Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 122328

Area Forecast Discussion
National Weather Service San Francisco Bay Area
328 PM PST Wed Dec 12 2018

.SYNOPSIS...Dry conditions are forecast to continue across our
region through Thursday night. Daytime temperatures will be mild
while nights will be seasonably cool. Patchy fog could redevelop
tonight across the area, especially in the valleys. A weak
weather system will likely produce light rain on Friday and Friday
night, especially for areas north of the Golden Gate. Periods of
light rain may continue into Saturday and Saturday night, mainly
in the North Bay. A stronger and wetter system is then expected to
produce widespread rain across our entire region Sunday and
Sunday night. Model differences increase by Tuesday and Wednesday.

&& of 03:00 PM PST Wednesday...A low amplitude
upper ridge just off the coast will keep the storm track to our
north through Friday. Occasional high clouds continue to stream
from the northwest into the region this afternoon. The stratus and
localized fog this morning dissipated from all areas by early
afternoon, allowing temperatures to rise into the upper 50s to
lower 60s. The upper ridge axis will move overhead by Thursday
morning, with continued mostly clear skies and light winds. This
will allow strong radiational cooling to commence overnight, with
lows expected to drop into the 30s in the inland valleys and 40s
near the coast and SF Bay. Patchy fog will again be possible,
especially near the SF Bay and and the North and East Bay Valleys.

As the ridge axis moves overhead on Thursday, temperatures will be
mild once again. Highs will warm several degrees into the mid to
upper 60s in the lower elevations in our southern areas, and low
to mid 60s north. Skies will be mostly sunny creating a nice day
for the area.

The next system will arrive on Friday as a trough approaches from
the west. A narrow band of high precipitable water will accompany
the trough and move into the Bay Area by Friday evening. As moist
southwest flow increases ahead of the trough, light rainfall is
expected to develop by early afternoon across the higher terrain
of the Central Coast in the warm sector. The main band of rainfall
will accompany a cold front which will push into the Bay Area
from northwest to southeast Friday late afternoon through
evening. The front will weaken and stall, likely not making it south
of Santa Cruz. Rainfall amounts with this system will be light,
with up to a quarter inch from the Bay Area north in the populated
areas and lesser amounts to the south.

Scattered showers will be possible on Saturday where the stalled
frontal band and high PWAT set up. Rainfall amounts with the
scattered showers will be light and no impacts are expected on
Saturday. A stronger storm system will then arrive on Sunday as a
deeper trough moves into the area. The Euro keeps the trough more
consolidated as it moves through, while the GFS brings a deeper
trough through, forming into a closed low by early Monday. Light
prefrontal precipitation will develop on Sunday morning north of
the Golden Gate. Heavier precipitation will move into the North
Bay on Sunday afternoon and spread south through Sunday night
across the area. The Euro is about 6 hours faster on the timing of
the precipitation and heavier than the GFS. Have leaned more on
the Euro for the time being. Rainfall amounts will be heavier with
this system, with widespread 0.5 to 1 inch in most populated
areas, 1 to 1.5 inches in the North Bay Valleys, Santa Cruz
Mountains, and Santa Lucia Mountains, and 1.5 to 2.5 inches in the
North Bay Mountains. Additionally, breezy to locally gusty winds
will develop ahead of the front near the coast and in the higher
terrain on Sunday.

Model differences increase on Monday and Tuesday. A ridge is
likely to develop in the East Pacific to our southwest, but the
strength remains uncertain with a disturbance moving over the top
of the ridge. The GFS builds a stronger ridge and pushes all
precipitation well to our north, while the Euro keeps the ridge
much flatter and allows precipitation to make it as far south as
the SF Bay. For now, have gone with a model blend and have kept
slight chances of showers across the north. Models do agree that
the ridge will further amplify by Wednesday, bringing a return to
dry weather through the middle of next week.

&& of 03:28 PM PST Wednesday...For 00Z TAFs. VFR
conditions through the period. Patchy fog will be possible late
tonight/early tomorrow morning in the North Bay. Valley fog may
develop and migrate into the East Bay in the early morning, but
keeping the tafs VFR for now as confidence is low. Winds will
remain mostly slight with high clouds overhead around
20,000-25,000 ft.

Vicinity of KSFO...VFR with high clouds and generally light winds.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR through the period with high clouds
and light winds.

&& of 02:32 PM PST Wednesday...Northerly winds will
gradually subside later this evening over the coastal waters, and
then will become southerly by late Thursday and Friday ahead of
system expected over the weekend. A series of west to northwest
swell trains will arrive across the waters over the coming days,
creating hazardous conditions for small vessels. The most
significant long-period swell is forecast to arrive by late Sunday
into Monday, which has the potential to bring open water swells
of 20 feet or more.


     .Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Rough Bar Advisory for SF Bar from 9 PM




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