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

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FXUS66 KMTR 170613

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1013 PM 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 9:00 PM PST Monday...Latest model guidance in
brings the start of the rain to the North Bay Wednesday morning,
down to San Francisco close to noon and southward to Monterey Bay
by the afternoon which is inline with the current package. Winds
will pick up through the day with gusts over 40 mph possible at
some urban spots. Current forecast package appears to right on
track so no updates needed.

Please see the video just posted to our social media pages
detailing the next 5 days.

.PREVIOUS DISCUSSION...A patch of low clouds remain over the
northern portion of the Sonoma Valley this afternoon with
temperatures beneath the cloud cover hold in the 40s. Do see signs
of these clouds burning off and should allow for additional
warming through the afternoon before cooling after sunset.
Elsewhere, widespread 50s reported over the region under mostly
sunny conditions. Tonight, patchy fog and low clouds will likely
return to the North Bay Valleys and around the Delta as well.
Could even see low clouds develop around the San Francisco Bay and
into the South Bay. Overall, conditions will be similar to the
past few days.

The forecast keeps conditions dry through Tuesday and likely even
into Tuesday night ahead of our next approaching system. The models
are in good agreement with widespread rainfall associated with a
frontal boundary spreading from north to south across the region
during the day Wednesday. Ahead of and along the boundary, southerly
winds will increase and become gusty at times. The strongest winds
and heaviest rainfall will likely occur during a short (3-6 hour)
period of time as the progressive system passes through. This should
help limit the overall hydro and wind impacts across the region.
None the less, wind gusts up to 40 MPH will be possible in the lower
elevations with gusts upwards of 50 MPH in the higher, typically
windier spots.

In wake of the first frontal passage, winds will diminish and
widespread rains will turn to lingering showers Wednesday night into
Thursday. A second system will then bring another round of
widespread precipitation from Thursday night into Friday. As with
the first system, winds will increase and become gusty at times
during the overnight and early morning hours as the system passes
through. Rainfall amounts from these two systems will be greatest in
the coastal ranges where 2.00" to 4.00" will be possible with
isolated amounts upwards of 5.00". Meanwhile, lower elevations will
likely pick up between 0.50" to 2.00" (greatest along the coast and
in the North Bay) while some of the driest inland valley locations
may receive less that 0.50". Lingering showers are then likely from
Friday night into much of Saturday. Snow levels are also forecast to
drop in wake of the second system and may result in some snow showers
in the higher peaks across the region.

A third system is then forecast to sweep through Saturday night into
Sunday and will bring another around of widespread rainfall. The
passage of this system should also be preceded by locally strong and
gusty winds. At this time, rainfall amounts are forecast to be
similar to the first two systems, yet may result in locally higher
totals. Will continue to monitor the latest guidance and update the
forecast as we get further into the workweek.


.AVIATION...As of 10:13 PM PST Monday...VFR except VLIFR/LIFR in
patchy fog tonight and Tuesday morning. Mainly light offshore flow
prevails tonight through Tuesday. Pressure gradients are presently
WMC-SFO 8.7 mb and SAC-SFO 0.8 mb, latest NAM forecasts these to
hold near steady into Tuesday.

Low clouds and fog coverage over Sonoma and Marin counties continues
to gradually expand this evening, visibilities lowering a bit now
at Petaluma and Gnoss Field. Tule fog not gaining much traction
yet over the Delta, however, this will likely change overnight with
clear skies and light offshore winds gentle mixing leading to fog
development. Fog could become dense in spots by morning. With
conditions seemingly more conducive to fog development /based on
recent obs and model forecasts/ for the overnight decided to add
in IFR conditions in 06z Bay Area tafs. Clearing likely by late
Tuesday morning, though clearing could be slow in spots where winds
remain light and variable and as high clouds begin to spread in.

Vicinity of KSFO....Light NE-E winds with patchy fog possible early
Tuesday morning. Tempo IFR 11z-15z IFR then prevailing til about 18z.
Clearing likely to return late morning.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR. E-SE winds 5-10 knots due to combination
of offshore winds and late night cold air drainage. Low clouds possibly
returning Tuesday evening with SW-W reaching across the Monterey Bay.

&& of 9:53 PM PST Monday...Light to moderate winds and
a moderate westerly swell will persist over the coastal waters into
Tuesday. Strong southerly winds will develop Wednesday just prior
to a quickly moving cold front passing through Wednesday evening.
The cold front will then reach southern california early Thursday
morning. Northwesterly swell near 30 feet will build by midday
Friday and last into Saturday. Expect more wet and windy weather
late this week and this upcoming weekend.






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