SFOAFDMTR (10/21/16 1121)

FXUS66 KMTR 211801

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1101 AM PDT Fri Oct 21 2016

.SYNOPSIS...Slight cooling is forecast for today as onshore flow
develops by afternoon, but inland areas are expected to remain
warm. Temperatures will cool off to closer to normal by 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 rain chances on Monday and Tuesday and then
again late in the week.


.DISCUSSION...As of 9:20 AM PDT Friday...Clear start to the day
with an offshore flow in place, however the gradient is weakening
and is expected to become neutral to onshore by later in the day.
Temperatures will be very comfortable with mid 60s to mid 70s at
the coast and mid 70s to mid 80s for most inland spots. 

Computer models continue to indicate a return to wet weather next
week as a longwave trough advance to the coast and moist air
becomes entrained in the flow. 0Z solutions continue to focus on
Monday into Tuesday for the first round and then Thursday into
Friday. Worth noting that the 12Z GFS operational run that just
came in has backed off on rainfall for both periods. Will be
interesting to see what the ensembles and ECMWF operational run

.PREVIOUS DISCUSSION...Satellite imagery is showing clear skies
except for some patchy stratus moving south along the coast north
of Point Reyes, and west of the coast to the south. Temperatures
are running several degrees warmer than yesterday at this time
with readings in the 50s to lower 60s across the district.
Offshore surface pressure gradients still persist, but they are
weakening. WMC-SFO is now 5.7 mb, but that gradient was nearly 2
mb stronger yesterday at this time. SFO-SAC is now barely onshore
at 0.1 mb. The onshore gradient is projected to increase by this
afternoon thus bringing a cooling seabreeze to the coast with
highs in the upper 60s to mid 70s expected. Inland areas are
forecast to be warm once again with highs expected to be in the
upper 70s to upper 80s.

Meanwhile, the upper level ridge currently over the west will
slowly edge eastward during the weekend as an upper level trough
approaches the coast. This will allow cooler air to move into the
district. Temperatures will cool to near normal values by Sunday
with lower to mid 60s at the coast, and 70s to near 80 inland.
Rain could spread into to the North Bay by late Sunday night or
Monday morning as the trough nears. 

Medium range models deepen the trough into a cut-off low off the
coast by the middle of the week. This will bring a break in rain
chances across the area before the low moves east by the end of
the work week bringing rain chances back into most of California.
Latest models still indicate that this cutoff low could tap into
subtropical moisture which would result in widespread significant
rainfall across our entire forecast area next Friday. However,
earlier model runs kept the low offshore with dry conditions. Will
have to wait and see how this progresses as cut-off lows are
often problematic to forecast.


.AVIATION...as of 11:00 AM PDT Friday...For 18z TAFs...Widespread
VFR today ahead of an approaching trough. High clouds will spread
southward through the day at or above 20000 ft. Winds will be
breezy along the coast in the afternoon but gradually weaken
overnight. A compressed marine further south ahead of the front
will bring patchy fog posbl to KMRY KSNS KWVI. Further north,
instead expect to see brief intrusion into the Bay Area around
sunrise around 600-900 feet around sunrise as marine layer springs

Vicinity of KSFO...VFR. light winds increasing this afternoon to
around 20 kt. Chance to see some brief IFR cigs around sunrise

SFO Bridge Approach...Similar to KSFO. 

Monterey Bay Terminals...VFR today with light winds. Chance to see
VLIFR due to patchy fog around sunrise tomorrow.


.MARINE...as of 10:55 AM PDT Friday...Moderate northwest winds
will continue today ahead of an approaching trough. The strongest
winds will be around Point Reyes and Point Sur. Winds will
gradually ease through the weekend as low pressure descends into
the region. This low pressure will bring a longer period
northwest swell to the area.


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


PUBLIC FORECAST: Bell/Sims AVIATION: DRP MARINE: DRP Visit us at www.weather.gov/sanfrancisco Follow us on Facebook and twitter at: www.Facebook.com/nwsbayarea www.twitter.com/nwsbayarea