025
FXUS66 KMTR 161129
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
329 AM PST Thu Jan 16 2025

...New AVIATION, MARINE...

.SYNOPSIS...
Issued at 1153 PM PST Wed Jan 15 2025

Quiet weather continues for the next several days, with some
marine layer stratus this weekend. There is an increasing
potential for strong offshore winds early next week.

&&

.SHORT TERM...
(Tonight through Friday)
Issued at 1153 PM PST Wed Jan 15 2025

Inland temperatures are dropping quickly again overnight, and the
dew points are a couple degrees higher than yesterday. As such,
there is a better chance for some mist and fog this morning across
the typical inland valleys. 3 of the ingredients are in place for
radiational fog; calm, clear and cool. The only missing piece is
moisture. While surface dew points have increased over the last
few days, light offshore winds are keeping the moisture below
average for this time of year. The 00Z sounding found a PWAT of
0.39", which is right at the 25th percentile for this time of
year. Thursday will be a rinse and repeat of Wednesday. Clear
skies, cool morning, and light offshore winds.

&&

.LONG TERM...
(Friday night through next Wednesday)
Issued at 1153 PM PST Wed Jan 15 2025

The cut-off low west of Baja will finally begin to move on
Friday, bringing more interesting weather. The subtropical jet
stream will move north and push the feature back inland, where it
will be picked up by the PFJ. At the same time, a short wave
trough will move downstream of the ridge axis at 500 mb, bringing
a weak, rainless cold front across the Bay Area. The combination
of the low evacuating and higher pressure returning behind the
cold front will bring back the typical onshore, NNW flow starting
Friday. This will also allow a marine layer to develop, with
periodic stratus and possible fog finally ending our sunshine
streak. This pattern will only last for a few days before offshore
winds return early next week.

The most interesting aspect of the forecast is how strong the
offshore winds will be next week. We`ll start with Monday. The
overall pattern has very cold air moving through the majority of
the US, supported by extremely high pressure at the surface. In
fact there is a area from Western Montana to SE Wyoming on Monday
where the NAEFS mean MSLP is outside of the 1979-2009 CFSR
climatology from January 10 through January 31. The surface
pressure looks to build to at least 1045 mb in that area. With
pressure that high inland, it makes sense that there will be
strong, down gradient flow, especially across the higher terrain.
While the global deterministic models have backed off, there is
still good support for a strong offshore event in ensemble space.
The SJSU WRF SFO-WMC pressure gradient still shows a bi-modal
solution with the more aggressive models dropping to around -20
mb. That`s about as low as we have ever seen, and would support a
major wind event across the North Bay Mountains. The uncertainty
is still quite high. There is now a second, possibly stronger push
of offshore winds possible next Wednesday. Unfortunately, both of
these events will likely bring strong dry winds back to southern
California.

When will it rain? It still looks dry through the forecast. For
what it`s worth, the GFS ensemble mean suggests the East Pacific
Oscillation will move back to a positive (wet) phase around the
29th, while the ECMWF ensemble mean holds on to the negative
phase through February 4th. We`re trying to find rain for you, but
just not seeing it yet.

&&

.AVIATION...
(12Z TAFS)
Issued at 323 AM PST Thu Jan 16 2025

VFR again most areas...with the exception of STS. MVFR vis for
KSTS this morning. VFR all areas this afternoon with relatively
light winds. Low level moisture increases tonight leading to
shallow cigs.

Vicinity of SFO...VFR today. Cigs AOB 1k ft late tonight and early
Friday.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR. Shallow cigs develop this evening
and will linger through Friday AM.

&&

.MARINE...
(Today through Tuesday)
Issued at 323 AM PST Thu Jan 16 2025

Light to moderate winds will persist over the coastal waters
Thursday. Winds will gradually increase and become more northerly
on Friday. Seas will increase over the weekend.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Frost Advisory until 9 AM PST this morning for CAZ506-510-513-
     516>518.

PZ...None.
&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...MM
MARINE...MM

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