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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 180404

Area Forecast Discussion
National Weather Service San Francisco Bay Area
904 PM PDT Mon Jun 17 2019

.SYNOPSIS...High pressure is strengthening over California,
bringing warmer temperatures and a compressing marine layer. 
Onshore winds will continue, keeping temperatures cool near the
coast and limiting 80s and 90s to inland locations. Seasonable 
temperatures, along with areas of night and morning low clouds, 
are forecast through the end of the week and into next weekend.

&& of 09:04 PM PDT Monday...High pressure centered 
around 37N 140W is slowly expanding eastward as an upper trough 
over southern California slowly drifts south and weakens. High 
pressure building overhead brought warmer temperatures to much of 
the region this afternoon, but onshore flow kept temperatures cool
near the coast with warmer temperatures inland. Highs ranged from
the upper 50s to the mid 60s at the coast, the upper 60s to 70s 
around SF Bay, and the 80s to lower 90s in the far inland hills 
and valleys away from any marine influence. These temperatures are
fairly close to normal for this time of year. Low clouds peeled 
back this afternoon to the coast with mostly sunny skies 
elsewhere. High clouds have streamed into the forecast area 
downstream of convection over the Sierra, but all thunderstorms 
are forecast to remain well to our east. 

The marine layer is forecast to compress tonight as high pressure
strengthens over the region, which combined with a strong north-
south gradient should reduce the inland extent of stratus
overnight in places such as the North and East Bay as well as the
Santa Clara Valley. Will likely continue to see patchy drizzle and
fog near the coast. The ridge will strengthen on Tuesday with
warming temperatures aloft and slightly weaker onshore flow. As a
result, high temperatures are expected to warm another couple of 
degrees. Onshore flow will continue, keeping temperatures near the
coast relatively cool (60s and 70s) with hotter temperatures 
inland (80s and 90s). Warm temperatures aloft will keep overnight 
lows in the hills mild, with 60s and 70s common while coastal and 
valley locations dip into the 50s. The forecast is on track this 
evening, and no updates are anticipated. For additional details, 
see the previous discussion. 


.PREV of 01:35 PM PDT Monday...Slightly compressed 
marine layer, building high pressure and increasing northerly flow
all helped for an earlier burn off of the stratus deck this 
morning. Even coastal section are seeing more sunshine today with 
a more disorganized cloud deck hugging the coast today. The 
increased sunshine and warmer airmass also translated to warmer 
temperatures today. The latest 24 hour trend has many locations 
running a few degrees warmer than yesterday and interior locations
as much as 10 degrees warmer. 

No major changes to the overall forecast. High pressure will
continue to build over the Bay Area keep a warming trend through
Tuesday then moderating temperatures Wednesday. Night and morning
low clouds and even patchy drizzle will continue. The coverage
will clouds will decrease however with the building 500mb heights
and increasing N-S gradient. Scaled back on clouds tonight across
the North Bay Valleys and inside SF Bay. San Mateo Coast southward
into the Salinas Valley will see good coverage. Not only will
there be warm temperatures during the day overnight lows in the
hills will remain mild. Given the airmass will still be in the mid
20C range at 850 mb overnight lows in the 60s and 70s will be easy
to get in the hills.

By Wednesday and more so on Thursday a trough in the Gulf of 
Alaska will pivot through the PacNW. This passing trough to the 
north will flatten the overhead ridge bringing much cooler weather
to the Bay Area. In fact, temperatures on Thursday will actually 
be at or below normal for many locations. A reverse in the 
temperature trend occurs again Friday and into the weekend the 
trough exits into the Great Basin. Not a real big warm up just 
returning closer to normal. 

&& of 4:52 PM PDT Monday...For 00z TAFs. A
strengthening northerly gradient coupled with a compressing marine
layer will keep low clouds from penetrating very far inland
tonight. As a result VFR conditions are expected at all Bay Area  
terminals through late this evening with the exception of KMRY 
and KSNS that will possibly see low cigs develop this evening. 
However confidence is low to moderate. Moderate onshore flow will
ease this evening becoming light and locally variable overnight. 

Vicinity of KSFO...VFR conditions through late tonight. MVFR cigs
expected to return around 13z Tuesday morning. Clearing is
anticipated around 17z Tuesday morning. Moderate onshore winds 
around 15 kt will ease this evening becoming light overnight. A 
short period of offshore flow is expected between 15z- 21z Tuesday
over KSFO.

SFO Bridge Approach...Approach will be clear through tonight.
Impacts to approach should be short lived Tuesday morning with
light offshore flow aloft and shallow marine layer.

Monterey Bay Terminals...VFR conditions will continue through this
evening followed by the return of a shallow marine layer between
03z-05z tonight. Clearing is anticipated around 19z Tuesday. 
Moderate onshore winds around 10-13 kt will ease this evening 
becoming light and locally variable overnight.

&& of 07:57 PM PDT Monday...Gale warnings have been
posted for the northern outer waters through late Tuesday night as
strong pressure gradients are setting up near Cape Mendocino that
will keep northerly winds in place with fresh seas. As the thermal
trough sets up near the coast light southerly winds and seas will
persist across the near shore waters with generally light winds in
the bays except for locally gusty afternoon seabreeze winds.


     .Tngt...GLW...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm until 3 AM




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