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

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FXUS66 KMTR 220034

Area Forecast Discussion
National Weather Service San Francisco Bay Area
534 PM PDT Thu Sep 21 2017

.SYNOPSIS...A cool, upper level trough will dominate the western
portion of the country through the workweek maintaining cooler
than average temperatures and breezy conditions. Aside from the
potential for isolated showers offshore or near the coast Thursday,
dry weather conditions will prevail. A gradual warming trend is
then expected to begin this weekend and persist into early next

&& of 1:05 PM PDT Thursday...Visible satellite
reveals a mostly sunny to sunny afternoon across the region. The
cold front that passed through overnight was able to sufficiently
mix-out the marine layer with mostly clear skies extending over
the waters as well. There have been a few areas of post-frontal
cumulus development over the Santa Lucias, Diablo Range, and
Santa Cruz Mountains this morning that will continue this
afternoon. Can`t rule out a stray sprinkle in these areas, but
aside that, dry conditions are expected across the San Francisco
Bay Area and central coast this afternoon. High temperatures will
be in the 60s along the coast and 70s for inland areas. Compared
to climatology, these highs are about 5 to 15 degrees below what
we would normally expect for a middle/late September afternoon.
Winds will be breezy again this afternoon with some gusts along
the coast or higher elevations reaching 30 to 40 mph.

Low temperatures tonight are expected to be chilly once again.
With 850 mb temperatures hovering around 4 to 6 deg C, the North
Bay can expect widespread 40s once again for Friday morning some
locations dipping to the mid/upper 30s. On Friday we`ll begin to
see a gradual and continuous warming trend as an upper level ridge
begins to build in the eastern Pacific. Expect Friday`s highs to
be about 2 to 5 degrees warmer than today with some communities in
the interior East Bay climbing to the upper 70s. Tomorrow also
marks the autumnal equinox, which will occur at 1:02 pm PDT.

One more seasonably cool night is forecast on Saturday morning
with lows in the 40s and 50s. The building Pacific ridge will
allow for 500 mb heights to continue to rise through the afternoon
and even into next week. Consequently, we`ll watch temperatures
go from below normal back to above normal by Sunday with
widespread 80s emerging for inland areas and even parts of the
South Bay. The warming will continue into early next week with 90s
starting to appear back on the map. With the north to
northeasterly flow, do expect drying conditions with decreasing
afternoon relative humidities by the second half of the weekend
and early next week. This will lead to increased fire weather
concerns across parts of central and northern California.


.AVIATION...As of 5:30 PM PDT Thursday for 00Z TAFs...
Postfrontal environment includes locally gusty westerly winds
along with mostly VFR conditions. Winds will gradually decrease
through the evening and overnight. Considerable low level moisture
remains across the region and so areas of MVFR ceilings are
likely to develop overnight and persist into Friday morning.
However, confidence is low to moderate on where ceilings may
develop. VFR expected for most of Friday along with lighter winds.

Vicinity of KSFO...VFR forecast through at least 12Z. West winds
23G33 kt early in period will gradually subside through the
evening. MVFR cigs possible Friday morning from 12-17Z.
Confidence low-moderate. Otherwise VFR on Friday with much lighter
afternoon west winds.

SFO Bridge Approach...similar to KSFO.

Monterey Bay Terminals...MVFR ceilings at times through the
evening as low level moisture is lifted along the coastal terrain.
MVFR ceilings will become more widespread overnight and then
continue through late Friday morning before dissipating. West
early evening winds gusting to 20 kt, gradually decreasing
by late evening.

&& of 02:33 PM PDT Thursday...Increasing northwest winds
today as a trough moves through the region. Winds subside slightly
late tonight through the weekend. Northwest swell will continue to
decrease through the coming days while steep wind waves continue
through Friday.


.FIRE 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.


     .Tngt...SCA...SF Bay until 9 PM
             SCA...Mry Bay until 9 PM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 9 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 9 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 9 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm




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