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FXUS66 KMTR 121152

Area Forecast Discussion
National Weather Service San Francisco Bay Area
352 AM PST Thu Dec 12 2019

.SYNOPSIS...Light rain will linger over the region through the 
morning before diminishing in coverage later in the day. Rain 
chances increase again Friday into Saturday with temperatures 
turning colder Sunday into Monday. An unsettled and wet pattern will 
likely return by middle of next week. 

&& of 03:14 AM PST Thursday...Zonal flow aloft 
continues to advect moisture across the region with light rain 
still being reported from the Santa Cruz Mountains northward 
across the San Francisco Bay Area. Meanwhile, mainly dry 
conditions prevail over the Central Coast with widespread cloud 
cover region-wide. These conditions have allowed for temperatures 
to hold in the 50s to even lower 60s with not much additional 
cooling before sunrise. Similar to the past few days, 
precipitation amounts will remain light and only amount to a few 
hundredths of an inch or so per hour. The forecast calls for 
conditions to dry out by this afternoon and into late this evening
as slightly drier air briefly settles into the region. 

By late Friday into Saturday, moisture advection will again increase 
over the northern and central California coast ahead of a mid/upper 
level system that will drop southward over the region. This will 
bring renewed chances for light to moderate rain first developing 
over the North Bay early Friday before spreading southward through 
Friday night. Similar to the last passing system, the best chances 
for rainfall will be over San Francisco Bay Area with lesser chances 
for the Central Coast. This will be a result of a likely dissipate 
frontal system as mid/upper level support will be limited. Rain 
chances may linger into Saturday as well with the NAM and GFS both 
showing a secondary system moving across the region while the ECMWF 
keeps conditions mostly dry on Saturday. Regardless, the ensembles 
generally agree with unsettled conditions likely persisting into 
Saturday before drying out late Saturday night into Sunday. 

Conditions will dry out and temperatures will turn cooler Sunday 
into early next week as a colder air mass to the north advects 
southward. Looking for more widespread 30s and 40s for overnight 
lows from Saturday night into Monday night. Will need to watch for 
the potential for areas of frost or even freezing temperatures in 
the region's coldest interior valleys early next week. 

While the operational models continue to differ in the details 
heading into the middle/late part of next week, the ensembles do 
indicate a return to unsettled and wet conditions. Initial thinking 
is there will be a potential for more widespread rainfall across the 
region during the extended forecast period.

&& of 3:50 AM PST Thursday...Unsettled pattern with
weak forcing/winds but abundant low level moisture thru the taf 
period. Main impact will be from LIFR to MVFR cigs (depending on
location and time of day). Winds will generally be light (10kt or
less). Isolated, very light on and off rain showers throughout 
the taf period as well but generally too weak to bring extended
periods of wet runways. Ceilings generally MVFR-VFR from midday to
sunset with MVFR to IFR, locally LIFR-VLIFR overnight.

Vicinity of KSFO...MVFR cigs with bases around 1500-2500 ft today.
Light winds under 10 kt. Light passing showers throughout the
duration. IFR ceilings (1000 feet or less) possible tonight.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals... Other than 8-11kt SE winds at Salinas
through the morning, light winds are expected. Ceilings generally
borderline MVFR/IFR this morning, improving through the day, but
then lowering to borderline LIFR/IFR tonight. A few passing light
showers through the duration.

&& of 03:46 AM PST Thursday...Light southerly winds will
prevail through Thursday before switching to west to northwest.
Northwest winds will increase on Saturday as a cool upper trough
passes to the north of the area.  Large long-period northwest
swell will impact the waters Thursday night through Saturday.
Hazardous conditions for small craft vessels will exist with this
arriving northwest swell due to steep large seas.


     .Tday...High Surf Advisory...CAZ006-505-509-529-530
             SCA...Pt Arena to Pt Reyes 0-10 nm from 10 AM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm from 10 AM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 10 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 10 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm from 10 AM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 10 AM




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