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

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FXUS66 KMTR 211735
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
935 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 9:00 AM PST Sunday...Chilly morning under
mostly clear skies, with several locations dropping to or below
freezing, including: Petaluma, Napa, Santa Rosa, Livermore,
Hollister, Santa Rita, San Martin, and other inland valley
locations. These temperatures are generally 5 degrees below
normal, with some areas closer to 10 below. Overnight temperatures
are forecast to remain several degrees above normal and above
freezing for the next few nights as increased overnight cloud
cover should act as a blanket, keeping daytime warmth/infrared
energy trapped near the surface.

The main focus today will be on a storm system currently well to
the north, centered just offshore of the British Columbia coast,
and its relatively shallow surface cold front, which is expected
to sweep across the area from later this afternoon through the
predawn hours of Monday morning. Forecast models have steadily
showed this storm system weakening, with less overall rainfall
and weaker winds. Because this model continues to trend weaker,
the models are struggling to come into agreement with each other
regarding the actual strength of the system, with some weakening
the system much more than others. That said, overall, this system
should be relatively weak by mid-winter storm standards. As per
typical with systems arriving from the northwest, the North Bay
will see the most overall precipitation, with up to 0.75-1.25" in
the North Bay valleys, 1.25-1.75" over the wettest coastal north
bay ranges (ie near Venado, Tamalpais, etc). Much lesser amounts
should be anticipated farther southward and inland, for example,
downtown San Francisco should expect around .25-.33", while San
Jose will range from .05-.20" in part due to rain shadowing. Winds
will primarily be breezy along the coast as the cold front pushes
through, however, some brief locally gusty winds could also
accompay the frontal passage inland as well. We are eagerly
awaiting the arrival of the latest runs of our short term models
since they should be in better agreement than the older runs,
given that the system is nearly on our doorsteps at this point.
Keep an eye out for updated rainfall amounts coming out by early
this afternoon.

Looking ahead, a wetter, more organized storm system will move
into the region from the northwest around Wednesday afternoon,
through the night, lingering through Thursday. Storm total
precipitation and winds should be of higher magnitude than
today/tonights storm system, so it may actually be the one to
watch this week. That said, the next storm looks like a run of the
mill mid winter system. Neither of these storm systems is likely
to help get us out of the precipitation deficit the region is in
as of today. The extended period is looking dry at this time, but
that may change as models recently broke up the potential for a
blocking rex block. This could help open the door for a slightly wetter
pattern in successive model runs, however, for now the forecast
remains dry.

Here`s a look at our current water year:
SITE             SINCE OCT01 (WATER YR)
                 OBSRV   NORMAL  %NORM
Santa Rosa       11.10    18.90     59
Napa              5.73    10.46     55
Concord           5.58     7.93     70
DT San Francisco  7.68    11.75     65
SFO Airport       7.26    10.11     72
Oakland           7.18    10.44     69
Hayward           4.77     8.65     55
Livermore         4.71     7.79     60
San Jose          4.23     7.08     60
Moffett Field     4.39     6.71     65
Watsonville       5.50    10.26     54
Monterey          6.08     7.65     79
Salinas           2.22     5.57     40
Santa Cruz        8.33    14.69     57

.PREVIOUS 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.

&&

.AVIATION...as of 9:35 AM PST Sunday for 18Z TAFs. VFR expected
today. Weak cold front moving in later today. Rain is expected to
begin after 00Z in KSTS then move through the SF Bay Area by
03-05Z. Rain not expected to affect Monterey Bay terminals until
after 12Z. Winds are expected to be mainly southwesterly/southerly
through the forecast period, between 08-12 kt.

Vicinity of KSFO...VFR expected through this afternoon. CIGs will
gradually lower 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 mainly high clouds. Generally
light winds. Rain chances increase after 12z Monday.

&&

.MARINE...as of 09:30 AM PST Sunday...Winds will increase and
become southwesterly later this morning, especially for the
northern waters, ahead of an approaching weak  cold front. The
cold front is forecast to pass through Monday  morning and winds
will switch out of the north. 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.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .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

&&

$$

PUBLIC FORECAST: DRP
AVIATION: Sims
MARINE: Sims

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