NWS Forecast Area Discussion - San Francisco Area

FXUS66 KMTR 101733

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1033 AM PDT Mon Aug 10 2020

.SYNOPSIS...A slight cooling trend is forecast to occur today and
Tuesday, especially across the interior. Meanwhile, little change
is anticipated near the coast with the typical ebb and flow of
low clouds each night/morning. Warming is likely late in the week
and into next weekend.

&& of 08:26 AM PDT Monday...A very `summer-like`
satellite image is visible this morning with widespread clouds
over the coastal waters and over the inland valleys. Profilers put
the depth at around 1,000 feet with some patchy fog being
reported for Monterey/Salinas. Above the marine layer temperatures
remain mild, but fortunately from a fire weather standpoint
humidity levels are in the moderate range with a few locally poor.

No update needed this morning. Once the clouds roll back to the
coast temperatures will warm up, but be slightly cooler than
Sunday. Nonetheless it should still be a pleasant August

Weather concerns for the upcoming week will be upper level
tropical moisture late in the work week and possible instability
and extent of warm up next weekend.


.PREV DISCUSSION...As of 2:30 AM PDT Monday...A broad upper ridge,
extending from the Eastern Pacific eastward across the southern
CONUS continues to compress the marine layer and generate
seasonably warm temperatures across the interior. Meanwhile,
persistent light southerly low level flow has kept marine layer
stratus banked up against our coast, and thus kept daytime
temperatures in most areas near the ocean in the 60s and lower

An upper trough, currently over the Gulf of Alaska, is forecast to
shift southeast and over the Pac NW during the next few days,
which will result in slight weakening of the upper ridge over
California and thus a deepening of the marine layer. In addition,
the latest local WRF forecasts increased low level onshore flow by
late today, and continuing through Tuesday. These developments
will result in cooling across inland areas over the next two days,
with high temperatures forecast to cool about 3 to 5 degrees each
day. Coastal areas likely won`t experience much change and will
continue to see widespread night and morning low clouds with only
partial afternoon clearing.

During the second half of the week, and into the upcoming weekend,
models generally agree that an upper ridge centered over the
Desert Southwest will expand westward and across Central and
Southern California, generating warming primarily for our inland
areas. However, the magnitude of warming during the extended
forecast period is somewhat uncertain since the ECMWF builds the
ridge more strongly over California compared to the GFS, and thus
produces more robust warming. The middle ground between these two
models, where the NBM forecast temperatures generally reside, is
probably the best forecast option.

Tropical Storm Elida, off the west coast of Mexico, is forecast
to become a hurricane later today. Some of the moisture from this
tropical system is forecast to be drawn northward around the
western periphery of the Desert Southwest High, and eventually
make its way across southern and central California by late in the
week. Models do not currently generate precipitation over
California as this moisture arrives, and the most we can expect to
see are some mid and high level clouds across the southern part
of our forecast area. But we will need to watch for any potential
mid-level instability which could trigger isolated high-based
convection and dry lightning.

&& of 10:30 AM PDT Monday...for 18Z TAFs. VRF
conditions have returned to the terminals as of 1730Z in response
to dissipating stratus. Look for VFR conditions to prevail through
the afternoon with a typical increase in onshore winds. Low clouds
likely to return inland slightly earlier this evening for
terminals closer to the coast while surface wind speeds also
diminish. A deeper marine layer will also likely bring lowering
ceilings to inland terminals by early Tuesday morning.

Vicinity of KSFO...VFR. W/NW winds will increase to around 12-15
KT around 20Z and persist through the early evening before
diminishing slightly. MVFR/IFR ceilings likely to develop beyond
10Z Tuesday and persist through about 18Z Tuesday.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. An increase in NW winds to 9-12 KT
is likely later this morning and will continue through the
afternoon with low clouds lingering near the coast. An early
return of stratus and diminishing winds is likely around 03Z with
IFR to LIFR ceilings persisting through Tuesday morning.

&& of 10:30 AM PDT Monday...A weak upper level low
pressure system just west of San Francisco and surface high
pressure to the north of this feature will bring light southerly
winds through Tuesday. Winds will switch to the west to northwest
Tuesday night through Friday but will remain generally light.
Mixed seas will continue through the period with a short period
northwest swell and a longer period southerly swell. Southerly
swell will increase Tuesday and Wednesday.


     .Tday...SCA...SF Bay from 11 AM




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NWS MTR Office Area Forecast Discussion