FXUS66 KMTR 210537

Area Forecast Discussion
National Weather Service San Francisco Bay Area
937 PM PST Sat Jan 20 2018

.SYNOPSIS...Dry and cool weather is forecast to continue across
our entire area through at least midday Sunday. The next system
is expected to spread rain across primarily the northern portion
of the forecast area late Sunday and Sunday night. The weather
pattern will remain active into next week, with more rain likely
by midweek.


.DISCUSSION...As of 8:55 PM PST Saturday...After a cool start
this morning, afternoon highs reached seasonable mid and upper
50s. Expect another cool night tonight, but not quite as cool as
last night since the airmass over our region has begun to moderate
and high clouds are starting to increase.

Evening IR satellite imagery shows a band of clouds associated
with a frontal boundary extending from near the CA/OR border west-
southwest across the northeast Pacific. Moisture within this
boundary is relatively plentiful with precipitable water values
approaching 1.5 inches. The models agree that this frontal
boundary will sag slowly to the southeast over the next 36 hours,
and bring rainfall to much of our forecast area from late Sunday
through early Monday. Recent model output has slowed the southward
progression of the boundary and thus delayed the onset of
precipitation. Rain is not expected to develop in the North Bay
until mid-to-late afternoon Sunday, with rain then forecast to
gradually spread across most of the rest of the SF Bay Area by
late Sunday night. Rain may then reach as far south as the
Monterey Bay Area by Monday morning before the frontal boundary

The bulk of the rainfall with this system is expected to be
generated by warm advection processes and fall across northern
California, generally north of I-80. Rainfall amounts are
projected to fall off precipitously to the south of I-80. In our
forecast area, only the North Bay is expected to pick up
significant precipitation with this system. Rainfall will
initially be light in the North Bay on Sunday afternoon, but rain
rates are then expected to increase Sunday evening with periods of
heavy rain possible across portions of the North Bay from late
Sunday evening through late Sunday night. However, model rainfall
amounts have recently been trending lower. For instance,
yesterday the NAM and GFS were forecasting as much as 2.5 inches
of rain with this system in the North Bay. Now, the latest NAM
and GFS (00Z runs) forecast maximum rainfall values of only about
half that much. And, rain totals across the North Bay burn scars
are now forecast by the NAM and GFS to be only slightly more than
a half inch. Of course, these courser-scale models tend to miss
some of the orographic enhancement and therefore tend to
underestimate rainfall maxima. So we still could see locally as
much as 2 inches of rain in northwest Sonoma County and as much as
an inch or more across the burn scars. In any event, concern for
debris flows/flash flooding in and near the North Bay burn scars
has decreased now that the models are trending somewhat drier.

Besides trending drier, the models have also shown a trend towards
less wind with this upcoming system. Southerly winds may gust
locally as high as 35 mph along the North Bay Coast late Sunday
afternoon and Sunday evening, but winds are not expected to reach
advisory levels.

Rain is expected to linger into Monday morning as the frontal
boundary dissipates over the southern portion of our area. Most
models end all rain by midday Monday. Dry weather is expected to
return by Monday afternoon and temperatures will be mild due to
lack of cold advection with this system.

Shortwave ridging is forecast to maintain dry and mild conditions
from Monday afternoon through Tuesday and Tuesday night. The next
system is expected to bring rain to our area area Wednesday and
Wednesday night. This midweek system is forecast drive a strong
cold front through our entire forecast area, which will mean a
potential for soaking rains regionwide. Brief heavy rain and locally
strong and gusty winds may also be factors with the midweek
system. Showers will linger into Thursday, with a return to dry
weather conditions expected by late in the week.


.AVIATION...as of 9:37 PM PST Saturday for 06Z Sunday TAFs. VFR
conditions are generally expected for the first half of the TAF
period before the next storm system begins to impact the area late
Sunday and Sunday night. Some low clouds may try to develop along
the San Francisco Bay shoreline in the late night/early morning
hours, but confidence remains low at this time. Precipitation will
initially begin to impact the North Bay terminals on Sunday
afternoon, and slowly progress southward on Sunday evening/night.
Decreasing ceilings are forecast as the precipitation chances

Vicinity of KSFO...VFR conditions are generally expected for the
first half of the TAF period, though there is a slight chance of MVFR
ceilings after 10Z Sunday. West winds around 5 to 10 kt overnight
will gradually back to the south on Sunday. Gusty southerly winds
are possible after sunset Sunday and into Monday as rain chances
increase and ceilings drop to MVFR levels.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions are expected to prevail
with FEW/SCT clouds overnight. High clouds will gradually increase
through the TAF period as the next storm system approaches.


.MARINE...as of 09:25 PM PST Saturday...Northwest winds will
gradually taper off overnight as a ridge weakens and moves
inland. Winds will increase and back to become southwesterly later
Sunday ahead of an arriving cold front. The wave train that
arrived earlier in the week continues to weaken, however,
hazardous conditions remain possible through the next couple of
days as wave heights align with the period. A new northwest wave
train arrives Sunday evening.


     .Tngt...SCA...Pigeon Pt to Pt Pinos 0-10 nm until 3 AM
             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...Rough Bar Advisory for SF Bar until 3 AM




Visit us at www.weather.gov/sanfrancisco

Follow us on Facebook, Twitter, and YouTube at:

NWS MTR Office Area Forecast Discussion