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

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FXUS66 KMTR 211800

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1100 AM 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 8:41 AM PDT Thursday...Water vapor satellite
imagery shows a cold front passing through central California and
continuing into Nevada and Idaho. In its wake, temperatures across
the San Francisco Bay Area and central California coast are
generally in the 50s with a few readings in the middle 40s for
parts of the North Bay, such as Santa Rosa and Napa. The 24 hour
temperature change varies depending on location. For the Monterey
Bay area temperatures are generally running at or slightly warmer
than this time yesterday, whereas the San Francisco Bay Area
temperatures are running about 5 to 10 degrees cooler. Go farther
north and temperatures are running even cooler with a 15 degree
change at Napa. Expect mostly clear, dry, and cool conditions
across the region with afternoon highs in the 60s along the coast
and 70s for interior areas. The exception to the dry weather would
be the higher elevations of Monterey County where locally patchy
drizzle is possible through mid morning. The cool weather won`t
last much longer as temperatures will begin to rebound tomorrow
with the warming trending continuing through the weekend and into
early next week. The short-term grids remains on track and don`t
anticipate any major changes to the forecast. For additional
details beyond today, please refer to the previous discussion


.PREV DISCUSSION...As of 2:45 AM PDT Thursday...A cold front passed
through the Bay Area on Wednesday bringing some light showers,
breezy conditions and cooler temperatures. The cold front is now
south of the forecast area, but continues to produce precipitation
across the Sierra. Behind the front and rather robust upper level
trough is currently swinging through NorCal. Latest satellite
imagery and radar both indicate showers over the coastal waters.
These showers are tracking mostly north to south. Model guidance
has not changed too much from previous runs regarding the shower
activity. Showers will continue early today, but remain offshore
with no impact to the land areas of the Bay Area region.

Do expect some clearing of clouds today, but temperatures will
remain cool as the upper trough moves through. Latest model runs
still show a cool airmass with 850mb temps in the 4-5 deg C range.
As a result, daytime highs will be in the 60s and 70s or about 5
to 10 deg below normal for late September. Overnight lows will be
chilly as well with the hills dropping into the upper 30s.
Additionally, hi-res models show some breezy winds today and
Friday. Gusts 30 to 40 mph along the coast and higher terrain
seems plausible.

Another pattern shift is forecast to develop this weekend and then
persisting into next week. In the wake of the upper trough high
pressure will build over the region this weekend. A noticeable
warming trend is forecast to develop this weekend and peak next
week. Widespread 80s to possibly mid 90s will be possible by early
next week. Pattern recognition and climatology also suggest
periods of north to offshore winds will possible this weekend and
next week. The offshore flow will further enhance any warming and
drying. Fire weather concerns may increase due to offshore flow
with hot and dry conditions.


.AVIATION...As of 010:55 AM PDT Thursday for 18Z TAFs...
Postfrontal environment bringing well mixed marine layer and
widespread VFR cigs this morning. Unstable postfrontal airmass is
firing some shallow convection over the higher terrain over the
South Bay, East Bay, Santa Lucia mountains and Santa Cruz
mountains. A few sprinkles possible today in these areas if
convection deepens, however this is unlikely. Otherwise, breezy
onshore winds forecast today, with sust winds 10-20 kt, with peak
gusts 20 to 30 kt. Some high clouds will arrive late today. Dirty
feed of SCT-BKN AOA 2000-4000 ft AGL will bring intermittent MVFR
cigs in a window from 10-18Z FRI, depending on location. High
confidence for today, low confidence on exact details for tomorrow
mornings dirty feed.

Vicinity of KSFO...VFR. Sustained onshore winds 15 to 20 kt, gusts
25 to 30 kt. MVFR cigs possible FRI morning from 12-17Z with dirty
onshore feed as trough shifts through.

SFO Bridge Approach...similar to KSFO.

Monterey Bay Terminals...VFR. Shallow convection distant south
and east with FEW-SCT at 3000 to 5000 ft AGL. Breezy onshore winds
10 to 15 kt with gusts up to 20kt possible, locally stronger along
the immediate coastline. Intermittent MVFR cigs posbl 12 to 18Z
FRI w SCT-BKN from 2000-4000 ft.

&& of 10:48 AM 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.


     .Tday...SCA...Mry Bay
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...SF Bay from 1 PM




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