Small Craft Advisory 234, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
153 PM PST Tue Nov 27 2018

.SYNOPSIS...Unsettled weather conditions are forecast to persist
through the upcoming weekend as a series of storm systems impact the
region. The most potent of these systems will push inland late
Wednesday into Thursday with widespread moderate to locally heavy
rainfall, isolated thunderstorms and strong, gusty winds. Lingering
showers will be possible into Thursday night with a lull in
precipitation on Friday before another system pushes inland this
weekend.

&&

.DISCUSSION...as of 01:42 PM PST Tuesday...The forecast period
remains active as a series of storm systems impact the region
brining periods of moderate to locally heavy rainfall, isolated
thunderstorms, strong/gusty winds and even the potential for some
wintry precipitation over the highest peaks.

A weak frontal boundary is currently approaching the northern
California coast with light precipitation occuring over the North
Bay. Rainfall since sunrise has ranged from a few hundredths of an
inch in Marin and Napa Counties with amounts of 0.50" to 1.00" in
the wetter coastal spots of Sonoma County. This band of
precipitation will continue to shift southward through the remainder
of the afternoon with light rain likely impacting the
afternoon/evening commute in the greater San Francisco Bay Area. The
short range models are in good agreement that the boundary will
"wash out" as pushes southward across the Santa Cruz Mountains this
evening. However, the deeper plume of moisture will remain aimed at
the Central Coast overnight. Rainfall is then forecast to increase
over the Monterey Bay Region early Wednesday morning as a strong
upper level jet streak approaches the coast. The latest thinking is
that widespread rainfall will develop just before sunrise on
Wednesday across this region while mainly dry conditions prevail
over the northern portion of the region. By midday Wednesday, the
precipitation over the Central Coast is forecast to shift southward.
However, cannot rule out lingering showers over the entire region
during the day Wednesday as the main mid/upper level system begins
to approach the coast and deep moisture continues to advect inland.

Widespread precipitation is then forecast to develop late Wednesday
night into Thursday morning as the core of the mid/upper level low
approaches northern California. Onshore winds will also increase
ahead an incoming frontal boundary and help increase rainfall along
the coastal slopes Thursday morning. A colder, more unstable air
mass will accompany this system and allow for the development of
isolated thunderstorms as well Wednesday night into Thursday. The
biggest change to the forecast and potentially greatest concern is
the development of a deepening surface low as it approaches the
coast. The 12Z GFS, NAM and local WRF track the center of this low
inland across the North Bay or San Francisco Bay area which would
result in a period of strong, gusty and potentially damaging winds.
Any lingering showers in wake of the frontal passage will have the
potential to mix down 50+ knot winds at 925mb. Thus, have issued a
High Wind Watch for areas along the coast as well as the coastal
ranges as 40-50 mph west to northwest winds will be possible with
gusts in excess of 60 mph not out of the question (cannot rule out
these stronger gusts also in the higher elevations across the
region). However, the exact track and evolution of this surface low
will have to be monitored as it will be the main factor for
if/when/where strongest winds may develop. Stay tuned...

Precipitation diminishes late Thursday night into Friday as the
upper level system shifts inland and the deeper plume of moisture
exits to the south. The models then show the potential for another
system dropping southward across the region late Friday night into
Saturday with another shot of widespread rainfall. However, the
latest ECMWF has backed off with regards to precipitation with this
system and overall confidence remains low to medium. Regardless,
much cooler conditions are forecast this weekend with even the
potential for wintry precipitation above 5k feet within any showers
that do develop. Confidence lowers even more beyond this weekend as
the models continue to diverge from one another and from run-to-run.
However, there appears to be the potential for unsettled conditions
during the first week in December as well.

&&

.AVIATION...As of 09:35 AM PST Tuesday...for 18Z TAFs. Radar shows
showers over the North Bay with KSTS having already reported rain
this morning. High clouds (10,000-20,000 ft) on the satellite are
obscuring the view of the surface. Obs show patches of low cigs
over the region with bases of around 1,500-2,000 ft. Generally
expecting MVFR cigs to prevail through the period with brief
MVFR/IFR vis with passing heavier rainfall. Showers will linger
throughout the period with this morning's high res models showing
another round of more steady showers for the Bay Area and southward
early tomorrow morning.

Light southerly winds through the period becoming locally breezy
at times. Winds will veer to a more westerly direction behind the
front, but still with some southerly component.

Vicinity of KSFO...MVFR with light southerly winds ahead of an
approaching front set to bring showers to the area this afternoon.
Latest models do not show much if any improvement in cigs
throughout the day. May be periods of VFR conditions, but
expecting BKN MVFR cigs to prevail until late tomorrow morning
into the early afternoon. Another period of steady rain is
forecast for early tomorrow morning.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...MVFR with periods of VFR conditions ahead
of an approaching front. Cigs expected to lift this afternoon
before lowering again this evening ahead of showers. Generally
light winds.

&&

.MARINE...as of 09:07 AM PST Tuesday...Southerly winds across the
waters will increase this morning ahead of an approaching cold
front. Winds will diminish and turn more westerly behind the front
this evening and tonight. Stronger southerly winds will return
later in the day tomorrow ahead of a second and more robust
frontal system. Occasional gale force gusts and scattered
thunderstorms will be possible Wednesday night into Thursday
morning with this second front. A significant west to northwest
swell arrives today and will continue through much of the week.
This swell along with moderate to gusty southerly winds will
create hazardous seas conditions into Friday, especially for
smaller vessels.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
      .Tngt......High Surf Advisory...Sonoma to Monterey Coast
                 SCA...Pt Arena to Pt Reyes 0-10 nm
                 SCA...Pt Arena to Pigeon Pt 10-60 nm
                 SCA...Pt Reyes to Pigeon Pt 0-10 nm from 3 PM
                 SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 3 PM
                 SCA...Rough Bar Advisory for SF Bar

&&

PUBLIC FORECAST: RGass
AVIATION: AS
MARINE: AS

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