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

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS66 KMTR 220547
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1047 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
week.

&&

.DISCUSSION...as of 9:00 PM PDT Thursday...Mainly clear skies
prevail this evening with the exception of a couple patches of
high clouds that are riding in on the northerly flow aloft as well
as a few patches of low clouds that have hung on to the hills
locally over the Santa Lucia, Diablo Range, and Santa Cruz
Mountains.

The cooler airmass that arrived last night resulted in cooler
temperatures across the board this afternoon. Temperatures cooled
anywhere from 3 to 5 degrees across most locations with as much as
10 to 15 degrees of cooling observed across Monterey and San
Benito counties. Temperatures ranged from 60s along the coast,
upper 60s to low 70s around the Bay and low to mid 70s inland. Not
one reporting station reached 80 degrees today. As mentioned
previously these highs are about 5 to 15 degrees below what we
would normally see for this time of year. This cooler airmass will
also affect overnight lows resulting in chilly conditions across
the district. Widespread 40s are forecast overnight with several
pockets of mid to upper 30s possible over elevated inland areas.

Breezy conditions were observed once again this afternoon with
some gusts along the coast or higher elevations of the North and
East Bay reaching 30 to 40 mph. A gust of 54 mph was observed by
the Big Rock RAWS around 6:30 pm this evening. Winds have since
ease a bit. Breezy conditions are expected once again Friday
afternoon.

The trough of low pressure that brought these cooler and breezier
conditions to the western U.S. will gradually push east as high
pressure builds in over the Eastern Pacific. A warming trend will
kick off Friday with 3 to 5 degrees of warming expected. More
significant warming is expected over the weekend with
temperatures returning to seasonal norms.

From previous discussion...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 11:45 PM PDT Thursday for 06Z TAFs...Low level
moisture is expected to increase across the region overnight and
the low levels should become more stable. This will likely result
in areas of MVFR ceilings later tonight, persisting into Friday
morning. However, confidence is low to moderate on where ceilings
may develop. VFR conditions expected for most of Friday. Winds are
forecast to be lighter on Friday than they were on Thursday.

Vicinity of KSFO...VFR forecast through at least 12Z. West winds
16G22 kt early in period will gradually decrease overnight. MVFR
cigs possible Friday morning from 12-17Z, but confidence only low
to moderate. Otherwise VFR on Friday with lighter afternoon west
winds compared to Thursday.

SFO Bridge Approach...similar to KSFO.

Monterey Bay Terminals...VFR early in period with MVFR ceilings
possibly developing by 09Z and then continuing through late
Friday morning before dissipating. Slight chance that low clouds
will persist into the afternoon. Light southwest winds through
most of the period.


&&

.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 9:00 PM PDT Thursday...Moderate and gusty
northwest winds will gradually subside tonight. Winds will
increase once again on Friday afternoon, but mainly over the
offshore waters. Northwest swell will continue to decrease through
the coming days while steep wind waves continue through Friday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 3 AM

&&

$$

PUBLIC FORECAST: CW
AVIATION: Dykema
MARINE: Dykema
FIRE WEATHER: RWW

Visit us at www.weather.gov/sanfrancisco

Follow us on Facebook, Twitter, and YouTube at:
www.facebook.com/nwsbayarea
www.twitter.com/nwsbayarea
www.youtube.com/nwsbayarea



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.