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

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FXUS66 KMTR 190556

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1056 PM PDT Wed Oct 18 2017

.SYNOPSIS...A cold front remains on track to push through our
region on Thursday and Thursday night, bringing our first
widespread rainfall event of the season, along with cooler
temperatures. Look for the greatest rainfall to be in the coastal
ranges and across the North Bay. Along with cooler temperatures,
breezy onshore winds can be expected through Friday. Dry
conditions will return by Friday and a warming trend is forecast
over the weekend and into the first part of next week.
Temperatures are forecast to be well above seasonal averages
during the first half of next week.


.DISCUSSION...As of 9:05 PM PDT Wednesday...Cooling occurred in
most areas today as onshore flow increased and a marine layer
became established along the coast. Afternoon highs today were 10
to 20 degrees cooler than yesterday along the coast. Inland areas
experienced more modest cooling, and some of the valleys farthest
inland and the hills above 1500 feet remained warm. But even
those warm areas will cool tomorrow as clouds associated with an
incoming weather system spread across the region and onshore flow
increases further.

Evening IR satellite imagery shows a cold front beginning to push
into the Pacific Northwest. The models remain consistent on the
timing of this system as it tracks to the southeast tonight and
Thursday. Rain is expected to begin along the Sonoma County Coast
as early as midday tomorrow, with light rain then expected to
spread across most of the rest of the North Bay during the
afternoon. Rain will then develop south and east across the
remainder of our forecast area from early Thursday evening through
late Thursday night. Up until yesterday, the models had been
trending drier with this system. But lately the models indicate
this system may manage to tap into the higher PW air to our
southwest, at least to a limited extent. Consequently, model QPF
has been trending slightly higher. The 00Z NAM forecasts local rain
totals up to 0.75" in the North Bay Mountains tomorrow afternoon
and evening, and up to 0.25" in the Santa Cruz Mountains tomorrow
night. Elsewhere in the hills and North Bay Valleys, expect rain
totals from 0.10-0.25", while all other valley locations will
likely see a tenth of an inch or less.

Most models indicate that rain will end in all areas by Friday
morning, although the latest WRF shows a few showers lingering
across the southeast portion of our area for a few hours after
sunrise Friday. Otherwise, Friday is expected to be a partly
cloudy and relatively cool day with brisk onshore winds.

A warming trend is forecast over the upcoming weekend as an upper
ridge begins to build over California. That ridge is then expected
to amplify and strengthen along the West Coast early next week,
resulting in robust airmass warming over California. 850 mb
temperatures are forecast to climb from only about 3 deg C on
Friday to around 22 deg C by late Monday. In addition, light
offshore flow is forecast to develop by Monday and Tuesday. The
result will be warm and dry conditions across our entire region on
Monday and Tuesday, with widespread highs in the 80s to around 90.
As is often the case with offshore flow this time of year, daytime
temperatures will be just as warm at coastal locations as inland
areas. Models indicate slight cooling at the coast beginning
Wednesday, but warmth continuing inland.


.AVIATION...As of 10:56 PM PDT Wednesday...For 06z tafs. Patchy
low clouds are beginning to impact area terminals with LIFR to IFR
cigs. The marine layer remains around 1000 ft and will allow low
clouds to penetrate well inland tonight. Low confidence with
respect to clearing times Thursday morning. Moderate onshore flow
will gradually ease overnight becoming light and locally
variable. An approaching front will bring chance of rain late in
the period Thursday evening and overnight into Friday.

Vicinity of KSFO...IFR cigs will prevail through tonight. Low
confidence with respect to clearing times Thursday morning. West
winds will continue through this evening then gradually ease
becoming light and variable early Wednesday morning.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...IFR cigs will prevail through tonight. Cigs
will continue to drop becoming LIFR between 08z and 09z. Low
confidence with respect to clearing times Thursday morning. West
winds will continue through this evening then gradually ease
becoming light and variable early Wednesday morning.


.FIRE WEATHER...As of 8:45 PM PDT Wednesday...Relative humidity
values continue to increase in the valleys and along the lower
coastal slopes as light onshore flow persists and a marine layer
becomes established near the coast. However, the airmass in the
hills above 1500 feet remains relatively dry and expect one more
night of poor humidity recoveries in the higher hills. Onshore
winds will continue to gradually increase into Thursday and
moderate and locally gusty winds are expected. Models continue to
move an early season cold front through the district late
Thursday and Thursday night. Rainfall will begin in the North Bay
Thursday afternoon and spread across most of the remainder of the
district Thursday night. The models have just recently trended
slightly wetter with this system, although rainfall amounts are
still expected to be mostly light. Wetting rains are most likely
in the North Bay Mountains and perhaps 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 begin 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 by Sunday
night and especially Monday night.

&& of 9:00 PM PDT Wednesday...An approaching frontal
boundary will bring increased winds and seas to the coastal waters
beginning Thursday afternoon into Friday resulting in hazardous
sea conditions. Winds will ramp up first Thursday afternoon with
large long period swells following Thursday night into Friday. The
front will also bring a chance of rain across the coastal waters.


     .Tngt...SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm




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