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

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FXUS66 KMTR 161159
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
359 AM PST Tue Jan 16 2018

.SYNOPSIS...A few lingering showers this morning otherwise dry but
continued cloudy today. Dry and continued mostly cloudy Wednesday
as moist flow comes in off the Pacific. Next front moves in
Thursday with rain spreading north to south across the district
reaching the Central Coast by Thursday night. Precip will turn
showery later Thursday night into Friday as the cold upper trough
passes over the region with one of the colder airmass so far this
season. Dry but cool on Saturday before clouds increase again on
Sunday as the next front arrives with rain later Sunday into
Monday.

&&

.DISCUSSION...as of 3:38 AM PST Tuesday...Radar still picking up
returns over Bay Area but most activity will be east of the area
by rush hour/sunrise. Coastal Sonoma county had a few sites report
upwards of half an inch at Sea Ranch on the coast and the Oak
Ridge RAWs located in the wet coastal hills. Most other locations
are reporting only a few hundredths with little or no rain south
of about the Dumbarton bridge. So by the time most people start
the day any precip will have ended. However a moist zonal flow is
set up across the Pacific and clouds will continue to stream
across the region today keeping daytime highs stuck around 60.

No changes of note for Weds with a forecast of partly/mostly
cloudy skies but continued dry and seasonably mild with highs
upper 50s to upper 60s.

The next front of interest is due to arrive later Thursday. The
models have been remarkably consistent with the timing and
intensity of this system, even beyond 240 hour forecasts! High
confidence that a well organized frontal boundary will move into
the North bay Thursday morning and then charge southward through
the Bay Area and reach the Central Coast by Thursday evening. The
fast moving nature of the system will limit rainfall potential
with current qpf forecasts showing 0.25-0.75 on average. So this
looks like a typical/moderate winter-time storm. Winds will
briefly gust to around 35 mph during frontal passage as rain
drags down higher momentum air. The fast nature of the frontal
passage and modest rain totals should preclude any significant
hydro issues outside of typical commute impacts and blocked storm
drains. As always will need to monitor for brief heavy rain-rates
over the North Bay burn scars, as its often the short duration
bursts versus long term heavy rain that can cause debris flows.

Precip will turn showery later Thursday night into Friday morning
as 1000-500 mb thickness values plummet to around 528 dm with 850
mb temps to around -2 celsius suggesting we should see snow on the
higher peaks of the Bay Area. On Friday there will be numerous
post-frontal showers, especially for the coastal hills that
benefit from cold northwest wind flow. 700 mb temps as cold as -13
celsius with sea surface temps in the upper 50s will allow for big
delta T in the lower levels. This type of pattern can produce
locally heavy rain for the Monterey coast and Big Sur hills where
the forecast will show accumulating snow above 3500 feet for any
hikers in the Los Padres Forest. Daytime highs on Friday will
struggle to reach 55 for nearly all areas with periodic cold rain
showers dragging down cold air. Although not in the forecast at
this time, small hail and graupel will certainly be possible as
well.

After a cold start Saturday morning, the forecast to start the
weekend looks dry but continued cool.

Will be watching for the next system to arrive later Sunday into
Monday with another round of soaking rain forecast for the
district. Latest model trends suggest front will be fairly fast
moving and pass through sometime Sunday night. Earlier solutions
showed the boundary stalling but not seeing those signals at this
time. These systems have fairly cold air associated with them and
are progressive so hydro issues should be minimal with beneficial
rainfall the main impact.

&&

.AVIATION...as of 4:00 AM PST Tuesday...MVFR cigs fairly
widespread across the North Bay and the SFO Bay Area this morning
as a weak front moves through. Light rain has been reported at
times. Models indicate drier air in the midlevels working its way
down to the surface later this morning. Expecting VFR conditions
for the North Bay and SFO Bay Area after 18Z. LVK is a tricky
forecast due to the influence of clouds from the Central Valley
which could keep cigs through the day.

Vicinity of KSFO...MVFR cigs through 18Z with some light rain
through 15Z.

SFO Bridge Approach...Same as KSFO.

Monterey Bay Terminals...Some fog around SNS will bring vsbys down
to 1/2 mile at times through 16Z. Otherwise MVFR cigs through 17Z.

&&

.MARINE...as of 02:41 AM PST Tuesday...Weak high pressure off the
California coast will keep light northwest winds for today. Winds
will switch southerly late  tonight and Wednesday as a low
pressure system develops off the British Columbia coast and
pushes the high south. Large swells will impact the beaches
through early tonight before decreasing. A larger swell with long
period will arrive Thursday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...High Surf Advisory...Sonoma County coast to Monterey County coast
             SCA...Rough Bar Advisory for SF Bar

&&

$$

PUBLIC FORECAST: RWW
AVIATION: W Pi
MARINE: W Pi

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