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

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FXUS66 KMTR 200544

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1044 PM PDT Tue Sep 19 2017

.SYNOPSIS...An upper level low will dominate the western US
through the work week. There is a slight chance of rain in the
far North Bay late tonight and Wednesday morning as a frontal
system moves through. Otherwise look for continued cooler than
average temperatures through Friday, along with breezy periods. A
slow warming trend is expected to begin over the weekend and
continue into early next week.


.DISCUSSION...As of 9:10 PM PDT Tuesday...Locally breezy
conditions developed once again this afternoon and continued into
the evening hours. Strongest winds were observed near the coast
and in the hills where wind gusts were locally as high as 35 mph.
Otherwise, our region enjoyed a mostly sunny day with mild
temperatures. High temperatures were generally within a few
degrees of normal.

An upper trough continues to dominate the western United States
this evening. Water vapor satellite imagery currently shows a
shortwave trough off the coast of Vancouver Island moving to the
southeast. A surface boundary out ahead of this trough is
currently pushing into far northwest California. The models have
been consistent in producing light rain with this boundary as far
south as northern Sonoma County late tonight and Wednesday
morning. Precipitation amounts are expected to be less than a
tenth of an inch and dry conditions should develop in all areas by
midday Wednesday.

After the boundary moves through our area tomorrow, west to
northwest winds will pick up once again and locally breezy
conditions can be expected by afternoon. A slightly cooler
airmass will arrive over our area tomorrow, so look for high
temperatures to about 5 degrees cooler than today.

A final shortwave trough is forecast to drop south into northern
California late Wednesday night into Thursday morning. No
precipitation is expected anywhere in our area with this
disturbance. However, it will usher in the coolest airmass we`ve
seen since last Spring, with 850 mb temperatures forecast to drop
below 6 degrees C by Thursday morning. This will result in a few
cool nights beginning Thursday night and continuing into the
weekend. Overnight lows are forecast to drop into the 40s in some
of our inland valleys by late this week and some of the wind-
sheltered valleys may even see upper 30s. Daytime highs, however,
will begin to warm by Saturday as the upper trough over the
western U.S. finally starts moving to the east. The warming trend
will then continue into early next week as an upper ridge over the
eastern Pacific builds over California. We may also see light
offshore flow develop early next week which could mean highs in
the 80s close to the coast by Monday or Tuesday.

From Previous Discussion...The first good long-period northwest
swell of the season will move through the coastal waters over the
next few days. This will bring coastal hazards to the northwest-
facing beaches with an increased likelihood of rip currents, large
shore breaks, and sneaker waves. As a result, a Beach Hazards
Statement will remain into effect through Thursday evening. If
your plans take you to the beach this week, please exercise beach
safety and never turn your back to the ocean.


.AVIATION...As of 10:44 PM PDT Tuesday...For 06z Tafs. Clear
skies prevail in the post-frontal/mixed boundary layer environment
tonight. High clouds are anticipated to increase Wednesday as an
upper level trough dig south over Northern California. Confidence
is low that MVFR/IFR cig may possible develop early Wednesday
morning. Have kept a TEMPO group from 12z-17z. Light winds will
prevail through tonight then increase Wednesday afternoon as the
trough swings through.

Low confidence with regard to cigs returning overnight.

Vicinity of KSFO...VFR conditions will prevail through tonight.
Broken MVFR cig possible between 13z-17z early Wednesday morning.
Low confidence. Sustained west winds of 15 kt will ease to around
10 kt overnight.

SFO Bridge Approach...similar to KSFO.

Monterey Bay Terminals...VFR conditions will prevail through tonight.
Broken IFR cig possible between 12z-16z early Wednesday morning.
Low confidence. Light winds.

&& of 9:00 PM PDT Tuesday...Moderate to locally gusty
northwesterly winds will persist across the coastal waters through
tonight. Winds will ease a bit Wednesday morning before
increasing once again Wednesday night. A long period northwest
swell arrived earlier today and will persist through midweek.


     .Tngt...SCA...SF Bay until 3 AM
             SCA...Pt Arena to Pt Reyes 0-10 nm until 3 AM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 AM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 3 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 AM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Mry Bay




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