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

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000
FXUS66 KMTR 172131
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
131 PM PST Sun Dec 17 2017

.SYNOPSIS...This morning`s breezy conditions in the North and East
Bay hills will continue to diminish this afternoon with dry
conditions region-wide. A Pacific weather system is forecast to
move through the area Tuesday night and into Wednesday, bringing
with it a chance of light precipitation.

&&

.DISCUSSION...as of 1:32 PM PST Sunday...Temperatures are
rebounding nicely after a cool December morning across the San
Francisco Bay Area and central California coast. Temperature
observations this early afternoon are generally in the 60s with
dewpoints in the upper teens to 30s as the airmass remains dry.
Preliminary and non-operational GOES-16 (tentatively scheduled to
become operational as GOES-East this week) shows mostly clear
skies across the state as an upper level low continues to move
east along the Arizona/Mexico border and an upper level ridge
builds over the eastern Pacific.

In the short-term low relative humidity values and locally breezy
conditions will continue, particularly in the higher elevations.
While some locations in the North Bay Hills and East Bay Hills are
still reporting sustained winds over 20 mph, expect these winds
to continue to subside through the afternoon hours. Because the
elevated wind speeds are no longer widespread and are primarily
confined to the ridgetops and peaks, the Wind Advisory was allowed
to expire earlier as scheduled at 10 am PST.

The Red Flag Warning that was issued earlier for the higher
elevations across the area was also allowed to expire at 10 am
PST. Though the humidity values will drop to 25 percent or less in
many elevated areas, the diminishing winds are what helped to
improve the fire weather conditions from "critical" to "near-
critical". That being said -- fuels still remain abnormally dry
for middle December.

The upper level ridge will begin to flatten late Monday and
Tuesday as an upper level low drops southeastward from the Gulf of
Alaska to the Pacific Northwest. The GFS, ECMWF, and Canadian
models remain optimistic that parts of northern California will
receive light amounts of precipitation from late Tuesday and into
Wednesday. The big questions at this point are 1) what exact track
will the upper low take, and 2) how far south will this moisture
make it? As witnessed earlier this season, the North Bay is
typically the favored location to receiving the most (if any)
rainfall. Models have trended marginally wetter over the past day,
so we`ll wait and see what the 00Z model runs advertise. While
rain amounts are expected to remain rather limited, any moisture
at this time would be much welcomed.

Heading into Friday and beyond, models are advertising a high-
amplitude ridge in the eastern Pacific with dry weather for much
of California. Climatology tells us that December is normally the
wettest month of the year for San Francisco (4.56" according to
the 1981-2010 normals). As it stands right now, downtown San
Francisco has observed only 0.02" of rain so far this month. With
records going back to 1850, the first 17 days of this month mark
the 7th driest start to December on record for San Francisco, and
the driest since 1989. With the Climate Prediction Center`s 8 to
14 Day Outlook now valid through the last day of 2017, confidence
remains fairly high that this month will conclude drier than normal.


&&

.AVIATION...As of 9:35 AM PST Sunday...Dry northerly flow will
result in continued VFR conditions through Monday. Stronger winds
aloft will result in low level wind shear, especially over the
northern portion of the region through this morning with
diminishing winds through 21Z.

Vicinity of KSFO...VFR.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR.

&&

.FIRE WEATHER...as of 1:32 PM PST Sunday...the Red Flag Warning
was allowed to expire earlier this morning as winds will continue
to diminish through the day. Dry relative humidity values will
persist again this afternoon, particularly in the higher
elevations. Dry conditions are forecast to continue early this
week before a chance of light precipitation arrives late Tuesday
or Wednesday.

&&

.MARINE...as of 09:35 AM PST Sunday...Winds will diminish through
the day as an upper level trough over the interior moves east.
High pressure will build off of the northern California coast
bringing light to moderate north winds tonight through Tuesday.
Northwest winds will increase Tuesday night and Wednesday after a
frontal system moves across the area.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm until 8 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm until 2 PM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 2 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 2 PM

&&

$$

PUBLIC FORECAST: Rowe
AVIATION: RGass
MARINE: RGass
FIRE WEATHER: Rowe

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