519
FXUS66 KMTR 190445
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
845 PM PST Sat Jan 18 2025

...New AVIATION, MARINE...

.SYNOPSIS...
Issued at 249 PM PST Sat Jan 18 2025

Cool weather continues with coastal and valley fog expected to
return again tonight. A Frost Advisory will be in effect from 1 AM
PST to 9 AM PST Sunday morning for portions of the interior
Central Coast. Gusty offshore winds remain likely Monday into
Tuesday with peak gusts between 35 to 40 mph across the interior
North Bay Mountains.

&&

.UPDATE...
Issued at 758 PM PST Sat Jan 18 2025

Not a bad way to finish a Saturday weather wise, relatively light
winds, clear skies and above normal temperatures. High
temperatures this afternoon topped out in the mid 50s to lower
60s, which is a few degrees within climatology.

Near term weather impacts will be the return of the marine layer
tonight and ongoing Frost Advisory. The marine layer retreated
westward during the afternoon and is slowly inching/reforming to
the coast. Lastest satellite imagery shows some lower stratus
expanding around the Monterey Peninsula and the San Mateo coast.
Latest guidance shows this stratus expanding and moving inland
(similar to latest night). Xsection put the developing marine
layer in the 1500-2000 foot range. That being said, do not expect
the stratus to reach the southern Salinas Valley. As such, clear
skies and light winds will be ideal for radiation cooling. A Frost
Advisory remains in effect for interior Central Coast with
temperatures below 36 degrees. In fact, the coldest sheltered
regions (Lockwood, Ft Hunter Liggett) will reach the upper 20s. As
noted below some lower confidence on temperatures outside of the
Frost Advisory, especially interior valleys locations. Similar to
last night`s concerns : marine layer clouds equals warmer
overnight lows and clear skies equals colder overnight lows.
Probabilistic guidance suggest N and E Valleys reaching the mid-
upper 30s, which is chilly, but not cold enough for a Frost
Advisory.

No update needed at this time.

A quick peek at the 00Z hi-res WRF guidance for Monday - still
looking offshore with peak winds late Sunday night and early
Monday. Gradient will yield peak winds of 50-60 mph over the
highest peaks of the N Bay and 35-50 mph for other higher terrain
the Bay Area. Will continue to fine tune this as Sunday night
nears.

MM

&&

.SHORT TERM...
(This evening through Sunday)
Issued at 249 PM PST Sat Jan 18 2025

Satellite shows stratus has primarily dissipated across the Bay Area
and Central Coast. Pleasant conditions continue through the
remainder of Saturday with widespread highs in the upper 50s to low
60s. Another chilly morning is on tap tomorrow with a Frost Advisory
in effect from 1 AM PST to 9 AM PST for interior Monterey and San
Benito Counties with temperatures in the low 30s expected. Portions
of far southeastern Monterey County may drop into the upper 20s.
Elsewhere, temperatures will hover in the mid 30s to low 40s with
most locations staying just above Frost Advisory criteria. In terms
of the forecast, temperatures were adjusted upwards by a few degrees
across the valleys (excluding the southern Salinas Valley) and along
the Monterey Bay Coastline. Temperatures in the straight NBM
forecast leaned too cold in comparison to the scenarios suggested by
WRF and MOS guidance. In fact, for some locations across the North
Bay the NBM deterministic and NBM 90th percentile (typically the
warmest temperature forecast) were equal to each other. An
additional complicating factor for tonight`s temperature forecast is
return of widespread stratus and fog tonight. A cut off upper level
low continues to linger just off of the California Coast, serving to
compress the marine layer and increase the likelihood of stratus and
fog across the region. The North Bay Valleys (highest confidence),
Bay Shoreline, East Bay Valleys, and Santa Clara Valley (lowest
confidence) all look likely to develop fog tonight. Any locations
where stratus or foggy conditions develop tonight are likely to
experience less radiational cooling and prevent low temperatures
from dropping as much as the models say they will. High resolution
models suggest that stratus and fog could develop as early as 9 PM
PST in the North Bay Valleys and between 12 AM PST to 3 AM PST for
the Bay Shoreline, East Bay Valleys, and Santa Clara Valley. The
combination of the NBM leaning too cold and widespread stratus/fog
expected tonight contributed to the decision to limit Frost
Advisories to the interior Central Coast. It is worth noting that if
stratus/fog does not develop as expected then temperatures may be
able to cool a few more degrees than they would have otherwise.

