Small Craft Advisory 14, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
427 AM PST Thu Jan 16 2020

.SYNOPSIS...A cold front will bring widespread rain, gusty winds,
high elevation snow and possible thunderstorms to the region
Thursday. Drier and slightly warmer conditions are likely for the
upcoming weekend. Rain chances could return by Monday or Tuesday
of next week.

&&

.DISCUSSION...as of 3:53 AM PST Thursday...Near term weather
concerns will be heavily focused a well advertised cold front. A
vigorous cold front is still on track bring moderate to brief
heavy rain, gusty winds, lowering snow levels, and possible
thunderstorms to the Bay Area today.

Latest radar imagery from KMUX shows a few pre-frontal showers
tracking across the Bay Area as of 3 AM. KSFO briefly reported
light rain around 2 AM. A few automated gauges across the North
Bay also tipped a few hundredths with the passing showers.

Latest surface analysis puts the aforementioned cold front
tracking SE across NorCal and just about to move through Ft Bragg.
KMUX radar imagery is now showing more widespread precip
developing over the northern coastal waters and Mendocino County.
The timing of the front has not changed a whole lot on the latest
hi-res short term models. Steady rain will increase in coverage
and intensity across the North Bay the next few hours and likely
impact the morning commute. Between 4-7 AM this morning the cold
front will begin its push through Santa Rosa, 7-10 AM San
Francisco, 10 AM-1 PM San Jose/Monterey, and finally 1-4 PM Big
Sur/San Benito. The fropa will definitely have a punch with it as
models suggest a potential Narrow Cold Frontal Rainband (NCFR).
Model derived frontogenesis is pretty solid along the boundary as
well. In layman's terms it means a burst of moderate to heavy
rain is possible with the front. Impacts could be brief ponding on
roadways, minor urban flooding, and hazardous conditions for
motorists. Winds will continue to pick up ahead of the front and
remain gusty with the fropa. Buoys along the coast currently show
winds of 20-25 mph with gusts to 35 mph. If a NCFR does develop
and enough bowing occurs will need to watch velocity signatures on
radar over the waters for any potential waterspouts.

Rain will quickly turn to showers behind the cold front from N to
S. A noticeable change in airmass will occur with front leading
to colder air aloft and dropping snow levels(More on that later).
The cold air aloft will help to destabilize the atmosphere. This
can already be see upstream behind the front with a few lightning
strikes being observed. Once the unstable airmass develops over portions
of the Bay Area thunderstorms will be possible with small hail
later this morning and this afternoon. Most likely locations for
thunderstorms will be the coastal waters, North Bay and the San
Francisco Peninsula.

Rainfall amounts will generally be 1-3" North Bay and Coastal
Mts(jackpot up to 4" in the Santa Lucia Mts) and 0.25-1.0"
elsewhere.

Snow levels are likely sitting close to 4,000 feet to the N and
5,000 feet to the south. The levels will waver ahead of the front,
but steadily drop from N to S with the colder airmass. By this
afternoon snow levels are forecast to be 3,000-3,800 feet. Timing
the precip and low enough snow levels will be like threading a
needle, especially in the Santa Lucia Mts. None the less, current
forecast still has a few inches for the North/East Bay peaks and
highest peaks of the Santa Cruz Mts. Biggest impacts could be for
Hwy 29 and Hwy 130. Wildcard will be the peaks above Big Sur as to
how high the snow levels go ahead of the front and if they drop
enough to accompany the heaviest precip. Current forecast says yes
with several inches above 4,000 feet and up to 10-12" above 5,000
feet. Impacts will be minimal there except for back country
visitors.

Drier, but cool weather on Friday. Dry weather conditions on
Saturday, but with a little warm up. Models bring another front
into NorCal, but it appears to wash out before reaching the Bay
ARea. Dry weather again on Sunday, but medium range models are
trending wetter early next week with precip returning
Monday/Tuesday.

&&

.AVIATION...as of 3:57 AM PST Thursday....For 12Z TAFs. Lowering
cigs, strong gusty winds, and periods of moderate to heavy rain
resulting in wet runways are expected with the passage of a cold
front forecast to move across the region today. Light prefrontal
rain has already begun over Bay Area terminals with heavier rain
and temporary IFR vis/cigs expected later this morning into the
early afternoon as the front moves through. Rain will taper and
gradually come to an end beginning late this afternoon/early
evening hours and continuing overnight as the front moves south.
In addition to the rain and winds, there is a slight chance of
isolated thunderstorms expected behind the front into the early
evening north of San Francisco. Kept VCTS out of the tafs for now
as confidence remains low.

Gusty winds of 15 to 25 kt with gusts to 30 kt expected later
this morning ahead of and during frontal passage. Along with
strong surface winds model continue to indicate strong winds aloft
with speeds between 30 to 50 kt. Have maintained LLWS in the TAFs
through this morning.

Vicinity of KSFO...MVFR cigs will prevail through the morning
with cigs lifting in the afternoon behind the front. Models remain
consistent with the timing of the cold front arriving between
17z-19z accompanied by a period of heavier showers. Periods of
moderate to heavy rain may reduce vis/cigs to IFR as the front
moves through. Strong southerly winds around 15 to 25 kt with
gusts to 30 kt expected later this morning. Winds will shift more
W/SW and gradually decrease this afternoon as rain tapers to
showers.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR cigs will lower to MVFR and prevail
through early afternoon with cigs lifting in the late afternoon
behind the front. Models remain consistent with the timing of the
cold front arriving between 17z-19z accompanied by a period of
heavier showers. Periods of moderate to heavy rain may reduce
vis/cigs to IFR as the front moves through. Strong southerly winds
around 15 to 20 kt with gusts to 30 kt expected later this
morning/early afternoon. Winds will shift more W/SW and gradually
decrease late this afternoon/evening as rain tapers to showers.

&&

.MARINE...as of 3:00 AM PST Thursday...A fast moving cold front
passing across the region will bring Gale Force winds to the
coastal waters today. The front will also bring periods of heavy
rain and isolated thunderstorms. These strong and gusty winds
will generate steep fresh swells and create hazardous seas
conditions. Moderate northwest swell will persist through the end
of the week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...GLW...Pigeon Pt to Pt Pinos 0-10 nm until 12 PM
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 12 PM
             GLW...Pt Arena to Pt Reyes 0-10 nm until 6 AM
             GLW...Pt Arena to Pigeon Pt 10-60 nm until 6 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 6 AM
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm from 6 AM until 3 PM
             GLW...Pt Reyes to Pigeon Pt 0-10 nm until 9 AM
             SCA...Mry Bay
             SCA...SF Bay until 9 AM

&&

PUBLIC FORECAST: MM
AVIATION: CW
MARINE: CW

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