Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
FXUS66 KMTR 300021
Area Forecast Discussion
National Weather Service San Francisco Bay Area
521 PM PDT THU SEP 29 2016
...LIGHT RAIN RETURNS FOR NORTHERN PARTS OF OUR AREA STARTING ON
.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 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
.AVIATION...As of 5:21 PM PDT Thursday...Area-wide lower level
cooling will persist through Friday night. Presently, the marine
inversion is 1,200-1,500 feet. 3C-4C cooling at 925 mb tonight
with an additional 4C-5C cooling Friday will be sufficient to
completely erode the already weakened marine inversion, probably
by early Friday morning at the latest.
A moderate onshore wind driven by a 3.6 mb SFO-SAC gradient gradually
subsides tonight then increases again to 4 mb Friday afternoon and
Peak of diurnal driven sea-breeze will coincide with the arrival
of a dry cold frontal passage moving south-southeast over the forecast
area Friday afternoon; gusty W-NW post-frontal winds expected mostly
later Friday into Saturday morning.
Vicinity of KSFO...Low-moderate confidence VFR holds til 04z-05z.
West winds gusting into the 20 knot range til 04z-05z. Lighter
winds tonight, then gusty W winds to 30-34 knots Friday afternoon
SFO Bridge Approach...Similar to KSFO.
Monterey Bay Terminals...IFR cigs due to stratus redeveloping this
evening between 02z-06z. IFR tonight then clearing returning by
.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 10:53 AM PDT Thursday...Weakening pressure
gradient today will bring generally light winds to the coastal
waters today. Winds beginning 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.
.Tngt...SCA...SF Bay until 9 PM
SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 PM
SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 9 PM
SCA...Pt Arena to Pt Reyes 0-10 nm
PUBLIC FORECAST: Sims
FIRE WEATHER: MM
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