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

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162
FXUS66 KMTR 220059
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
559 PM PDT Tue Aug 21 2018

.SYNOPSIS...After a period of cooling, a slight warming trend will
begin tomorrow as the upper trough influencing our current weather
exits to the northeast. This will allow temperatures to rebound to
near seasonal averages by late this week. Coastal clouds are
still expected to persist and penetrate inland each night.

&&

.DISCUSSION...as of 02:11 PM PDT Tuesday...Stratus has mainly
dissipated across the region with some lingering clouds around the
San Francisco Bay and northward. Satellite imagery also shows
smoke drifting across the North Bay from various wildfires. Thanks
to persistent cloud cover and an upper trough stretching across
northern California, temperatures this afternoon are around 5 to
10 degrees cooler than this time yesterday. Expect another round
of overnight and morning low clouds as the marine layer is
forecast to remain above 1500 ft.

The upper low over Idaho and associated trough that brought us
cooler weather will move to the northeast tomorrow, allowing the
ridge of high pressure over the desert southwest to strengthen.
This will kick off a warming trend through the rest of the week
and into the coming weekend. A broad upper trough over British
Columbia and an upper low well to the west of California will
prevent the aforementioned ridge from strengthening enough to
cause any significant warming. Most locations will remain near
seasonal normals through the week. This translates to afternoon
high temperatures in the 60s to low 70s along the coast and 80s to
90s inland. Overnight and morning low clouds are expected to
continue as well.

Looking into the medium range, CPC outlooks continue to show near
normal temperatures over California. This morning`s 8-14 day
outlook even shows below normal temperatures for much of the west
coast.


&&

.AVIATION...as of 5:57 PM PDT Tuesday...A moderately deep marine
layer up to 2400 and 2800 feet will likely compress tonight into
Wednesday as lower level thermal ridging strengthens; 925 mb temps
warm 6C-12C from midday earlier today through Wed afternoon. Bodega
Bay, Fort Ord, Point Sur profilers starting to show some warming and
compression of the marine layer on recent check.

The 18z NAM shows increasing surface dewpoint temperatures sweeping
SE-E across the coastal waters and a weak low pressure circulation
setting up just west of Mendocino/Sonoma counties tonight thru Thu
morning. This supports ongoing widespread ocean based stratus feed
along with at least patchy fog (4:20 pm visibility 1/2-1 mile in fog
at the Farallon Islands). Based on recent high-res model output
wildfire smoke both aloft and prob some smoke extending to the
surface will advance inland and add to presently smoky conditions
over the Bay Area through this evening into Wednesday; clearing
Wednesday could be a very slow process, many locations may have
stratus and a mix of fog/smoky/hazy conditions all day.

Expect moderate to poor slant range visibility through mid-late
week, and surface visibilities reduced to MVFR-IFR due to combo
fog and smoke, along with lowering stratus ceilings to IFR-VLIFR.

Vicinity of KSFO...Stratus coverage is more extensive late this
afternoon, plenty of stratus feed off the ocean, and the marine
layer is moderately deep. Stratus ceilings are likely to lower as
lower level warming steps up through the evening into Wednesday
morning. Scattering of stratus may hold off until Wednesday
afternoon. Horizontal visibilities are good at the moment, but
should generally decrease through the evening; KHAF vsby and cig
(IFR) have been steadily lowering.

SFO Bridge Approach...Moderate to poor slant range visibility for
the period. Otherwise similar to KSFO.

Monterey Bay Terminals...MVFR gradually lowering to IFR tonight,
patchy LIFR-VLIFR is possible depending how quickly the marine
layer compresses; may need to adjust vsbys/cigs lower by 06z taf
issuance. Slow mixing process on Wednesday, clearing prob delayed
at least til late morning if not afternoon with brief window of
clearing in the afternoon. Stratus/fog likely rolling back inland
early Wednesday evening and night.

&&

.MARINE...as of 4:52 PM PDT Tuesday...Light onshore winds over
the coastal waters through the coming days. Breezy onshore winds
will develop near coastal gaps through the afternoon and evenings.
Winds will increase over the northern waters Friday into the
weekend. Light mixed swell overall.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...None.

$$

PUBLIC FORECAST: AS
AVIATION: Canepa
MARINE: DRP

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