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

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FXUS66 KMTR 211349

Area Forecast Discussion
National Weather Service San Francisco Bay Area
549 AM PST Sun Jan 21 2018

.SYNOPSIS...Dry and cool conditions today will become replaced by
wet weather this evening and overnight as a cold front moves
southeastward over the area. Briefly gusty winds will accompany
this front. Rain will gradually dissipate as the front moves south.
The weather pattern will remain active this week, with more rain
likely by midweek.


.DISCUSSION...As of 5:45 AM PST Sunday...Mainly clear to clear
skies overnight have allowed temperatures to cool down to the 30s
to mid 40s, including reports of mid to upper 20s in the southern
Salinas Valley. IR imagery indicates steadily increasing clouds
associated with a cold front 450 miles to the northwest of the Bay
Area. The result of warm air advection and elevated inbound
moisture can be seen on this morning`s 12z Oakland sounding, a
weak warm frontal inversion is near the 600 mb level.

Recent model trends have been going with comparatively less qpf with
tonight`s cold frontal passage. 06z GFS focuses max qpf 1.25" over
coastal Sonoma county to 1/2" Marin county and 1/4" or less south
to southern Monterey and San Benito counties. Likewise the 00z
ECMWF favors these aforementioned areas as wettest to driest with
less qpf though, varying from 3/4" north to less than 1/10" southern
counties. The NAM has also cut back on qpf, by as much as 50% from
just 1-2 days ago. The models have even trended toward this being
a shallower, cold, dense modified polar air mass primarily confined
at or below the 850 mb level. When rain begins in the North Bay later
today KMUX will have trouble seeing it because the precipitation will
be relatively robust, but fall at a very low altitude. This front may
still demonstrate some vigor as some of the stronger fronts can be
those that are not elevated rather confined between a well developed
upper boundary (850 mb in this case with subsidence indicated above
this pressure level) and the lower boundary being ocean and ground.
Winds will still pick up and be gusty near frontal passage, but the
models have indicated less wind compared to previous output. There`s
very good agreement in the model output that this front`s rain band
will steadily diminish as it moves southeastward overnight. Most if
not all of the rain will be over by Monday morning`s commute.

The next chance of rain will be Wednesday into Thursday. This rain
will be generated by a much more vertically developed low pressure
system in the Gulf of Alaska and its cold front will sweep through
the area with moderate to heavy rain. With a long wave trough in
place over the west this weather system should produce more of a
widespread soaking rain in a more robust dynamic environment; at
least a couple inches or more in the Bay Area to an inch or so in
the southern counties. Orographic forcing on windward slopes may
enhance rain totals with this system as well. The air mass behind
this system will be modified polar air from the Bering Sea/Alaska.
Some of the coldest air available at high latitudes is presently
emptying out across southwestern Alaska. Nome to the north for
instance has been -15F to -20F this morning. Below normal temperatures
are likely in our forecast area by the end of the week. Cloud cover
will play a role in night temperatures specifically it could get quite
chilly to cold over inland valleys with a period of near freezing
to sub-freezing temperatures.

Lastly, the 00z ECMWF has backed off on the omega block solution
for eastern Asia to Alaska late this week. We still may have ridging
return to the west coast, but if the pattern has a chance to be more
progressive than stationary or retrogressive than it offers more of
a chance of periodic disturbances bringing wet weather to California
versus a long stretch of dry weather. We may be a little the past
climatological peak of blocking patterns too which helps.

&& of 3:59 AM PST Sunday for 12Z TAFs. Current fog
product shows streaming high clouds overhead, with no signs of
stratus across the district. However, lastest KSTS and KAPC obs
are now reporting light/patchy fog. Expect intermittent fog
through 15-16z for North Bay terminals. Still monitoring for low
clouds/fog development around the Greater San Francisco Bay Area
over the next couple of hour, hinted by the 00z MTRWRF model. Kept
TEMPO group in TAF for KOAK and KSFO through 15z, just in case.
Dewpoint depressions at KSFO and KOAK are 6F and 0F, respectively.

An upper level system approaching the Bay Area will introduce
rain beginning this afternoon for the North Bay, therefore
lowering CIGs in the process. Current thinking is rain will begin
around or slightly after 00z Mon at KSTS, then progress southward
through the SF Bay Area by 03-05z. It looks like rain will affect
Monterey Bay terminals after 12z Mon. Rainfall may be moderate
and occasionally heavy at times. This will likely drop CIGs/VSBY
down to IFR or LIFR during heavier showers. Winds are expected to
be mainly southwesterly/southerly through the forecast period,
between 08-12 kt.

Vicinity of KSFO...Slight chance of MVFR CIGs through 15z this
morning. Otherwise VFR expected through this afternoon. CIGs will
gradually lower through the afternoon and are expected to be MVFR
this evening. Light east/southeast winds this morning will turn
more southerly/southwesterly and increase by afternoon.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions are expected to prevail
through the forecast period, with high cloud. Generally light
winds. Rain chances increase after 12z Monday.

&& of 2:35 AM PST Sunday...Winds will increase and
become southwesterly later this morning, especially for the
northern coastal waters, ahead of an arriving weak cold front. The
cold front is forecast to pass through Monday morning and will
switch winds out of the north again. Small Craft Advisories are in
place for most coastal water zones mainly due to squared and
hazardous seas over the next couple of days. A new northwest swell
train arrives by this evening.


     .Tday...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...SF Bay from 3 PM




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