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

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FXUS66 KMTR 240528

Area Forecast Discussion
National Weather Service San Francisco Bay Area
928 PM PST Thu Nov 23 2017

.SYNOPSIS...Dry and mild weather is forecast from Friday through
most of Saturday. Rain is expected to spread across the region
from late Saturday evening through Sunday. Scattered showers on
Sunday night will taper off and end by midday Monday. Dry weather
is then expected for the remainder of next week.


.DISCUSSION...As of 9:05 PM PST Thursday...Radar shows that
isolated very light showers have mostly exited the region and
shower chances for the remainder of the evening are negligible.
Considerable mid and high level cloudiness persists across the
region and the airmass remains quite warm for this time of the
year. 8 pm temperatures will still in the 60s at most locations.
Expect mild overnight lows tonight.

The strong upper level ridge that covers the southwestern U.S.
weakened slightly over northern California today, but the ridge
continues to dominate our weather by sustaining a warm airmass
over our region. This ridge is forecast to remain in place through
the early part of the weekend and maintain dry weather conditions
across our entire region through Saturday afternoon. The moisture
plume responsible for today`s clouds is forecast to dissipate
somewhat by tomorrow, so expect more sun compared to today,
although periods of high clouds will likely persist. Daytime
temperatures over the next two days will continue to be about 7 to
12 degrees warmer than normal.

A trough in the Gulf of Alaska is forecast to dig south offshore
over the next few days and merge with wet system currently
developing much farther to the south. This merged system is then
expected to impact our region with widespread rainfall later in
the weekend. The 00Z GFS depicts a narrow band of precipitable
water in excess of 1.5 inches arriving across our area by Saturday
night in advance of the incoming trough and the models have been
trending wetter with this system. As far as timing is concerned,
the model consensus indicates that rain will begin in the North
Bay by late Saturday evening, spread across most of the rest of
the SF Bay Area by mid morning Sunday, and reach the Monterey Bay
Area around midday Sunday. Rain rates with this initial frontal
rain band will be light to moderate for the most part, but could
be briefly heavy in the coastal ranges, especially across the
North Bay. It appears there will be a brief break in the action
late Sunday afternoon and early evening before an upper low moves
in from the northwest and triggers widespread shower activity on
Sunday night. Showers may linger into Monday morning but are
expected to end by midday Monday.

Yesterday the models were predicting no more than about a half
inch of rain in our area from late Saturday through Monday
morning. Latest models are forecasting as much as three times that
amount. Rainfall totals are now forecast to range from 0.5-1.5
inches in the North Bay and from 0.25" to 1.00" for most of the
rest of our forecast area, except as little as a tenth for far
southern inland areas.

Gusty southerly winds are expected with this system from late
Saturday through midday Sunday, with local wind gusts up to 35
mph possible. Gusty westerly winds will then follow on Sunday
night. Winds are expected to remain below advisory levels.

Longer range models agree that dry weather can be expected from
midday Monday through the remainder of next week.


.AVIATION...As of 9:28 PM PST Friday...For 06z Tafs. Mid and high
level clouds remain over the area. Winds have decreased and
remain W-NW across most sites. Cigs are expected to lower late
this evening and overnight across the region down to MVFR/IFR.
KSTS is already seeing LIFR cigs with occasional fog. Confidence
in timing of development and clearing of MVFR/IFR cigs is low.

Low-moderate forecast confidence.

Vicinity of KSFO...Currently VFR and should remain as such until
later tonight. Light NW winds tonight expected to be less than 10
kt. Current thinking is still MVFR CIGs rolling in around 08-09z.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR, with IFR CIGs possible around
midnight and lasting into the early morning; low confidence on
CIG timing. Light winds overnight at around 5 kt.

&& of 08:21 PM PST Thursday...Moderate to occasionally
strong northerly winds will continue  over the southern waters
through tomorrow, and possibly into  Saturday morning. By Saturday
winds will increase across the  coastal waters and turn southerly
as a low pressure systems moves  into the Pacific Northwest. This
will cause an increase in  northwest swell late Saturday night
that will then continue to  build over the weekend. Chances of
rain also return to the area  this weekend.


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




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