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

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FXUS66 KMTR 200557

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1057 PM PDT Thu Oct 19 2017

.SYNOPSIS...A weak cold front will continue to move through the
region overnight, producing widespread rainfall. Rainfall will
mostly be light except locally moderate in the North Bay
Mountains and locally elsewhere in the hills. Along with
precipitation, expect cooler temperatures and brisk onshore winds
through Friday. Dry weather will return by mid morning Friday and
dry weather is expected through the remainder of the forecast
period. A warming trend will begin over the weekend and continue
into early next week. Temperatures are forecast to be well above
average through most of next week.


.DISCUSSION...As of 8:50 PM PDT Thursday...Light rain began to
spread into the North Bay around mid afternoon and has since
developed over nearly all of Sonoma and Marin counties as well as
across portions of the San Francisco Peninsula and the East Bay.
Rain totals thus far (as of 8 pm) have mostly been less than a
tenth of an inch, although a couple of spots in the North Bay have
picked up between a quarter and a third of an inch, including
Mount Tamalpais and Mill Valley. Rainfall so far has been
associated with an area of warm advection in advance of the main
cold-frontal rain band which is still offshore. Rain rates will
likely increase somewhat later this evening as the primary rain
band moves through. The latest HRRR shows this band moving through
the North Bay between 10 pm and midnight, through the rest of the
SF Bay Area between midnight and 3 am, and through the Monterey
Bay Area between 2 and 5 am. Additional rainfall from this system
overnight is expected to range from a tenth to a quarter of an
inch except as much as an additional half inch in the Sonoma
county hills and locally elsewhere in the coastal hills and East
Bay Hills. Less than a tenth of an inch is expected in southern
and eastern inland valleys.

This system is moving somewhat slower than previously forecast and
so we may see some isolated showers linger for a few hours past
sunrise on Friday. Otherwise, expect clearing skies on Friday with
cool temperatures and locally brisk northwest winds.

A warming trend will get underway over the upcoming weekend as an
upper ridge begins to build over California and onshore flow
gradually weakens. Much warmer temperatures are then forecast
early in the work week as the upper ridge strengthens along the
West Coast and light offshore flow develops. On Monday and Tuesday
expect widespread high temperatures in the 80s to around 90, with
these warm temperatures developing not only inland, but in
coastal areas as well. Slight cooling is projected for Wednesday,
mainly near the coast, as the ridge temporarily weakens.

Earlier model runs indicated cooling during the second half of
next week. But the latest GFS and ECMWF now both show renewed
strengthening of the upper ridge late next week which will mean
continued warm and dry weather.


.AVIATION...As of 10:57 PM PDT Thursday...For 06z Tafs. Scattered
to broken MVFR to VFR cloud decks have filled in across area
terminals ahead of the approaching cold front. Light rain
resulting in wet runways are expected through tonight as the front
moves across the region overnight. The front has progressed a bit
slower than anticipated therefore timing of the rain has been
adjusted of the 06z Tafs. Light to Moderate southwesterly winds
will continue through this evening ahead of the front with winds
anticipated to veer to the west and become temporarily gusty
around the time of FROPA. Gusts to 25 kt possible.

Overall forecast confidence: moderate

Vicinity of KSFO....Borderline VFR/MVFR cigs currently being
reported with light rain anticipated to begin shortly. Moderate
southerly winds will veer to the west with FROPA. Gusty
conditions are also expected with FROPA. Wet runways expected
tonight with improving conditions expected by morning. LLWS also
remains a possibility with FROPA (light at surface gusting to
25kt at 2000ft AGL).

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...MVFR cigs as well as light to moderate
west to southwesterly winds anticipated to continue through
tonight. Light rain will accompany FROPA. Wet runways expected
tonight with improving conditions expected by morning.


.FIRE WEATHER...As of 5:55 PM PDT Thursday...Cooling occurred
today as onshore flow increased ahead of an approaching cold
front and widespread cloud cover began to overspread the area.
Relative humidity values increased significantly as well,
especially across the North Bay Hills. By late Thursday afternoon
light rain had begun to fall across the North Bay. Expect light
rain to gradually spread south across the remainder of the
district Thursday night. Rainfall amounts will mostly be a quarter
of an inch or less, except locally more than a half inch in the
North Bay Mountains. Wetting rains are most likely across the
North Bay and locally in the Santa Cruz Mountains. Rainfall should
end in most areas by Friday morning. Friday will be a cool day
with brisk northwest winds.

A warming trend will get underway over the upcoming weekend as a
high pressure ridge begins to build over California. This ridge
will strengthen into early next week and result in warm and dry
conditions districtwide from Monday through Wednesday. Will need
to be wary of possible gusty offshore winds in the hills beginning
late in the weekend and continuing into Monday night.

&& of 10:45 PM PDT Thursday...An approaching cold front
will move through the coastal waters tonight bringing the first
rain of the season. Increased winds with occasional gusts to
around 30 kt and a long period northwesterly swell are also
forecast to accompany this system. Operators of small crafts are
urged to use extreme caution if out at sea as hazardous seas are
forecast to persist over the next  few days.


     .Tngt...High Surf Advisory...CAZ006-505-509-529-530
             SCA...Mry Bay
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Rough Bar Advisory for SF Bar
             SCA...SF Bay until 3 AM




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