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

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FXUS66 KMTR 161226

Area Forecast Discussion
National Weather Service San Francisco Bay Area
426 AM PST Mon Jan 16 2017

.SYNOPSIS...Dry weather will continue through Tuesday. An active
weather pattern returns midweek and persist into next weekend as
a series of storm systems impact California. Moderate to heavy
rainfall will be likely at times as well as locally strong and
gusty winds associated with the passage of each system.

&& of 4:00 AM PST Monday...Currently an upper level
ridge is off the California coast and extends into the Pacific
Northwest. Surface high pressure over the northern Great Basin is
bringing an offshore east to northeast flow over the SFO Bay Area.
Tule fog from the Central Valley is trickling through the delta
and into the North Bay and East Bay Interior. The greater SFO Bay
Area will see some light fog and haze today but the main effect
will be cool temperatures today with highs in the 50s.

The ridge breaks down tonight as the jet stream advances eastward
from the central Pacific. We are expecting 3 storm systems to
impact the area from Wednesday through the rest of the week. The
first system brings warm advection rain to the North Bay and
spreads into the SFO Bay Area Wednesday afternoon. Models are in
good agreement that the main front moves through Wednesday evening
(00Z-06Z Thursday) and loses its punch as it spreads south during
the night. There will be a period of moderate to locally heavy
rain in the North Bay and the coastal mountains but the system
moves through rather quickly so the duration of heavier rains
should be about 3-6 hours. Gusty south to southwest winds can be
expected as the front passes with 40-50 kt possible in the coastal
hills and 20-30 kt in the lowlands.

A few showers will linger around on Thursday then the next system
arrives in the North Bay early Thursday night and spreads south
overnight. The main front will be similar to the first one which
moves through quickly and brings a brief period of moderate to
heavy rain along with another period of gusty south winds.
The jet stream will be stronger behind this system than the first
and this will result in more widespread showers on Friday and
possibly into early Saturday.

A third system is expected around Sunday and is also accompanied
by a strong jet stream. Thus we can expect the main front to move
through Sunday with unsettled weather into Monday.

These systems have an atmospheric river connection with them but
unlike last week the connection is further out in the Pacific and
loses moisture as approaches the California coast. So we are not
expecting the rainfall amounts in our CWA that we received last
week. However the ground is still pretty saturated and some minor
hydro issues can still be expected.

Medium range models indicate another upper ridge building off the
coast towards the middle of next week for a drying period.


.AVIATION...As of 4:26 AM PST Monday...For 12z tafs. ksts still
showing 1/4 mile fog this morning and expect that to last
through the morning hours. There is bust potential this morning
with easterly flow in place and low clouds from the central valley
pushing towards the Bay Area. However, its interesting that cigs
at ksac, ksck and kmod are all 2000 feet or higher with 1800 feet
at Livermore. So even with strong ridge building it appears
widespread dense fog into sf bay is unlikely. Will continue to
monitor obs and trends as 14-15z is climatology window when these
events can form. Should we avoid the low cigs just expect light
easterly winds with perhaps some high clouds. No synoptic changes
of note until Weds.

Vicinity of KSFO....Right now obs remain clear with some gusty
east winds preventing fog formation. Still some concern for fog or
low cig formation towards sunrise but taf will show vfr conditions
for now.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Hills along the San Benito county line
are deep enough to prevent any central valley sludge from sneaking
into monterey bay terminals. Tafs remain vfr through the period.
High confidence.

&& of 04:15 AM PST Monday...Some dense fog will be
possible this morning mainly over the west delta and into san
francisco bay. otherwise, generally light nw winds over the ocean
will continue today into tuesday over a long period swell. a front
will approach late tuesday night into wednesday with increasing
south winds that will turn southwest by wednesday night. large nw
swell will build midweek and beyond with potential for 20 foot
plus seas by friday and into the weekend with at least two more
strong storms passing over the waters.





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