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

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FXUS66 KMTR 300512
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1012 PM PDT THU SEP 29 2016

...LIGHT RAIN RETURNS FOR NORTHERN PARTS OF OUR AREA STARTING ON
SUNDAY...

.SYNOPSIS...A cooling trend will continue through the weekend as
the marine layer and onshore flow become better established with a
trough deepening along the West Coast. In addition, a dry cold
front will move through on Friday leading to breezy conditions. A
system will drop down from the north over the weekend which will
bring rain to our area especially over the North Bay.

&&

.DISCUSSION...as of 8:58 PM PDT Thursday...Marine layer 1500 feet
with low clouds pushing into Sonoma county and the SF peninsula at
this hour. SFO-SAC gradient is 3.1 mb with 20-25 kt winds from SFO
and out into the Delta suggesting low clouds will continue to
surge inland overnight. Upper trough arrives on Friday with a
strong seabreeze/onshore push forecast for Friday afternoon. It
will be downright blustery at the coast, especially from the
Golden Gate north as a potent upper trough approaches with strong
surface gradients and cool advection aloft. Strong northwest winds
down the Santa Clara Valley will be of concern for the Loma Fire
despite the cooler temperatures and higher humidity as models show
solid 30 knots of wind around 2000-3000 feet. Saturday looks
seasonably cool with highs from the upper 50s beaches, 60s most of
the district and low to mid 70s inland...quite a cool down from
earlier in the week.

00z nam is in and shows some light rain chances for the north bay
on Sunday but nothing impressive. The cooler temps will be the
bigger story with cold air cumulus across the sky. Will await the
full 00z gfs and ecmwf before making any changes to the mid and
long range forecasts.

&&

.PREV DISCUSSION...as of 1:38 PM PDT Thursday...Satellite imagery
shows most of the low cloudiness has cleared to the coast.
Exceptions include most of San Francisco, Point Reyes, and much of
the Monterey Bay area. Afternoon temperatures are running much
cooler than those yesterday at this time with many readings in the
60s and 70s, with some areas as many as 16 degrees cooler than 24
hours ago. Locations right along the immediate coast are
struggling to hit 60 degrees, while even the warmest, well-inland
locations have only warmed into the 80s.

The cooling trend will continue Friday and into the weekend as an
upper level trough approaches the coast. A dry cold front will
move across the district on Friday resulting in an increase in
wind speeds, mainly over the coastal waters and areas near the
coast as well as in the near-coastal hills. Wind speeds of 15 to
25 mph with some higher gusts are possible in these areas during
the afternoon and evening.

On Sunday, the main upper trough will move through bringing rain
chances to mainly the North Bay but some showers could occur from
around Monterey Bay northward. The proximity of the upper low will
also give the possibility of thunderstorms with these showers
late on Sunday. Rainfall estimates of around 0.20" in the North
Bay still look reasonable, with around a tenth of an inch across
much of the San Francisco Bay. A few post-frontal showers are
expected to linger into Monday.

A flat ridge will rebuild over the west into the middle of next
week and possible longer bringing a slight warming trend to the
area. The 12Z run of the ECMWF brings another round of showers to
the district possibly by next Sunday. Low confidence in that at
this time.

&&

.AVIATION...As of 10:12 PM PDT Thursday...Satellite shows plenty
of stratus over the coastal waters north and west of the Monterey
Bay, however there`s only patchy stratus seen south along the Big
Sur coast. Lower level cooling persists through Friday night, and
the marine layer inversion will steadily lift and weaken likely
mixing out entirely Friday morning with increasing vertical mixing
eventually overturning the stratus layer trapped under the marine
inversion; categorical cig forecasts tonight as well as forecasting
the return of VFR Friday will be challenging.

Strong and gusty W-NW winds Friday afternoon and evening due to 4
mb SFO-SAC onshore pressure gradient and peak of diurnally driven
sea-breeze coinciding with a dry cold frontal passage. Gusty lower
level winds overlapping coastal terrain linger into Saturday morning
with some de-coupling of area surface winds in the valleys probable
due to near surface inversion redevelopment, thus wind shear on a
localized scale could be an issue later Friday evening into the
overnight hours.

Vicinity of KSFO...MVFR cig tonight and Friday morning. Timing VFR
returning to KSFO Friday is a low confidence forecast. Gusty westerly
winds to 30-34 knots Friday afternoon and evening.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR has persisted longer than forecast at
KMRY, IFR cigs still forecast tonight, slow to arrive. Clearing
returning by mid-late Friday morning.

&&

.FIRE WEATHER...As of 3 PM PDT Thursday...The main weather
message will be cooler and wetter conditions heading into the
weekend. The marine layer will deepen as a trough approaches the
region tonight. The biggest fire weather concern the next few days
will be a passing dry cold front Friday afternoon/evening. The
front will be gusty northwest winds 10 to 20 mph with gusts to 30
mph. Despite the cooler/wetter conditions history shows this could
be of concern given the current state of the fuels. Winds will be
peak Friday night and Saturday. Next item of concern will be
precip...including thunderstorm chances. Loma Fire appears more
likely to receive some light precip where the Soberanes will be on
the edge. Instability increases Sunday afternoon over the North
Bay then spreading south. Slight chc of thunderstorms will be
poss.  LAL of 2.

&&

.MARINE...as of 8:05 PM PDT Thursday...Northwest winds increase
early Friday for the north coast and strengthen through the day.
The strongest winds will be located along the Sonoma coast north
of Pt Reyes and along the Big Sur coastline by tomorrow afternoon.
Strong winds will persist along the Big Sur coast...especially
around Point Sur through Saturday. Winds decrease Sunday morning.
Small mixed swell with a long period southerly swell through the
period.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm

&&

$$

PUBLIC FORECAST: RWW/sims
AVIATION: Canepa
MARINE: DRP
FIRE WEATHER: MM


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