Small Craft Advisory 229, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
208 PM PST Wed Nov 21 2018

.SYNOPSIS...An approaching weather system will bring widespread
moderate to locally heavy rain and gusty winds to the region
through the day, with scattered showers continuing through
tonight and into Thanksgiving morning. A second system is
expected to bring additional rainfall to the area from Thursday
evening through Friday. Dry weather is expected over the weekend
before another chance of rain returns by the middle part of next
week.

&&

.DISCUSSION...as of 02:05 PM PST Wednesday...Upper level trough
moving ashore and bringing substantial rainfall and breezy to
gusty winds with it. Rainfall amounts so far have varied
considerably across the area as individual bands of rain have
favored coastal regions so far. The Santa Cruz Mountains have
received the most so far with well over an inch in several
locations and over 2 inches on Ben Lomond. The San Francisco
Peninsula and points north have also received substantial rainfall
with 0.5 to 1 inch in most locations. A frontal band is moving
through the SF Bay and will bring rainfall to most locations,
including the East/South Bay and points south and east which
haven't received as much rainfall so far. Convection within the
cold upper low is occuring offshore as evident by satellite, and
showers will continue in the post frontal environment across the
region this evening and overnight. Isolated thunderstorms could
form in the unstable environment as well. An area of concern that
could see high precipitation totals over the next couple hours is
the Big Sur coast and Santa Lucia Mountains, where the HRRR and
local WRF are showing the potential for an additional 1-2+ inches
of rainfall through this evening.

Showers will taper off overnight and most areas will see a break
in precipitation Thanksgiving morning. The next trough will bring
a return to rain by Thursday afternoon across the North Bay as
warm advection, high precipitable water, and strong onshore flow
act to create significant precipitation. Models have come into
better agreement about the band of rainfall sagging southward into
the Bay Area Thursday evening. Rainfall with this system will
make it as far south as the Monterey Bay on Friday, but amounts
will be much less than the North Bay as the heaviest rainfall will
stay focused on our northern zones. Rainfall with this system
will bring an additional 2-3 inches in the North Bay Mountains
with 0.5 to 1.5 inches in the North Bay Valleys. The Bay Area will
see rainfall amounts around 0.25 to 0.75 inches, with lesser
amounts points south.

Expect a return to quiet weather this weekend as rain showers
taper off on Friday night. A shortwave ridge will build across the
eastern Pacific Saturday through Monday with a weak shortwave
trying to move through on Sunday. However, this is not expected to
bring any precipitation at this time. A return to wet weather is
likely the middle by the Tuesday into Wednesday timeline as both
the GFS and Euro are in agreement about another trough forming in
the East Pacific.

&&

.AVIATION...As of 10:20 AM PST Wednesday...Regime shift from
stagnant smoky air to wet, breezy storm system this morning.
Generally borderline MVFR to VFR ceiling as storm system moves
inland. Visibility is more of an issue again today as some
lingering smoke and rain combine to bring lower than typically
seen visibility, particularly in the North/East bay where
particulate counts are elevated versus other areas. IFR to MVFR
visibilitys thus expected thru morning at KSTS KAPC KCCR KOAK KLVK
but should steadily increase as new air mass enters the region by
the afternoon. Otherwise, generally light southerly, except in
favored locations such as the Salinas valley where stronger winds
have been observed already. Brief improvement early tomorrow,
however, a different storm system arrives late tomorrow night to
bring another round of lowered cigs and stronger winds.

Vicinity of KSFO...Generally MVFR cigs/visbys with brief IFR visby
posbl with heaviest rain. Conditions improve to VFR after frontal
passage. Showers linger later today. Winds generally 12 kt or
less. A different storm system arrives late Thu-early Fri.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Generally VFR with borderline MVFR
cigs/visbys with the heaviest rain later today. Light winds near
the coast with gusty S-SE winds through the Salinas Valley incl
KSNS.

&&

.FIRE WEATHER...as of 2:44 AM PST Wednesday...Southerly winds
will increase into the morning hours, especially along the coast
and in the hills. Some strong gusts are likely just before rain
begins this morning. Periods of light rain on today with some
brief heavy rainfall possible as the front moves through. The
typical rain shadowed valleys of the East Bay, Santa Clara Valley
and Salinas Valley will see the lightest rain amounts. Thursday
will be mainly dry with just some lingering showers. Next warm
front arrives by Thursday night with rain developing in the North
Bay and then slowly sliding southward on Friday. The heaviest rain
will be in the North Bay and Santa Cruz mountains. By Friday
night most areas should have received season-ending rains. Long
range looks dry Saturday through the middle of next week but
already signs of another storm before the end of the month.

&&

.MARINE...as of 10:09 AM PST Wednesday...Increasing southerly
winds will precede a cold front that will  sweep southeastward
over the coastal waters and bays today  bringing rain and possibly
thunderstorms. Winds weaken by late  afternoon, but another storm
system will once again strengthen  southerly winds Thursday night.
Mixed swell, predominately moderate northwest with a light
southerly component.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 9 PM
             SCA...SF Bay until 3 PM
             SCA...Mry Bay until 3 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm until 3 PM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 PM

&&

PUBLIC FORECAST: ST
AVIATION: DRP
MARINE: DRP
FIRE WEATHER: Rowe/RWW

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