143
FXUS66 KMTR 111715
AFDMTR

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

...New UPDATE...

.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.

&&

.UPDATE...
Issued at 906 AM PST Sat Jan 11 2025

The Frost Advisory was allowed to expire at 9AM this morning.
Temperatures ended up being a bit tricky, with the NBM running to
cool in some places and even a few degrees too warm for where the
Frost Advisory was. Will need to look into temperatures for
tonight since NBM runs even colder than last night.

&&

.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...
(12Z TAFS)
Issued at 421 AM PST Sat Jan 11 2025

VFR through the TAF period. Light and variable winds turn northerly
and breezy in the mid to late morning with some of more favored
windy areas seeing gusty winds through much of the day. Winds reduce
into the night, becoming light into early Sunday.

Vicinity of SFO...VFR through the TAF period. Northerly winds
increase the mid morning with gusts around 25 kts expected. Gusts
reduce into the early night, but winds stay moderate through early
sunday before becoming light and variable.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR through the TAF period. Winds stay
light through much of the morning, but become breezy in the
afternoon as they turn north and northwest. Winds reduce in the
evening and become light into the night.

&&

.MARINE...
(Today through Thursday)
Issued at 421 AM PST Sat Jan 11 2025

Expect strong and gusty northerly winds through much of the day.
Wave heights build to 10 to 15 feet 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 reduce later into the week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Coastal Flood Advisory until noon PST today for CAZ006-506-508.

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

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

     Small Craft Advisory until 9 AM PST Sunday for Mry Bay-Pigeon Pt
     to Pt Pinos 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.

     Small Craft Advisory until 3 PM PST Sunday for Pt Arena to Pt
     Reyes 0-10 nm.

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

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

     Small Craft Advisory until 3 AM PST Monday for Pigeon Pt to Pt
     Pinos 10-60 NM.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Murdock
MARINE...RGass

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