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

Home | 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
FXUS66 KMTR 220423

Area Forecast Discussion
National Weather Service San Francisco Bay Area
923 PM PDT Fri Oct 21 2016

.SYNOPSIS...Temperatures will cool off to closer to normal this
weekend as high pressure currently over the West Coast moves to
the east. An unsettled weather pattern is expected to develop
early next week with highest rain chances on Monday and Tuesday
and then again late in the week.


.DISCUSSION...As of 8:40 PM PDT Friday...A return of a marine
layer along the coast and light onshore flow by afternoon resulted
in anywhere from 5 to 15 degrees of cooling near the ocean and
bays today, with slight cooling in most coastal valleys as well.
The inland valleys and hills remained warm, and in some cases
warmed a few degrees over yesterday.

Cooling will become more widspread over the weekend as the upper
ridge continues to move off to our east, allowing marine air to
filter farther inland and higher into the hills. Most locations
are expected to cool to near seasonal averages by Sunday. Expect
patchy low clouds during the night and morning hours, mainly near
the coast, otherwise skies should be mostly clear with passing
high clouds.

A forecast update was completed earlier this evening to reduce the
amount of cloud cover from tonight through the weekend.

Evening satellite imagery shows an upper trough dominating the
northeast Pacific. This trough is forecast to dig to the south
over the weekend and then push a fairly moist frontal boundary
into northern California late Sunday Night into Monday. The 12Z
ECMWF...18Z GFS...and 00Z NAM all show rain developing over much
of the North Bay by Monday afternoon. These models also show
little, if any, rain making it south of the Golden Gate. Rain may
make it as far south as Santa Cruz and San Jose by Monday Night or
Tuesday morning but the models indicate the front`s southward
progression will halt by then as a potent shortwave drops into
the base of the longwave trough offshore, causing the frontal
band downstream over our area to stall and then lift back to the
north by Tuesday Night. Across the North Bay, rain totals from
late Sunday Night through Tuesday evening could range anywhere
from a half inch to 2 inches. Less than a half inch of rain is
expected across the central portion of the SF Bay Area given
current model projections. Meanwhile, the southern half of our
forecast area may end up seeing no rain during the first half of
next week.

Wednesday looks like a dry day as a cutoff low develops several
hundred miles west of the Central California coast...resulting in
ridging downstream over CA. The cutoff low is then forecast to tap
into subtropical moisture and advance eastward on Thursday.
Depending on the exact track and timing of the cutoff low, we
could see significant precipitation across much of our area
sometime between midday Thursday and midday Friday. The timing on
this precip event later in the week has trended faster. The 12Z
ECMWF spreads rainfall uniformly across the entire state of
California late next week while the 18Z GFS focuses the heaviest
rainfall across southern CA. As can be expected this far in
advance with a cutoff low, the models are having a tough time with
the specifics and it may take a few more days to gain forecast
confidence on the rain event later next week.

&& of 9:20 PM PDT Friday...Patchy IFR cigs along the
immediate coast, VFR elsewhere. Generally looking at light winds
with high pressure aloft steadily weakening through the period.

Vicinity of KSFO...VFR. Tempo IFR cig possible around sunrise

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Patchy IFR developing this evening, areas
of IFR tonight and Saturday morning. Clearing returns by late Saturday

&& of 9:20 PM PDT Friday...Gusty northwest winds will
continue tonight into Saturday as an upper level trough approaches.
winds will gradually ease later this weekend. A moderate sized
fresh swell train with a 15 second period will arrive early this


     .Tngt...SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 AM




Visit us at

Follow us on Facebook and twitter at: is the U.S. government's official web portal to all federal, state and local government web resources and services.