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

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FXUS66 KMTR 230346
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
846 PM PDT Fri Sep 22 2017

.SYNOPSIS...High pressure building near the West Coast will result
in dry weather, along with a warming trend, through the middle of
next week.

&&

.DISCUSSION...As of 8:45 PM PDT Friday...Autumn officially began
at 1:02 pm PDT this afternoon. Our region will experience classic
early fall weather through the weekend, with clear and cool nights
followed by sunny and warm days.

The upper trough that brought cool and locally breezy conditions
to our area over the past few days is centered over the Great
Basin this evening and is slowly moving east. The trough is
forecast to continue moving to the east through the weekend.
Meanwhile, an upper level ridge offshore will slowly build
eastward along the west coast. This will result in a gradual
warming trend over the weekend. High temperatures tomorrow are
forecast to be about five degrees warmer than today, with another
few degrees of warming expected going into Sunday. However, nights
will continue to be rather cool through the weekend, especially
in the interior valleys where lows will drop into the mid 40s at
some of the cooler locations. A forecast update was completed
earlier this evening to lower some of tonight`s low temperatures
(mainly in the valleys) and to raise some of tomorrow`s highs.
With light offshore flow forecast to develop just above the
surface late tonight, and then continue off and on through the
weekend, daytime temperatures in coastal areas may end up being a
bit warmer than expected.

The northerly (offshore) winds that are forecast in the hills
over the weekend will raise fire weather concerns, especially
given the expected warming and drying. Concerns are greatest
across the North Bay Hills where drying winds are forecast to be
the strongest. A Fire Weather Watch is in effect for elevations
above 1000 feet in the North Bay from Saturday afternoon through
Monday evening. See the fire weather discussion below for complete
details.

The warming trend is forecast to continue through the middle of
next week as the upper ridge strengthens over California. As is
often the case with warm weather in the autumn during light
offshore flow events, temperatures will be rather uniform across
our region, with highs by Tuesday and Wednesday forecast to be
80s near the coast and in the mid 80s to mid 90s inland. We may
even see some locations near the coast creep up to near 90 by the
middle of next week. Fortunately, the longer autumn nights and dry
airmass will allow temperatures to cool back nicely overnight,
especially at lower elevations, which will help mitigate heat
risks.

The models agree that the warming trend will end by late next week
as the upper ridge axis shifts inland. And, there will likely be
at least modest cooling in coastal areas by Thursday and Friday as
light onshore flow develops.

&&

.AVIATION...As of 4:14 PM PDT Friday for 00Z Saturday
TAFs...Aside a few possible SCT and perhaps locally BKN clouds at
sunrise tomorrow along the coast, VFR conditions are expected to
prevail throughout the San Francisco Bay area and central
California coast over the next 24 to 30 hours. Breezy onshore
winds will diminish after sunset, and increase again on Saturday
afternoon. High confidence.

Vicinity of KSFO...VFR conditions expected. Breezy west winds this
evening around 15 kt will decrease after sunset.

SFO Bridge Approach...similar to KSFO.

Monterey Bay Terminals...VFR conditions expected, though there is
a slight possibility of some brief SCT or even BKN skies around
sunrise Saturday. Likelihood of occurrence at KMRY or KSNS is too
low to put into TAF, but will monitor and amend if necessary.
Breezy onshore winds will decrease after sunset.

&&

.FIRE WEATHER...As of 2:39 PM PDT Thursday...Clear and cool
overnight with easing northwest winds. Continued dry and
seasonable on Friday. By late Friday night northerly winds will
develop across the North Bay hills into early Saturday morning but
humidity should stay fairly moist. Noted warming and drying trend
over the weekend with periods of gusty northeast winds across the
north and east bay hills later Saturday night and then again
Sunday night. Afternoon highs well into the 80s and lower 90s
inland areas with humidity lowering into the teens. Long range
trends show a prolonged yet seasonable warm and dry pattern
through the weekend and into next week with high pressure aloft
and a thermal trough near the coast. Fine fuels will continue to
dry given the lack of marine air. Climatology suggests potential
for near critical fire weather conditions for much of the next 5-7
days starting this weekend. Fire Weather watches and or warnings
may become necessary should the pattern develop as expected,
however no strong offshore wind events are forecast at this time.

&&

.MARINE...as of 4:08 PM PDT Friday...Eastern Pacific ridge will
gradually build towards the region, bringing breezy northwest
through the coming days, especially over the northern outer
waters. Northwest swell train will continue to weaken over the
coming days.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .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 until 3 AM

&&

$$

PUBLIC FORECAST: Dykema
AVIATION: Rowe
MARINE: DRP
FIRE WEATHER: RWW

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