system off the CA coast. At the same time, this system will 
bring a renewed transport of moisture into California with IVT 
values in excess of 700-800 kg/ms likely. Between Tuesday and early 
Wednesday, an additional 1.5-3.5" are expected across the North Bay, 
1-3" across the Santa Cruz Mountains, 2-3" across the Santa Lucia 
Range and 1-1.5" across the interior Bay Area, Monterey Bay region, 
and interior Central Coast. Late Wednesday into Thursday, a second 
low pressure system will start to deepen and move northwards 
parallel to the CA coast. This system does not deepen as much as the 
first system so it is not considered a bomb cyclone. It will bring 
another round of widespread moderate to heavy rain and widespread 
totals of 2-4" (locally up to 5" in the mountains) between late 
Wednesday through Friday. After Friday, rain amounts start to 
diminish with light rain to persist into the weekend. In terms of 
flooding impacts, flooding impacts will get worse throughout the 
week as multiple rounds of moderate to heavy rain occur and soils 
saturate. This will result in increased nuisance flooding across 
urban areas and see sharp rises in minor streams and creeks. Minor 
flooding is likely across at least a few of the mainstream rivers 
(such as the Russian River and Napa River in the North Bay) by late 
in the week. Anyone who lives in the vicinity of a river or a small 
creek known to rise rapidly should pay careful attention to the 
forecast and timing of heaviest precipitation. Flooding can often be 
hard to see at night so exercise extreme caution when driving at 
night. If you see roadway flooding, it cannot be expressed enough 
that it is absolutely not safe to drive through. Turn Around, Don't 
Drown is the most important motto for you to remember this week.

High Winds: In addition to heavy rain and flooding concerns, the two 
strong systems Tuesday-Wednesday and late Wednesday-Thursday will 
result in dangerously strong winds. As the first low pressure system 
moves parallel to the coast Tuesday, it will bring a strong low 
level coastal jet directly against the Bay Area and Central Coast 
coastlines. High resolution models suggests 925 mb winds could be in 
excess of 90 knots (103 mph) along our coastline. At the surface, 
this will result in gusts to at least 70 mph (potentially higher) as 
the main cold front moves through. Winds will be strongest over the 
marine environment where storm force to isolated hurricane force 
winds are possible. A High Wind Watch, to be upgraded to a High Wind 
Warning, is in effect from 7PM PST Tuesday to 4AM PST Wednesday. The 
good news is these winds will diminish fairly quickly Wednesday 
morning. The bad news is this reprieve will be short lived as winds 
strengthen Wednesday evening as the second low pressure system 
approaches. Winds looks to be slightly weaker late Wednesday-
Thursday with gusts peaking between 40-60 mph. Right now, everything 
is leaning towards a Wind Advisory being necessary for the late 
Wednesday-Thursday storm over a second High Wind Warning. The 
combination of heavy rain and strong winds will result in trees 
falling down and widespread power outages. It will additionally 
result in incredibly dangerous conditions to any mariners or anyone 
who goes to the beach while high winds are occurring.

Thunderstorms: The final piece of the puzzle is the possibility of 
thunderstorms, potentially severe, this week. The potential for 
thunderstorms Tuesday into Wednesday and again late Wednesday into 
Thursday (all day) has increased from last nights forecast. 
Probability of thunderstorms is still between 15-25% but models are 
starting to support a more favorable environment for thunderstorm 
development with these systems. The most likely scenario would be 
for thunderstorms to be embedded within the main rain bands and then 
the potential for scattered thunderstorms in the post cold frontal 
environment. The SPC has issued a general mention of thunderstorms 
for our entire CWA Tuesday into Wednesday. The SPC additionally 
issued a marginal risk of severe thunderstorms for the coastal 
Central Coast (including the city of Monterey and Big Sur) and a 
general mention of thunderstorms for the rest of the CWA Wednesday 
into Thursday (Christmas Day). While not in the range of high 
resolution models yet, the ECMWF and GFS are both suggesting good 
instability (several hundred J/kg of MUCAPE) associated with both 
systems, good low level wind shear thanks to the strong winds, and 
the cold front acting as a source of lift. Both systems have the 
potential for thunderstorms and severe weather but the Wednesday 
into Thursday system initially appears to be the most favorable. 
Have multiple ways to receive warnings this week in the event that 
we see strong thunderstorm development with either system.

