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

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FXUS66 KMTR 111131

Area Forecast Discussion
National Weather Service San Francisco Bay Area
431 AM PDT Tue May 11 2021

.SYNOPSIS...A Red Flag Warning remains in effect through 6 pm
Tuesday for the North and East Bay Hills as well as the East Bay
Interior Valleys. Otherwise, expect continued sunny and warm
weather through Tuesday. A cooling trend begins Wednesday and
lasts through the end of the week and into next weekend as the
marine layer develops and also deepens, and as onshore winds


.DISCUSSION...As of 3:32 AM PDT Tuesday...Satellite fog product
shows the lingering coastal clouds from Monday evening have become
more established and returned as a shallow marine layer.
Satellite fog product shows low clouds along the entire coast
from Big Sur to the Golden Gate. Fort Ord profiler near Marina,
CA, shows a shallow layer less than 1k feet. Salinas Airport is
also showing low ceilings with patchy fog as the marine layer
pushes up the valley. Do expect coverage to increase along the
coast through sunrise. With the exception of the Salinas Valley
not expecting much of an inland push of the marine layer this
morning. One of the main reasons for this is the lingering
offshore flow. Gradients still indicate offshore flow pressure
gradients. The offshore flow is reflected across the higher
terrain as weather stations continue to show breezy conditions and
low relative humidity. Winds may have eased since their peak
readings earlier, but breezy winds and low RH are contributing to
critical fire weather conditions. Critical fire weather conditions
are expected to continue through this afternoon so no change in
existing Red Flag Warning. See Fire Weather section below for more

Otherwise, another mild to warm day is expected across the Bay
Area on Tuesday. Warmest conditions are expected across the
interior and the higher terrain.

Offshore flow will end by Wednesday and onshore flow will return
ushering in more widespread cooling and higher humidity. The
cooler weather will persist into the upcoming weekend with night
and morning clouds becoming more widespread. One weather item of
note that will be watched over the next few days will be the
possible development of an upper level low Friday night and
Saturday. Medium range models and ensembles show the development
of an upper level over the region Friday night/Saturday. The
detail that is less certain is precip. A few models do develop
some showers with the low, but confidence is too low to include
them in the forecast just yet.


.AVIATION...As of 04:31 AM PDT Tuesday...For the 12z TAFs. Low
cigs and fog stay confined to the coast and over the Monterey Bay
at this hour, but are not moving farther inland, leaving VFR
conditions elsewhere. Conditions will improve at the MRY Bay after
17Z, while VFR is expected through Tuesday elsewhere. As the
prevailing pattern weakens, winds will remain onshore and breezy
Tuesday afternoon, but less so than previous days. Models are
projecting a more established marine layer over the ocean,
allowing for more SCT low/mid level clouds to move into the SF
Bay Tuesday night, while the Monterey Bay will return to IFR or

Vicinity of KSFO...VFR with onshore winds. Clear skies through
Tuesday with max onshore winds 15 to 20 kts. Winds will stay
breezy Tuesday night as FEW low clouds move near the terminal, but
stay VFR.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Low clouds and patchy fog surround the
bay and encapsulates the terminals for LIFR/VLIFR conditions.
Expect some gradual lifting after 15Z, but the low clouds will
remain through 17Z. VFR Tuesday afternoon with breezy onshore
winds. Low clouds are set to return Tuesday night for IFR/LIFR
conditions expected.


.FIRE WEATHER...As of 4:00 PM PDT Monday...Red Flag Warning
remains in effect through 6 pm Tuesday for zones 507/510/511.
Winds in the hills subsided somewhat on Monday afternoon, but were
still gusting between 25 and 35 mph while RH values have dropped
as low as the teens. Offshore winds in the hills are forecast to
increase once again tonight, although models have backed off on
wind speeds tonight. In any case, a reinforcing burst of dry air
arriving late this afternoon and evening will result in little or
no RH recovery overnight. Very dry air, combined with moderate
offshore winds in the hills, will maintain critical fire weather
conditions into Tuesday morning. Winds will subside by mid
morning Tuesday, but warm and very dry conditions will persist
through the afternoon hours with RH values dropping as low as the
single digits.

Onshore breezes should return to all areas by Tuesday evening.
Moderating conditions are forecast into Weds in terms of cooler
temps and an uptick in humidity. A cooling trend will continue
through the end of the week with temps and humidity returning to
more seasonal values.

&& of 02:48 AM PDT Tuesday...Winds along the coast and
southern waters continue to diminish while staying breezy to
moderately gusty over the northern outer waters. Seas continue to
subside, but will remain hazardous for smaller vessels through
Tuesday afternoon. By mid-week, winds will be light with
prevailing mixed seas from light northwest and southerly swells.
Winds will increase over the weekend increasing the wind driven
waves when the next system approaches the waters.


     .Tday...Red Flag Warning...CAZ507-510-511
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 PM
             SCA...SF Bay from 3 PM




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