FXUS66 KMTR 120532

Area Forecast Discussion
National Weather Service San Francisco Bay Area
932 PM PST Tue Dec 11 2018

.SYNOPSIS...A weak system may bring a few showers to northern Sonoma
County tonight. However, for the most part our forecast area is
expected to remain dry and seasonably cool through Thursday.
Another disturbance will bring rain to the region on Friday and
Friday night, especially for areas north of the Golden Gate. A
potentially wetter system will then bring widespread rain to the
region on Sunday.


.DISCUSSION...As of 8:55 PM PST Tuesday...Current satellite shows
a weak cold front pushing onshore near Cape Mendocino. The 00Z
NAM keeps all precipitation with this front north of our forecast
area, but the latest HRRR brings a few showers into the northwest
corner of Sonoma County later this evening. So will maintain
slight chance POPs in that area tonight, while maintaining a dry
forecast elsewhere. At least patchy fog is expected later tonight
and into Wednesday morning. However, fog is not expected to be as
widespread or dense tonight as last night/this morning for a
couple of reasons. First, there isn't as much moisture near the
surface as dewpoints are at least a few degrees lower this evening
compared to last evening. And second, the incoming dissipating
front will bring in enough clouds to decrease fog potential,
especially across the North Bay.

The weak system moving inland to our north tonight will quickly
move off to the east, allowing a shortwave ridge to build over
California on Wednesday. This ridge will result in dry weather and
slightly warmer daytime temperatures on Wednesday and Thursday.
However, overnight temperatures will be cool due to mostly clear
skies and long nights as we approach the winter solstice. In
addition, patchy night and morning fog will persist, especially in
the valleys.

Rain chances return by Friday afternoon as a Pacific system
reaches northern California. The models have been slowing
precipitation onset with this system and it now appears rain won't
reach the North Bay until about midday Friday, with most of our
area expected to remain dry through early afternoon. The models
agree that the frontal boundary will rapidly weaken over our area
late Friday afternoon and evening and produce mostly just light
rain. The models also agree that the remnants of the frontal
boundary will then remain draped across our area into Saturday and
Saturday night, and produce additional scattered light
precipitation as warm advection develops ahead of an approaching
stronger system. The models disagree on rain totals with this
system, with the ECMWF being much wetter compared to both the GFS
and NAM. The ECMWF forecasts widespread half inch rainfall amounts
across our area from the North Bay down to Monterey Bay, while
the NAM and GFS forecast a tenth of an inch or less, and mainly
confine measurable rain to the North Bay. Given the fact that
there will be little cold advection behind the front, feel the
ECMWF is likely too wet, especially across the southern portion of
our area. Also, there are model differences regarding the
location of the stalled frontal boundary and associated light
precipitation on Saturday and Saturday night, with the NAM and GFS
forecasting the boundary to be across the SF Bay Area (mainly
North Bay) while the ECMWF places the boundary farther south
across Monterey Bay. Needless to say, forecast confidence is
rather low from Friday through Saturday. However, in any case,
this system is forecast to be weak with only light rain and light
winds, so impacts will be rather minimal.

Models are in better agreement with the following system that is
forecast to bring widespread rainfall to our forecast area from
Sunday afternoon through early Monday. Preliminary rainfall totals
with that system range from 0.5-1.5". Locally gusty winds are also
likely on Sunday, but current model projections indicate winds
will remain below advisory levels.

Longer range models indicate upper level ridging over California
next week, resulting in dry and mild weather conditions from late
Monday through much of the rest of the week.


.AVIATION....as of 09:32 PM PST Tuesday...For 06Z tafs. MVFR/IFR
cigs beginning to develop over the south San Francisco Bay and
along much of the coastline. Expecting widespread MVFR/IFR
conditions overnight due to low cigs lasting through around 16z-
19z tomorrow. Patchy fog remains a possibility in the North Bay
overnight. Recent guidance has backed off on fog development in
the East Bay, but wouldn't rule it out. Should fog develop, not
expecting it to be as extensive or as long lasting as last night.
Generally light winds through the period.

Vicinity of KSFO...MVFR/IFR cigs have developed along the San
Mateo coast and are drifting over the terminal. Another patch of
low clouds has developed over the south San Francisco Bay and is
expanding towards the approach per recent satellite imagery.
Incoming high clouds will obscure the view further for much of
the night. Overall, expecting MVFR/IFR cigs overnight lasting
until 17z-19z tomorrow, although confidence is low on timing.

SFO Bridge Approach...similar to KSFO, may see more cloud
development over the approach.

Monterey Bay Terminals...VFR with increasing high clouds. Low
clouds have formed over the waters and are making their way
towards the Peninsula. Expecting MVFR/IFR cigs prevail tonight and
into tomorrow morning. Low confidence on timing.


.MARINE...As of 8:06 PM PST Tuesday...Northerly winds will
increase overnight across the waters as a cold front moves
through. Winds will diminish tomorrow night and into Thursday
before turning southerly into Friday. A building northwest swell
will impact the waters late this week result in hazardous
conditions. A more significant northwest swell is then forecast to
arrive Sunday into Monday.


.Tngt...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 Arena to Pt Reyes 0-10 nm from 3 AM
SCA...Pt Reyes to Pigeon Pt 0-10 nm from 3 AM




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