&&

.LONG TERM...
(Sunday night through next Friday)
Issued at 249 PM PST Sat Jan 18 2025

An upper level cut off low lingering off the California Coast will
weaken and gradually merge with upper level ridging over the Pacific
Ocean. At the same time, a strong upper level trough will move
inland over the Central United States. As the inland trough
strengthens, it will become positively tilted (trough axis oriented
from the northeast to southwest) and dig into the Southwest United
States. This will lead to an "inside slider" type setup which is
likely to bring gusty offshore winds to our area. Confidence has
continued to increase that we will see strong offshore winds across
the North Bay interior mountains as the trough digs/the pressure
gradient strengthens Monday into Tuesday. The strongest winds are
likely to occur between Monday morning and early Tuesday morning.
Sustained winds across the North Bay interior mountains are likely
to be between 30 to 40 mph with gusts to around 50 mph possible on
the highest ridgetops. Local, high resolution WRF guidance suggests
that isolated stronger gusts may be possible across the highest
ridgetops in the North Bay. Gusty winds to around 25 mph are likely
to develop across the East Bay hills and along the San Francisco/San
Mateo Peninsula during the same time frame. Fire concerns remain low
as fuel moistures remain high thanks to rainfall from earlier this
winter.

High temperatures will stay in the upper 50s to low 60s through
midweek. A gradual warming trend is expected to occur mid to late
week with most inland sites warming to the mid to upper 60s. Cold
mornings will continue throughout the rest of the week with lows
generally in the 30s to low 40s across the region. Additional Frost
Advisories are likely to be considered as we move through the week
for the coldest locations. Now a response to the age old question:
"where`s the rain?" Long range guidance shows an upper level
trough moving through the Western United States the weekend of the
25th through the 28th. NBM guidance has started to populate low
PoP values on the 25th/26th but confidence in this event remains
low given that most ensembles still show either no rain or only a
few hundredths of an inch. Potentially more exciting news on the
rain front, however, is that the CPC has introduced a 50-55%
chance of above normal rainfall for much of our area in their
weeks 3-4 outlook (February 1-14). While this is unfortunately a
few weeks away, it is still a good indication of when we can
expect the rain to return again.

&&

.AVIATION...
(06Z TAFS)
Issued at 845 PM PST Sat Jan 18 2025

Mostly VFR with some MVFR ceilings building along the coastal
terminals. MVFR/IFR/LIFR conditions are expected overnight with
higher confidence of fog/low stratus over the North Bay and Monterey
Bay as the marine layer compresses. Confidence remains moderate that
the remaining terminals will see low VIS and CIGs overnight but
several models show MVFR/IFR conditions until 19-20Z. There is some
models that show terminals clearing as late as 21Z with Monterey
Bay terminals seeing clearing in the late afternoon. Confidence is
low at this moment on timing of low CIGs and VIS Sunday night,
but models show a stronger signal for return after 06Z. Winds will
continue to remain light to moderate through the TAF period.

Vicinity of SFO...VFR with a FEW low status building. There is
moderate confidence that MVFR/IFR conditions will build overnight
and clear by 20Z. Models show there is a possibility of a return
of VFR conditions after 21Z. Light winds build to moderate and
onshore by Sunday afternoon.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...MVFR conditions with a chance for CIGs to
lower to IFR. Confidence is low to moderate on ceiling height and
visibility but expect MVFR/IFR conditions through most of the TAF
period for KMRY with a break during the afternoon hours for KMRY.
KSNS is expected to see VFR through the remainder of the TAF period
after 18Z, but there is a chance for MVFR conditions to return
towards 05-06Z.
&&

.MARINE...
(Tonight through next Friday)
Issued at 845 PM PST Sat Jan 18 2025

Moderate northwest breezes continue through Sunday with high
pressure continuing to support dry conditions. Compression of the
marine layer promote fog and low clouds during the overnight
periods. Moderate offshore breezes early next week will limit fog
and low cloud development.

&&

.BEACHES...
Issued at 432 AM PST Sat Jan 18 2025

A long period westerly swell with a period up to 22 seconds will
result in a moderate to high risk of sneaker waves and bring an
increased risk of rip currents beginning today and lingering
through Monday. For beachgoers to remain safe it is vital to keep
off of jetties and rocks, keep pets on a leash, observe the ocean
for 20 minutes before recreating on the beach or in the water, and
never turn your back on the ocean. A Beach Hazards Statement has
been issued for all Pacific Coast beaches from 7 PM today through
7 PM Monday.

Sarment

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement through Monday evening for CAZ006-505-
     509-529-530.

     Frost Advisory from 1 AM to 9 AM PST Sunday for CAZ516>518.

PZ...None.
&&

$$

SHORT TERM...Kennedy
LONG TERM....Kennedy
AVIATION...SO
MARINE...SO

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NWS MTR Office Area Forecast Discussion