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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 202042

Area Forecast Discussion
National Weather Service San Francisco Bay Area
142 PM PDT Sun May 20 2018

.SYNOPSIS...Upper low over California will keep temperatures below
normal at least through Tuesday. Weak ridge builds around midweek
with temperatures near normal before another trough approaches
Friday through Sunday with onshore winds keeping temperatures on
the cool side of normal while mainly dry conditions are forecast.

&& of 1:42 PM PDT Sunday...Below normal 
temperatures and breezy onshore winds continue to be the main 
forecast trend. So far this afternoon temps generally running from
the upper 50s beaches, 60s around the bay and 70s well inland. 
Cloud decks are around 2500 feet deep along the coast due to an 
upper low over the region. The Ben Lomond RAWS up in the Santa 
Cruz mountains is sitting in the cloud layer and so far reporting 
a high of only 49 degrees so far this afternoon. Onshore gradient 
of 2.5 mb from sfo to sac is pushing some gusty winds from the 
Golden Gate out into the Delta with SFO gusting to 33 mph last 

Looking forward the last few nam runs have generated some coastal
drizzle tonight with cyclonic flow aloft and moist onshore flow
lifting the boundary layer. Coastal hills from San Mateo southward
to Monterey that are exposed to nw winds off the ocean is where
the modeling is showing some measurable drizzle type qpf that
should it develop will be localized.

Main forecast theme to start the work week will be continued below
normal temps and breezy onshore winds. The North Bay may see the
most sunshine on Monday with drier north/northeast flow around 
the upper low. MOS data showing 79 degrees for Santa Rosa on
Monday while most cities around the Bay will see 60s once again
with some lower 70s for the Santa Clara Valley.

The upper low slowly ejects inland by Tuesday but a deep marine
layer and onshore flow will likely remain in its wake with
continued trend of below normal temps and onshore breezes. 

Some weak ridging or zonal type flow develops Weds afternoon into
Thursday sending temps back to near normal.

Longwave trough approaches by Friday afternoon with onshore winds
and approaching upper low sending inland temps back below normal
once again for Saturday and Sunday of the upcoming Memorial Day
weekend. Latest ecmwf does indicate system may be progressive 
with warmer high pressure by Memorial Day Monday but that scenario
may be dubious with developing tropical system in the Gulf of 
Mexico potentially slowing the large scale west to east flow 
across the US.


.AVIATION...As of 10:35 AM PDT Sunday...For 18z TAFs. MVFR 
ceilings continue to impact the San Francisco Bay and Monterey Bay
terminals this morning. Should see some partial clearing this 
afternoon for the San Francisco Bay terminals with low clouds 
likely to remain in place over the Monterey Bay terminals though 
much of the day. Onshore winds will also increase and become gusty
at times through the afternoon and evening hours. Low clouds make
an early return once again given the deep marine layer in place 
with winds diminishing slightly into Monday morning. 

Moderate forecast confidence. 

Vicinity of KSFO...MVFR ceilings through at least 20-21z, with 
the possibility of only partial clearing this afternoon. Westerly 
breezy/gusty winds will continue through the day and evening. Low 
MVFR ceilings, if they do scatter out, likely to make early return
late this evening.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...MVFR ceilings likely to persist over the
Monterey Bay terminals through the forecast period. Slight chance
of brief clearing at KSNS this afternoon, but confidence is low. 
Ceilings may lower to IFR tonight and into early Monday morning. 
Breezy onshore winds, occasionally gusty this afternoon and 

&& of 01:17 PM PDT Sunday...A strong north to south
pressure gradient along the California coast will maintain
moderate to strong northwesterly winds over  the coastal waters
through Monday. As a result, steep wind waves  will produce
hazardous conditions for small crafts. A long period  southerly
swell will increase and mix with northwesterly swell by  Monday
morning, gradually increasing through much of the week.


     .Tngt...SCA...SF Bay until 9 PM
             SCA...Mry Bay until 9 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm




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