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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 180543

Area Forecast Discussion
National Weather Service San Francisco Bay Area
943 PM PST Fri Nov 17 2017

.SYNOPSIS...Dry and seasonably cool weather can be expected
through the weekend. The next system will bring rain chances late
Sunday night through Monday, mainly across the North Bay. 
Additional rainfall is possible at times in the North Bay for 
much of next week, but most of our area will likely see dry and 
warm weather.


.DISCUSSION...As of 9:00 PM PST Friday...A low amplitude upper 
level ridge, currently centered offshore along 130W, is forecast 
to shift over California by tomorrow. This ridge will maintain dry
weather conditions across our entire forecast area through the 
weekend. High temperatures today were mostly in the 60s. Expect 
more of the same through the weekend with a few lower 70s possible
in Monterey County. Mostly clear skies will mean cool nights
through the weekend, with inland valley locations expected to see
overnight lows in the 30s. May need to consider a frost advisory 
in the North Bay Valleys for Saturday night/early Sunday morning. 

An upper trough currently in the Gulf of Alaska is forecast to
move slowly to the southeast over the next few days, reaching the
Pacific Northwest by late Sunday. Meanwhile, southwesterly flow 
across the eastern Pacific will direct a moist airmass to the West
Coast by late in the weekend. However, the trough's southward 
advancement will be stymied by an upper ridge west of Baja 
building slowly northward over southern and central California. 
What this means for our area is that although precipitable water 
values will increase significantly by late Sunday night and 
Monday, most dynamics will be focused well to our north. The 
00Z NAM and 00Z GFS forecast only light to moderate warm 
advection rainfall across the North Bay on Monday, with perhaps 
spotty light precip as far south as San Mateo and Alameda 
Counties. The 12Z ECMWF forecasts more widespread and heavier 
rainfall with the Monday system, but wouldn't be surprised if the
00Z ECMWF trends drier too. 

During the first half of next week the synoptic pattern is
forecast to become more amplified with a trough deepening well
offshore and an upper ridge strengthening over the southwestern 
US. What this likely will mean is that nearly all precipitation 
after Monday will be focused across the Pacific Northwest and far 
northern California, while our area remains mostly dry. The ECMWF,
which as recently as yesterday had been forecasting a wet and 
windy storm across our area late on Thanksgiving, now shows almost
no rainfall in our area that day. The general model consensus for
next week is for periodic rain chances across the far northern 
part of our forecast area, but dry and warm weather elsewhere with
widespread high temperatures in the 70s. 

A forecast update late this afternoon included reducing rain
chances across the southern portion of our forecast area from 
Sunday night through the middle of next week. If the 00Z ECMWF 
trends drier like the other models, there may be good reason to 
reduce rain chances even further.


.AVIATION...As of 9:42 PM PST Friday...For 06z Tafs. Only major 
change from 00z TAF package is the removal of FG at KSTS 
tonight/overnight. However, if temperatures cool down enough, 
patchy dense FG is still possible at KSTS before sunrise. 
Otherwise and elsewhere, VFR will prevail overnight and through 
the TAF period. Generally light and variable winds through 
tomorrow morning.

Vicinity of KSFO...VFR. Light and variable winds overnight,
becoming WNW tomorrow afternoon and staying below 10 kt.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. E/SE winds around 5 kt overnight.


.MARINE...as of 09:26 PM PST Friday...Light to locally moderate
northerly winds will continue tonight, decreasing through the
weekend. Winds turn southerly and increase late Sunday night into
early Monday ahead of a weak frontal  boundary. Seas light to
moderate, easing through the coming days.





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