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

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FXUS66 KMTR 220036
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
436 PM PST Sun Jan 21 2018

.SYNOPSIS...Increasing cloud cover this afternoon ahead of an
approaching storm system. Wet weather this evening and overnight
as a cold front moves southeastward over the area. Breezy, 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 1:40 PM PST Sunday...The day started with
near to subfreezing temperatures in many inland valley locations,
including near Santa Rosa, Napa, Livermore, and Hollister, but has
transitioned into a fair weather day thus far. Temperatures
tonight should be much more mild as a result of a blanket of
clouds over the area.

Visible satellite imagery shows mostly cloudy to overcast high
clouds blanketing the greater San Francisco Bay area and pushing
southward into the Monterey Bay region. These high clouds are
arriving ahead of an approaching cold frontal boundary that will
bring generally light rains and winds to the area from this
evening through the predawn hours tomorrow morning. The parent
upper low associated with this frontal boundary is absent from the
local region, instead it is centered well to the north offshore
of the British Columbia coast.

Each successive forecast model run has suggested this storm
system would weaker, with a widening gap between the parent upper
low and surface cold front. This trend continued with todays short
term runs and the latest forecast data has cut forecast
precipitation amounts by half in some instances. As a result, this
system will be on the weak side of winter storm systems, bringing
1.25-1.75" to the wettest spots in the North Bay coastal ranges,
0.50-.75" to the coastal North Bay valleys, .25-.33" to the San
Francisco, Oakland, and the Santa Cruz areas, around .15" to the
South Bay/East Bay & their valleys, and a tenth or less to much of
the Monterey Bay region excluding Santa Cruz. Southwest winds
will breezy as the frontal boundary moves through tonight,
especially along the coast and over higher terrain, with sustained
winds generally around 20mph and peak gusts around the windiest
spots up to around 30mph. Those in the South Bay and Monterey Bay
areas could see some light rain during the Monday morning commute,
so be prepared for wet roadways if commuting in those areas.

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 (especially in the southern half of the
forecast area), so it may actually be the one to watch this week.
Neither of these storm systems is likely to help get us out of the
precipitation deficit the region is in as of today as it would
take about 1.5 to 2 stronger systems to pull water year
accumulations closer to normal. 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

&&

.AVIATION...as of 4:30 PM PST Sunday...Lower cigs and brief light
rain are moving into the North Bay. Lower cigs expected into the
SFO Bay Area after 03Z. Deeper moisture arrives after 03Z in the
North Bay and 06Z in the SFO Bay Area with widespread light rain
and reduced vsbys to 4-6 miles. The trailing end of the system
reaches the MRY Bay Area after 12Z.

Vicinity of KSFO...MVFR cigs after 03Z. Light rain with vsbys down
to 4-6 miles after 06Z. Southeast winds 5-10 kt becoming more southerly
tonight.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...MVFR by 12Z as the tail end of a weak
frontal system approaches. Some light rain from 12Z-18Z.

&&

.MARINE...as of 03:46 PM PST Sunday...South winds will increase
ahead of an approaching 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...
     .Tngt...SCA...Pigeon Pt to Pt Pinos 0-10 nm from 10 PM
             SCA...Pt Arena to Pt Reyes 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...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Mry Bay from 4 AM

&&

$$

PUBLIC FORECAST: DRP
AVIATION: W Pi
MARINE: Sims

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