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

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544
FXUS66 KMTR 210544
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1044 PM PDT Tue Aug 20 2019

.SYNOPSIS...A warming trend is forecast through Thursday as high
pressure builds over Central and Southern California. Inland
areas will see less in the way of night and morning low clouds as
the marine layer compresses. Seasonable temperatures are forecast
through the upcoming weekend, along with continued dry conditions.

&&

.DISCUSSION...as of 9:53 PM PDT Tuesday...No forecast updates
planned at the moment though current cloud coverage is currently
slightly less than originally anticipated. Synoptic pattern
features a fairly strong early season front approaching the Oregon
coast overnight while the Four-Corners ridge is dominating the
pattern to our south. The Bay Area is stuck in between. Currently
there is only a 3 mb northerly gradient while the sfo to
Sacramento onshore is about 2.5 mb. Some low clouds are already
starting to bank up across the East Bay Hills as well as along the
San Mateo coast and down around Monterey Bay. In addition some
high clouds continue to pass to our north.

Its a little odd that at 500 mb we will be going into a
trough/cyclonic flow over the next day or so (due to the front
well to our north). However near the surface we will see warming
as the 590 dm contour stays over the region and the southern
ridge remains stubborn. The boundary layer winds will turn more
northerly on Weds which will induce some adiabatic warming. Model
guidance suggests many of the North and East Bay valleys will warm
into the upper 80s and lower 90s on Weds which will be as much as
10 degrees warmer than what was observed on Tuesday.

One item to watch on Weds night is some potentially gusty
northerly winds in the hills. 00z NAM has 20-30 kt winds at 925 mb
(around 2500 feet) from Sonoma to Monterey late Weds afternoon
into Weds night. Not expecting any wind advisories but could be a
brief fire weather concern.

Pattern looks to continue with above normal/warming trend for
inland areas Thursday and Friday as the ridge builds over the
region with potential for the thermal trough to straddle the coast
at times keeping marine stratus to a minimum. Long range trends
remain warm and dry. Will be watching the tropical activity as it
heads north from Baja next week. Current ECMWF would bring
remnants of our coast by about next Thursday...a long ways off.

&&

.PREV DISCUSSION...as of 02:30 PM PDT Tuesday... California is
positioned between two synoptic scale features early this
afternoon -- an advancing trough to the northwest and a
retrograding ridge to the southeast. Both of these features have
played a role in driving the local weather today with the
advancing trough encouraging a deeper marine layer overnight into
this morning and the retrograding ridge suppressing the marine
layer and forcing warmer temperatures and the mostly skies early
this afternoon. While the ridge has taken over as the dominate
feature early this afternoon, the advancing trough will wrest
control from the ridge, at least across the North Bay, through the
remainder of the day as cooler, moister air mass advects inland from
the west. Latest satellite imagery shows scattered to broken high
clouds on the leading edge of this air mass boundary now shifting
inland across the Mendocino coast and northward at this hour. Expect
to see some of these clouds spread into the North Bay and parts of
the immediate Bay Area later this afternoon into the evening but
will probably arrive too late to drastically impact todays afternoon
high temperatures.

Forecast models have struggled with the specifics of what happens
next but generally agree that the ridge will deflect the advancing
trough northward into the PacNW/extreme NorCal after attempting to
invade the Golden State.

For tonight, the marine layer will become suppressed by the
retrograding ridge across the Central Coast and parts of the Bay
Area but may instead mix out across the Sonoma/Marin coastlines due
to the presence of the cooler air aloft in the advancing trough. As
such, would not be surprised to see patchy fog and periods of
drizzle along the coastline from the San Mateo coastline southward
through Big Sur with fractured cloud decks across the North Bay.

The long range forecast models then depict a steady rise in
atmospheric heights over the 7 plus days however that rise will
not necessarily dictate a steady rise in temperatures.

For tomorrow, the ridge will become the dominate player after the
trough has deflected to the north into the Pacific Northwest and
promote the warmest temperatures of this work week for most areas.
That said, the North Bay high temperatures may somewhat lag behind
the rest of the forecast area tomorrow after a secondary vort
pulse within the broader parent trough over the PacNW forces it to
briefly dip back into Northern California.

Temperatures tomorrow will range from near normal along the
shorelines to up to 10 degrees above normal for the inland areas.
Afternoon highs of 60s to low 70s are forecast along the coast,
upper 70s to upper 80s for areas in some proximity to a
coast/shoreline, and into the low to mid 90s for the extreme
inland areas far from the coast. The marine layer should further
compress tomorrow night into Thursday morning which will allow
Thursday morning`s temperatures to slightly outpace Wednesday`s.
By the afternoon, Thursday temperatures will come into line with
Wednesdays readings though the North/East Bay should run slightly
warmer than the previous day given the absence of a trough on
their northern periphery.

500mb heights continue to increase into Friday as the ridge shifts
offshore and amplifies. Despite this, temperatures are forecast
to stall or even drop primarily due to a shift in wind direction
to the southwest. This shift in wind direction should increase
the onshore advection of cool, moist air to inland regions despite
the warmer air subsiding from aloft. A warming trend is then
expected through the weekend and into early next week with the
hottest temperatures anticipated to arrive early to mid next
week.

The one potential gotcha in the long range forecast is the
presence of a potential tropical disturbance (80-90% chance of
organization) currently off the southern coast of Mexico. GFS/EURO
ensemble members give this system a roughly 33-40% chance of
moving into the broader vicinity of the SF/Monterey Bay areas by
early next week, potentially introducing increased a wetter, more
unstable air mass underneath a broader high pressure ridge.
Needless to say, there remains a fair amount of uncertainty in the
forecast over the next week.

&&

.AVIATION...As of 10:45 PM PDT Tuesday...Ft Ord Profiler shows a
marine layer developing at around 1000 feet as the airmass aloft
warms. This is in response to an upper level ridge over the
southwest expanding westward into California. Stratus has been
slow to develop tonight as a light northerly flow has developed
over the area. There is a small area inside the bay which could
spread out into OAK and SFO. Still expecting stratus to reform
along the coast later tonight with the marine layer but confidence
is not high as the northerly flow could keep clouds reduced. The
northerly flow will increase on Thursday so there is less of a
chance for stratus Wednesday night.

Vicinity of KSFO...MVFR cigs after 12Z with bases around 1500 ft
possibly as low as 1000 ft. Confidence is not high as it could
stay VFR. Any cigs will clear out by 17Z.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...MVFR/IFR cigs after 12Z clearing after
17Z.


&&

.MARINE...as of 10:31 PM PDT Tuesday...Light to moderate northwest
winds will persist across the coastal waters through mid-week as
high pressure sits over the eastern Pacific and a surface low
moves into western Canada. Locally  gusty coastal jets are
forecast south of Point Arena, Pigeon  Point, and Point Sur that
will create hazardous seas conditions  for smaller vessels. Breezy
afternoon and evening winds are also  forecast over the San
Francisco Bay around the Golden Gate and  through the Delta as
well as over the Monterey Bay. Mixed seas  will continue with
shorter period northwest waves at around 8 to  10 seconds, a light
longer period west swell, and a light  southerly swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm

&&

$$

PUBLIC FORECAST: RWW
AVIATION: W Pi
MARINE: W Pi

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