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

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FXUS66 KMTR 271734
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1034 AM PDT Tue Oct 27 2020

.SYNOPSIS...Critical fire weather conditions will persist in the
North Bay Mountains, East Bay Hills and the Diablo Range into
this afternoon where gusty winds persist. Elsewhere, winds will
remain light, yet dry conditions will persist through midweek.
High pressure will bring sunny, mild afternoons and cool
overnights through the remainder of the week.

&&

.DISCUSSION...as of 08:59 AM PDT Tuesday...Morning visible
imagery shows smoke from the active fires along the Sierra Crest
lingering over much of the Central Valley, while winds have
remained light across much of the San Lucia Mountains. Winds in
the East Bay hills were slightly weaker than what was observed two
nights ago, with the Mt. Diablo station reporting a max gust of
49mph (a far cry from the 72 mph gust that was observed Sunday
night). The same can also be said for the North Bay Mountains,
where the Mt. St. Helena station reported a max overnight gust of
78mph (compared to the 89mph Sunday night). It goes without saying
that these were still strong overnight winds, and some of these
strong winds did begin mixing down to lower elevations.
Thankfully, they did not become as pronounced this time around,
with the East Bay Foothills gusting around 5-10 mph lower than
what we observed two nights ago. Given that the strongest of these
winds remained predominantly over the higher-terrains of the CWA
and that moderate winds continue across many of the peaks this
morning, Red Flag Warning will be allowed to continue for the
North Bay Mountains and the East Bay Hills until its expiration
time at 5PM today.

In terms of humidities, the air mass is still rather dry across much
of the CWA, with widespread teens across the East Bay and Central
Coast, but regionwide we observed overnight substantial humidity
recoveries (keeping in mind this time yesterday our region was
experiencing widespread single-digit relative humidities all the way
down to the coast. RH values are in the 20s and 30s across much of
the North Bay, while the Half Moon Bay Station has bounced back up
to 30% this morning. The dry air mass also resulted in substantial
radiative cooling in the interior valleys of the East and North Bay,
resulting in morning temps in the low 40s across much of the valley
floors. The culmination of lower RH values and lighter winds in the
Central Coast resulted in even more overnight radiational cooling
along the peaks of the San Lucia Range and the Salinas Valley,
resulting in some locations starting their day in the 30s F.

Overall, the trend for the next couple of days is for the gradual
weakening of winds regionwide as the trough exits the Mountain West.
As it does so, the upper-level ridge off the West Coast will
gradually build and move over California, resulting in some subtle
warning across the CWA as well, with temps in the 70s to low 80s
across the region. Given the shorter days and the time of the year,
overnight lows will range from the 40s to lower 50s for most
locations while higher-terrain locations will trend a few degrees
warmer. Medium-range guidance also replaces the upper-level ridge
with a PAC shortwave trough during the second half of the week,
which should greatly aid in the reintroduction and deepening of the
marine layer. Nonetheless, RH values will likely remain rather low
across much of the interior in the coming days, and medium-to-long
range guidance is trending towards dry conditions through the end of
the month and first week of November. ECMWF also picks up on the
possibility for some offshore flow sometime around the second half
of next week. Will be monitoring but for now still several days out.
Fire season is not over yet.

&&

.AVIATION...as of 10:34 AM PDT Tuesday...For 18z tafs. VFR
conditions through the period with mostly clear skies as a dry air
mass remains in place. Some localized LLWS may remain possible at
about 20-25 kt as NNE linger on the lee side of hills or
mountains around the SF Bay Area, though it is expected to subside
later this morning. Onshore winds expected to return along the
coast late this afternoon and early this evening. Generally light
winds overnight.

Vicinity of KSFO...VFR through the period. Light winds this
morning becoming onshore around 10-15 kt late this afternoon.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. East to southeast winds this morning
to turn onshore 10-15 kt this afternoon. Locally breeze SE winds
may return down the Salinas Valley tomorrow morning after winds
diminish tonight.

&&

.FIRE WEATHER...as of 02:30 AM PDT Tuesday...A red flag warning
remains in effect for the North Bay Mountains, East Bay Hills and
the Diablo Range until 5 pm PDT Tuesday. Moderate winds with gusts
of 45-60 mph are likely in the higher elevations of the interior
North and East Bay into this afternoon before gradually
diminishing late in the day. High pressure keeps things dry all
week with warm days and cool nights. While a well established
marine layer may not develop until late week, humidity levels
should gradually improve in the region`s lower elevations.

&&

.MARINE...as of 08:08 AM PDT Tuesday...Generally light to locally
moderate northerly winds will prevail across the waters. Winds
will turn southerly along the immediate coastline tonight, though
will remain relatively light. Mixed seas will continue through the
period with a pair of northwest swell trains in addition to a
lighter southerly swell and some weak wind waves.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Red Flag Warning...CAZ507-511

&&

$$

PUBLIC FORECAST: Diaz
AVIATION: AS
MARINE: AS
FIRE WEATHER: RGass

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