&&

.AVIATION...
(18Z TAFS)
Issued at 1128 AM PST Mon Dec 22 2025

Frontal boundary is hanging over the area this morning bifurcating
the Bay Area at the Golden Gate. This boundary will slowly lift to
the north through the day. This will allow for some clearing with
scattered showers south of the boundary while widespread rain 
will ride the boundary north. Winds south of the boundary will
generally be out of the south while north of the boundary will
generally be out of the north. Eventually the boundary will lift
out of the Bay Area allowing for scattered showers across the
region. By late Tuesday morning, expect strong gusty winds from
the south to develop, accompanied by widely scattered showers.

Vicinity of SFO...Showers will be the main story at SFO today.
Winds will remain out of the south, but on the lighter side of the
coin. CIGS will bounce a bit with showers in the area, but
generally should remain VFR or MVFR. The only concern I have over
the next 24 hours would be light winds overnight, wet grounds, and
radiational cooling, leading to some localized fog development.
This remains a spot of contemplation, but needs to be noted. The
big story is on Tuesday as a potent system approaches from the
southwest and brings rapidly increasing winds from the south.
Expect winds to peak late in the day and overnight with gusts 
exceeding 40 knots.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Light to moderate winds out of the SE
with an occasional passing shower. Winds ramp up Tuesday
afternoon. 

&&

.MARINE...
(Today through Saturday)
Issued at 500 AM PST Mon Dec 22 2025

Winds and rain rates subside slightly as the front moves
northward. A significant change takes place Tuesday as a strong
surface low  traverses the waters from the southwest towards the
Northern  California coast. This surface low will undergo rapid
intensification as it approaches the coast Tuesday leading to
nearly widespread gales, with storm-force winds expected along
coastal jets near Point Pigeon and along the Big Sur Coast. There
is a chance for isolated thunderstorms across the waters through
Wednesday before another strong surface low enters the waters late
Wednesday night. 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 from Tuesday morning through Friday evening for 
     CAZ006-508>510-512>518-528>530.

     High Wind Watch from Tuesday evening through late Tuesday night 
     for CAZ006-502>506-508>510-512>518-528>530.

     Flood Watch through Friday evening for CAZ502>506.

PZ...Gale Warning from 9 AM Tuesday to 3 AM PST Wednesday for Mry Bay-
     Pt Arena to Pt Reyes 10-60 NM-SF Bay N of Bay Bridge.

     Gale Warning from 3 AM Tuesday to 3 AM PST Wednesday for Pigeon 
     Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm-Pt 
     Reyes to Pigeon Pt 0-10 nm.

     Small Craft Advisory until 3 PM PST this afternoon for Pigeon Pt 
     to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM-Pt Pinos 
     to Pt Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.

     Gale Warning from 3 AM to 9 AM PST Tuesday for Pigeon Pt to Pt 
     Pinos 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm.

     Storm Warning from 9 AM Tuesday to 3 AM PST Wednesday for Pigeon 
     Pt to Pt Pinos 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 
     nm.

&&

$$

SHORT TERM...Kennedy
LONG TERM....Kennedy
AVIATION...BFG
MARINE...Murdock

Visit us at www.weather.gov/sanfrancisco

Follow us on Facebook, X, and YouTube at:
www.facebook.com/nwsbayarea
x.com/nwsbayarea
www.youtube.com/nwsbayarea


  
Prepared by Boulder-Creek.com Weather at: Mon Dec 22 12:30:02 PST 2025