FXUS66 KMTR 191926
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1126 AM PST Fri Dec 19 2025
...New AVIATION...
.KEY MESSAGES...
Updated at 315 AM PST Fri Dec 19 2025
- Mostly light, beneficial rainfall today
- Moderate to heavy rainfall returns Saturday and will persist
through at least Thursday
- Impactful wind returns Tuesday and persists through at least
Thursday
&&
.SHORT TERM...
Issued at 315 AM PST Fri Dec 19 2025
(Today and tonight)
KMUX is in 215 which means we are tracking precipitation across the
region. Light rain showers are possible through the morning in the
Bay Area, primarily favoring the North Bay and coastal areas. Other
coastal/higher terrain areas can expect drizzle. High clouds are
beginning to stream into the region from a surface low and its
attendant cold front in Oregon. Surface high pressure near the
California/Mexico border and surface low pressure in the Gulf of
Alaska are creating a conveyor belt of subtropical moisture into
California. By this afternoon, the main rain band will trek south
into the North Bay, losing its strength as it treks farther south.
The passage of the cold front will also increase winds with gusts up
to 25 mph expected along the coast, across ridgelines, and through
gaps and passes. Today's rainfall is expected to be beneficial. The
one critical exception is the Pickett Fire burn scar. There are some
hi-res models showing the burn scar approaching its hourly rain rate
threshold. Upon further investigation, it seems to be keying on
precipitation in the Mayacamas Mountains. Still, it is too high of
an impact to not mention. Those who live or travel through the area
should be aware of the threat and know that it will persist through
the forecast. The aforementioned cold front will become stationary
tomorrow. This along with the persistent moisture will allow for
rain to continue through the day.
&&
.LONG TERM...
Issued at 315 AM PST Fri Dec 19 2025
(Saturday through Thursday)
Saturday is the earliest that we expect rainfall to transition from
beneficial to impactful with at least some impacts expected by
Sunday. As such, we are monitoring the need for a potentially
prolonged Flood Watch starting somewhere in the Saturday/Sunday
timeframe, but no issuance has been made yet. For now, you should
monitor later forecasts and those living in areas prone to flooding
should be prepared to take action. The exact onset time of impactful
rainfall is still uncertain as it will be highly dependent upon
antecedent conditions and how this system performs in comparison to
its forecast. The most likely rainfall impacts as of now look to be
flooding of low-lying, poor drainage, and urban areas as well as
flashy creeks/streams such as Mark West Creek in Sonoma County. Once
again, these impacts may start as early as Saturday, with at least
some impacts expected by Sunday. These impacts can be expected
through the duration of the event, with impacts expected to worsen
through the event as soils become saturated.
A surface low developing off the North Bay Coast will be responsible
for Sunday's weather. The closer proximity of the low (as compared
to the Pacific Northwest) will bring greater impacts than the first
system. The firehose of subtropical moisture will continue to take
aim at the state, only this time around IVT will remain in excess of
500 kg/ms for near 24 hours rather than briefly peaking like the
first system is expected to. Moderate to heavy rainfall is expected
across most of the region on Sunday with the Interior Central Coast
likely remaining in the light rainfall category. In true atmospheric
river fashion, rainfall totals will be highest in southwest facing
terrain such as the North Bay Coast, Santa Cruz Mountains, and Big
Sur Coast as the moisture fetch runs orthogonal to the terrain,
providing the most efficient output. Winds will increase, although
they are expected to remain sub-criteria for any product as of now.
Gusts up to 40 mph can be expected along the coast, across
ridgelines, and through gaps and passes. 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. 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. There's also a low chance (15% or less)
for thunderstorms. The accompanying cold front will pass through on
Monday, bringing yet another day of rain and wind. As of now Monday
does not look as impactful as Sunday, but there is still uncertainty
in rainfall totals and cumulative impacts.
A developing surface low off the California/Mexico border will tap
into the persistent conveyor belt of moisture from the subtropics
and bring us our third system on Tuesday. It is going to trek near
parallel the Bay Area Coast. This in conjunction with antecedent
conditions has the potential to make this the most widespread
impactful system. Notable features with this system include a
negative tilt that's often a sign of an intensifying system and a 70
knot 925mb jet. There is still uncertainty in the location,
strength, and timing of the feature, but confidence is increasing
and the above still stands. As of now I would expect this to be the
strongest windmaker. More tree impacts may be possible due to the
non-dominant wind direction of southerly winds as compared to
westerly winds. The reason is that trees build resiliency to their
"normal" winds. Add in a strong wind from a non-dominant direction
and loose soils and that increases the risk for downed trees, and
thus downed power lines. The attendant cold front will sweep through
on Wednesday bringing more rain and wind.
A digging surface low with upper-level support from the
Washington/Canada border will bring our Christmas Day (Thursday)
system. More rainfall and wind can be expected with this system.
Hazardous beach conditions are also expected to return around this
time, so make sure to exercise caution if near the water. This will
not be the end of the rain, this is merely the end of the long term
forecast. Wet conditions are expected through at least next
Saturday. When all is said and done, locations can generally expect
a December's worth of rain or a quarter of their annual averages,
that's a lot of rain no matter how you slice it. If you are
travelling by car for the Christmas holiday, I urge you to take
the weather into account. Whatever you do, do not drive around
barricades and remember turn around, don't drown!
&&
.AVIATION...
(18Z TAFS)
Issued at 1127 AM PST Fri Dec 19 2025
Initial boundary will drift north to south with light rain
currentlyu falling KSTS. Showers and cigs will drift southward
this afternoon and evening. Boundary will wash out overnight
across the Bay Area and Central Coast with extensive left over
moist boundary layer meaning more cigs and or fog overnight
through Saturday am with light southerly winds. Looks like next
organized boundary arrives late Saturday afternoon/evening with
moderate to heavy rain for KSTS southward towards SFO.
Vicinity of SFO...Cigs have currently lifted at SFO. Incoming
boundary will bring lowering cigs and rain to the terminals closer
to 00z and lasting through the evening hours. Left over moisture
will keep low cigs and reduced vsbys in place to start the day
Saturday.
SFO Bridge Approach...Similar to SFO
Monterey Bay Terminals...VFR skies this afternoon and early
evening. Incoming boundary brings a period of light rain
overnight. Low cigs and vsbys to start the day Saturday.
&&
.MARINE...
(Today through Wednesday)
Issued at 315 AM PST Fri Dec 19 2025
A series of storm systems arrives today and continues through the
extended forecast. Moderate winds continue into the weekend with
occasional fresh gusts possible as a weak cold front moves through
the coastal waters. The first system arrives Friday into Saturday
with widespread light rain expected. Moderate to heavy rain is
expected beginning Sunday and continues through at least mid next
week as two stronger systems develop. Hazardous seas return early
to mid next week as strong winds develop over the coastal waters.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 3 PM PST this afternoon for Pt Arena
to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM.
&&
$$
SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...RW
MARINE...RW
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Prepared by Boulder-Creek.com Weather at: Fri Dec 19 14:30:02 PST 2025
From the National Weather Service
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