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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 231038

Area Forecast Discussion
National Weather Service San Francisco Bay Area
338 AM PDT Fri Aug 23 2019

.SYNOPSIS...A shallow marine layer along the coast with onshore
winds will bring seasonable temperatures to the Bay Area today.
High pressure begins to strengthen and rebuild over the state this
weekend allowing for inland areas to warm back into the 90s.
Milder conditions will persist near the coast with a shallow
marine layer. The ridge will weaken slightly by Tuesday and
Wednesday as the remnants of Tropical Storm Ivo pass west of the
Golden Gate. This may bring some increasing clouds and more
humidity to the region. 

&& of 3:38 AM PDT Friday...24 hour temps and 
humidity trends are noteworthy this morning with dewpoints running
5-10 degrees drier than yesterday at this time with readings back
down into the 50s while temps hover in the lower 60s. The moist 
airmass has been pushed south of our region per the latest tpw 
satellite scans. A more typical summertime regime is returning to 
the Bay Area and Central Coast. A shallow 500-1000 foot marine 
layer is hugging the coast from the Golden Gate southward to Big 
Sur. Cloud bases are shallow from 400-600 feet from Half Moon Bay 
to the Monterey Bay region. There is steady onshore pressure 
gradients in place with about 2 mb SFO to Sacramento allowing the 
cooler marine air to ooze inland. The 850 mb temps are around 22 
celsius this afternoon which should keep the warmest inland areas 
in the lower 90s while 70s/80s persist around the bay with 60s for
the beaches.

A broad 594 dm ridge is forecast to build on Saturday, extending
from the Eastern Pacific, across California and all the way
towards Arizona. This will bring some more inland warming with mid
and upper 90s returning to the inland valleys by Sunday and Monday
as 850 mb temps warm to around 25 celsius. The surface onshore
gradients will remain fairly light with the thermal trough trying
to hug the coastline night and morning hours. Given the strength
of the ridge dont expect the marine layer to deepen too much
through the weekend and into early next week. There will likely be
bouts of morning dense fog near the coast and coastal slopes that
the marine layer covers as it gets squashed by the warm upper
ridge. Locations in the hills above 1500 feet will likely see
little night time variation with warm and dry weather persisting
day and night right through early next week.

In general the pattern the next few days looks to be fairly
typical but inland highs look to be about 4-8 degrees warmer than
normal due to the strong ridge and expected lighter afternoon

Perhaps the most interesting forecast item to watch will be as the
remnants of Tropical Storm Ivo shift northward by early next week.
The models have been fairly consistent in bringing the remnant
upper level disturbance west of the Golden Gate by about next
Weds. We should see some increasing high clouds as early as
Tuesday night and feel some higher dewpoint air getting ushered in
under a gentle southerly flow.

Conceptually the pattern screams watch out as the tropical
remnants push north and bump up against the previously mentioned
stout upper ridge. As this occurs a mid-latitude trough will be
crossing the East Pac and approaching from the Northwest. This 
will induce a negatively tilted upper trough. For days now the 
models have been consistent in not really showing much moisture or
instability over our region. This should be viewed with some 
skepticism. At this time its clear that the areas from Mendocino 
northward up towards Mt Shasta and Modoc as well as much of Oregon
look to see much better chances for t-storms as diffluent flow 
aloft will develop. The latest 06z gfs implies a large t-storm 
outbreak later Weds night up near the Oregon border. Will need to 
watch all these trends closely in the coming days.


.AVIATION...As of 11:14 PM PDT Thursday...VFR except VLIFR-IFR 
along the coast, extending eastward over the San Francisco Bay,
and also moving into nearby coastal valleys tonight and Friday 
morning. On Friday diurnal mixing should lift ceilings to VFR at 
most, mainly inland, terminals, however in terms of areal coverage
of coastal stratus and fog it's a very low confidence forecast for
Friday into Saturday morning; a 4 mb SMX-SFO pressure gradient 
and wind will be juxtaposed to a diminishing 3 mb or less ACV-SFO 
pressure gradient and wind resulting in any combination of coverage.

Vicinity of KSFO...VFR, west wind has diminished. Seeing signs that
the aforementioned SMX-SFO pressure gradient is getting a foothold
(usually the NAM is slow with this forecast). Under a compressed 
marine layer this combination should keep VFR going for the period.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VLIFR-IFR due to increasing stratus and 
fog coverage extending into the Salinas Valley. Low ceilings and 
visibilities lift to VFR by late Friday morning, then low confidence
stratus and fog return Friday evening and night.

&& of 03:10 AM PDT Friday...Northwest winds will
decrease across the area today but remain breezy over the northern
outer waters. Steep northwesterly swell at 8 to 10 seconds will
decrease today through the weekend with period becoming 7 to 8
seconds over the weekend. A coastally  trapped southerly wind
reversal will lead to light southwesterly  winds over the waters
this weekend. A moderate southerly swell  will arrive on Sunday
through early next week with light  northwesterly swell continuing
through the period.


     .Tday...SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm until 9 AM
             SCA...SF Bay from 9 AM




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