Small Craft Advisory 235, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1102 PM PDT Tue Oct 10 2017

.SYNOPSIS...Seasonably cool weather conditions will persist through
late week with breezy northerly winds developing in wake of a weak,
dry frontal passage. A gradual warming trend is expected during the
upcoming weekend as high pressure builds over the region

&&

.DISCUSSION...as of 9:15 PM PDT Tuesday...An upper level trough
approaching the Pacific Northwest has caused winds to switch to a
west to southwest direction. Smoke from the fires in the North Bay
blew into the SFO Bay Area earlier today but has now blown off to
the east. Winds in the hills were decreased from Monday but there
was still a gust reported to 40 mph at Atlas Peak this afternoon.
Patchy fog has developed along the coast and there have been
reports of visibilities down to 1/4 mile. So watch out for patchy
dense fog along the coast tonight.

The onshore winds should allow for improved RH recoveries tonight.
However fire conditions are expected to worsen by Wednesday
morning as a dry cold front moves across the district. Latest
models forecast the front to move through between 09Z and 12Z.
Winds will switch to northerly and increase behind the front.
Latest hi-res models show 925 mb winds in the North and East Bay
hills in the 20-25 kt range Wednesday morning with gusts to 35
mph. Winds decrease Wednesday afternoon but will increase again
and peak Wednesday night into Thursday. It is worth noting that
wind speeds associated with this system are not forecast to be
nearly as strong as those that impacted the region Sunday night
into Monday morning but hi-res models show 925 mb winds up to
30-35 kt with gusts as high as 45 kt in the North Bay hills with
25-30 kt in the East Bay hills. RH values will be lower as the
airmass becomes drier. The combination of strong winds and drier
conditions warrants a red flag warning for the North and East Bay
Hills Wednesday night and Thursday. See the latest product
SFORFWMTR for details.

Winds are forecast to decrease Thursday afternoon and evening.
However medium range models are showing another upper level trough
deepening over the Great Basin Friday night and Saturday morning
and there could be yet another offshore wind event.

Temperatures will be a few degrees cooler the next couple of days
due to cold air advection behind the front. Overnight temperatures
will be modified by winds Wednesday night but then will be colder
late in the week as winds relax. A slight warming trend is then
forecast for the weekend. Dry weather conditions are also forecast
to prevail through early next week with temperatures rebounding
back to near seasonal averages.

&&

.AVIATION...as of 11:02 PM PDT Tuesday...GOES-16 and metar obs
show stratus and fog /IFR/ over the coastal waters and immediate
coast from southern Marin county to the Big Sur Coast and into the
Salinas Valley. Smoke continues to reduce visibilities mainly over
the North Bay, and slant-wise and horizontal visibilities may
become reduced again over the remainder of the Bay Area Wednesday.
Otherwise VFR is reported this evening.

The SFO-SAC pressure gradient is 2.1 mb thus inducing seabreezes
resulting in a temporary inland intrusion of stratus and fog prior
to the arrival of a fairly fast moving and mainly dry cold front
tonight. The ACV-SFO gradient is 5.2 mb and forecast to strengthen
some more tonight and Wednesday resulting in increasing N-NW winds
over the coastal waters with minor overlap onto land possible from
the North Bay to the SF Peninsula. Guidance forecasts gusty W-NW
winds Wednesday and Wednesday evening. Ongoing lower level cooling
will steadily erode the marine temperature inversion overnight,
Wednesday, and Wednesday night. Moderate to high confidence VFR
for the terminals Wednesday.

Vicinity of KSFO...West winds with a few gusts to 20 knots possible
overnight, depending on arrival of the cold front. 06z taf leaning
toward optimistic non-ceiling forecast few-sct008, but this is low
confidence as the ACV-SFO is only marginally strong enough late tonight
and Wednesday morning to prevent an inland intrusion of stratus and/or
fog. Moderate to high confidence VFR forecast during the day Wednesday
as a drier W-NW wind develops.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...IFR continues tonight and Wednesday morning,
spotty light drizzle is possible into Wednesday morning in a deepening
and eventually weakening marine layer. Clearing /VFR/ forecast to
return by late morning Wednesday.

&&

.FIRE WEATHER...As of 9:15 PM PDT Tuesday...Conditions are a
little better this evening due to lighter winds and an onshore
west to southwest flow. A dry cold front will move through the
area very late tonight with winds switching to northerly
and increasing behind the front. RH recoveries should still be
decent tonight but will become poor by Wednesday night and
Thursday morning as northerly winds continue and drier air
filters in. Winds will decrease during the day Thursday.

&&

.MARINE...as of 10:17 PM PDT Tuesday...A cold front over northern
California will move southeastward over the coastal waters tonight
and early Wednesday morning. Winds will become gusty from the
northwest through midweek as high pressure builds over the eastern
Pacific and northern California. Locally steep fresh swell will
accompany the northwesterly winds.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm from 3 AM

&&

PUBLIC FORECAST: W Pi
AVIATION/MARINE: Canepa
FIRE WEATHER: W Pi

Visit us at www.weather.gov/sanfrancisco

Follow us on Facebook, Twitter, and YouTube at:
www.facebook.com/nwsbayarea
www.twitter.com/nwsbayarea
www.youtube.com/nwsbayarea