738
FXUS66 KMTR 110934
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
134 AM PST Sat Jan 11 2025

...New SYNOPSIS, SHORT TERM, LONG TERM...

.SYNOPSIS...
Issued at 1209 AM PST Sat Jan 11 2025

Dry weather continues through the week with periods of strong
offshore winds and cold morning temperatures.

&&

.SHORT TERM...
(Today and tonight)
Issued at 1209 AM PST Sat Jan 11 2025

The wind advisory went into effect at midnight and verified within
10 minutes in the North Bay Mountains (PGE Station Healdsburg
Hills North 2480 ft, N35G54 mph at 1210 AM). The 06Z surface
analysis from the Weather Prediction Center shows the dry cold
front has moved south of the Monterey Bay, with a surface low
developing near Sacramento. This analysis makes sense as the dew
point has been falling at Santa Rosa, Napa and San Francisco. Some
of this can be explained with dry offshore winds, but the pressure
has also been rising everywhere north of Monterey, (compared to
24 hours ago to eliminate the diurnal pattern). This means the
front has been much faster than expected, likely due to the
cyclogenesis near Sacramento helping to move things along. Since
this back-door cold front came from the NE instead of the typical
NW, it has not brought any rain. It has, however, encouraged
areas of low stratus across the SF Peninsula Coast, East Bay,
North Bay, Santa Clara Valley, the Monterey Coast and Salinas
Valley. These clouds are expected to clear in the next 2-6 hours
from N-S as dry air above and below the cloud layer begins to mix
in.

Comparing the current winds to various models, I am encouraged to
lean fully into the local WRF solution through this event. The new
forecast agrees well with the current wind advisory, if not a bit
stronger along the ridgelines. The big question going into this
shift was a possible extension into Sunday and/or Monday night,
which will be addressed in the Long Term section below. &&

.LONG TERM...
(Sunday through Friday)
Issued at 1209 AM PST Sat Jan 11 2025

The latest trend has been stronger winds Sunday night and weaker
winds Monday night. If this holds, the wind advisory may be left
to expire as scheduled Sunday morning, or only extended for the
North Bay Mountains. It`s less likely we will need something out
Monday night, but we can`t rule that out yet. Due to the frontal
passage, the SFO-WMC gradient has been on a roller coaster from
-11 mb to +3 mb and now back to slightly negative and quickly
 falling. This all happened over the last 24 hours. As the surface
 high builds behind the front, the gradient will stabilize through
 the weekend, somewhere around -8 to -12 mb. This will continue to
 encourage dry, offshore winds and clear skies. By Monday, a E-W
 oriented short wave trough will move quickly through California
 and become cut-off west of the Channel Islands. This disturbance
 won`t bring much more than a pretty sunrise to the region,
 however, as PWAT percent of normal will only jump from 30-50%
 over the weekend to 50-70% Monday and quickly fall again Tuesday.
 The biggest effect will be on wind direction. The gradient
 between the unmoving low pressure off the coast of S.
 California/N. Mexico and high pressure over the Intermountain
 West will continue to support 925 mb NE winds across the North
 Bay, but shift to E winds in the Central and South Bay, and
 support uncommon SE winds along the Central Coast. These dry,
 offshore winds will support clear skies which means cold nights.
 Many areas will drop into the mid 30s for several days this week.
 The wind speed should decrease through the week as the low
 pressure system gradually weakens without any upper level
 support. Cluster analysis shows the following system will likely
 be another positively tilted trough next weekend, but
 fortunately none of the ensemble clusters have it getting cut-
 off, so hopefully it will be a little more progressive. Overall
 though, the long wave pattern will continue to support an omega
 block centered over the E Pacific, and there is no rain in the
 forecast through at least next weekend. Both GFS and ECMWF
 ensemble members support this, though the uncertainty really
 balloons in the following week (starting Jan 20th). The mostly
 likely outcome is that January will fall well short of normal
 rainfall, possibly averaging around 10% if we don`t get much help
 in the final week of the month.

&&

.AVIATION...
(06Z TAFS)
Issued at 954 PM PST Fri Jan 10 2025

Mix of VFR and MVFR conditions overnight with some potential for IFR
conditions to develop across the North Bay. Continued onshore flow
and higher RH values will result in patchy/temporary lowered
CIGs/VIS across the Bay Area through 10Z and through 12-13Z for the
Central Coast. Winds are expected to shift offshore again (out of N
to NE) after 10Z which will bring in drier air to the region and
improve CIG/VIS conditions. Patchy fog is possible at STS through
10Z with chances decreasing after as drier air filters in. Gusty,
northerly winds will return by mid to late morning and continue into
the evening hours. Most sites will see winds starting to weaken by
the late evening but a few may not see more substantial weakening
until shortly after the end of this TAF period.

Vicinity of SFO...Moderate confidence that VFR will persist through
the TAF period. MVFR CIGs may develop between 06-10Z but confidence
is low to moderate. Satellite shows an incoming push of low level
clouds with SFO continuing to report onshore flow. By 10-12Z, winds
will shift offshore and bring with it drier conditions which
increases confidence in VFR conditions from this point onwards.
Gusty northerly winds will increase during the day before weakening
by the late evening.

SFO Bridge Approach... Similar to SFO.

Monterey Bay Terminals...Patchy broken clouds remain possible
through 10-12Z as onshore flow brings more moist air into the
region. Winds will become more northerly and gusty by mid to late
morning with wind gusts between 15 to 18 knots expected. Winds will
start to weaken by the evening hours and into the overnight
period.

&&

.MARINE...
(Tonight through next Thursday)
Issued at 954 PM PST Fri Jan 10 2025

Strong and gusty northerly winds are developing as a dry frontal
boundary moves through the region tonight into Saturday morning.
Wave heights build to 10 to 15 feet in the wake of the passing
front and remain elevated through the weekend. The strongest
winds will be located over the northern outer waters where gale
force gusts are likely through the weekend. Seas reamin elevated
into the start of the upcoming workweek, but winds look to
diminish.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Coastal Flood Advisory from 5 AM early this morning to noon PST
     today for CAZ006-506-508.

     Wind Advisory until 10 AM PST Sunday for CAZ504-512-514-515.

     Frost Advisory until 9 AM PST this morning for CAZ516>518.

PZ...Gale Warning until 3 AM PST Sunday for SF Bay N of Bay Bridge.

     Small Craft Advisory until 3 AM PST Sunday for Mry Bay-Pigeon Pt
     to Pt Pinos 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm-
     Pt Reyes to Pigeon Pt 0-10 nm.

     Small Craft Advisory until 3 AM PST Sunday for Pigeon Pt to Pt
     Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm.

     Gale Warning until 3 AM PST Sunday for Pt Arena to Pt Reyes 10-
     60 NM.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Kennedy
MARINE...DialH

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