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

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FXUS66 KMTR 221734
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1034 AM PDT Fri Jun 22 2018

.SYNOPSIS...A robust warming trend is forecast through Saturday
afternoon as high pressure builds over the region. This will lead to
hot conditions on Saturday with moderate to locally high heat risks
across mainly the interior. Additionally, offshore winds will
develop and result in heightened fire weather concerns to portions
of the region on Saturday. Cooling is then expected on Sunday with
increased onshore flow.

&&

.DISCUSSION...as of 9:30 AM PDT Friday...Visible satellite
reveals areas of low clouds along the immediate Pacific coast from
San Francisco south through the Monterey County coast. The marine
layer remains quite shallow as high pressure builds over the
region. Temperatures are already beginning to heat up as most
airports are running about 5 to 7 degrees warmer than this time 24
hours ago. One significant change that was made this morning was
the upgrade of the Fire Weather Watch to a Red Flag Warning across
the North Bay Mountains and East Bay Hills, valid from 11 am
Saturday through 8 pm Sunday; more details with fire weather can
be found in the Fire Weather section of the discussion. For
additional details with tomorrow`s heat event, please refer to the
previous discussion section.

&&

.PREV DISCUSSION...as of 04:23 AM PDT Friday...A marine layer of
around 1,000 feet in depth remains in place this morning. This has
resulted in low clouds and patchy fog from the San Francisco
Coast down along the Big Sur Coast and into the Salinas Valley. In
addition, some low clouds can be seen developing over Marin
County at this hour. Meanwhile, temperatures are generally in the
50s region-wide. These conditions will likely improve through the
morning as low clouds retreat to the coast in response to building
high pressure aloft. A warming trend will also continue today,
especially across the interior and the marine presence becomes
compressed. Look fore widespread 90s across the interior this
afternoon with 80s around the San Fransisco Shoreline and upper
60s to 70s near the coast.

The warmest day of the week is forecast to occur on Saturday as
offshore flow develops over the North Bay and East Bay
Hills/Mountains. Widespread 90s are likely with many of the warmest
inland areas reaching to between 100 and 105 deg F. Given the hot
daytime temperatures and mild overnight conditions in response to
the offshore flow, the risk for heat related illnesses will increase
tonight into Saturday. Thus, a Heat Advisory has been issued for
inland areas as well as in the hills/mountains on Saturday. Coastal
areas will also warm to above seasonal averages, however weak
onshore flow should limit overall heat risks.

Breezy to locally gusty offshore winds are forecast to be strongest
in the North Bay and East Bay Hills/Mountains on Saturday. Northerly
winds of 15 to 30 mph with gusts up to or exceeding 40 mph will be
possible across these areas and will combine with humidity values in
the upper single digits to middle teens to create increased fire
weather concerns. As a result, a Fire Weather Watch remains in
effect from Saturday into Sunday for the aforementioned areas. Be
mindful of these fire weather conditions and avoid any activities
that may lead to fire starts, such as lawn or automotive work,
driving motorized vehicles on dry grass, lighting campfires or
fireworks, or discarding hot or burning objects such as lighter
coils or cigarettes.

Onshore flow is then forecast to return on Sunday as a southerly
surge develops first along the Big Sur Coast and spreads northward
through the afternoon and evening. This should cool conditions near
the coast and locally inland as onshore winds increase. With this
said, inland areas will likely remain warm to hot through Sunday
afternoon before the marine airmass advects further inland. Further
cooling is then likely region-wide on Monday with temperatures
dropping to near seasonal averages. Seasonably conditions are likely
next week with the presence of a more established marine layer and
onshore flow.

&&

.AVIATION...As of 10:34 AM PDT Friday...For 18Z TAFs. VFR is
expected through the day and overnight most locations. IFR
cigs/vsbys expected by dawn Saturday vicinity of the Monterey Bay.
Light to moderate west/northwest winds this afternoon.

Vicinity of KSFO...VFR expected through the forecast period.
Breezy onshore winds this afternoon, sustained around 20 kt with
higher gusts possible.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR expected to dominate through the
rest of the forecast period, but there could be a couple of hours
of IFR/LIFR cigs/vsby near the Monterey Bay (MRY and SNS) around
sunrise Saturday. Generally light onshore winds this afternoon,
around 10 kt.

&&

.FIRE WEATHER...as of 9:30 AM PDT Friday...Fire Weather Watch has
been upgraded to a Red Flag Warning for the North and East Bay
hills from 11 am Saturday through 8 pm Sunday evening. The main
concern is hot temperatures and low humidity coupled with some
moderate north/dry winds. To be clear the weather set-up is very
different and not nearly as critical in terms of winds to the
north bay firestorm last October.

Hot temps will develop on Friday and continue to dry out fuels
that are currently running near average in terms of seasonal
dryness. After a mild and dry night Friday another hot day is in
store Saturday with lots of 90s and even some triple digit heat
forecast. As this occurs some dry north winds will come down the
Sac valley and first spread over Napa county Saturday morning.
Wind gusts should be in the 35-45 mph range. Winds will ease
Saturday afternoon and then another round of North winds will
spread over the north and east bay hills Saturday night. This will
coincide with little or no rh recovery and very warm overnight
temps in the 70s and 80s. Warm to hot weather will continue into
Sunday afternoon but onshore flow will return by late in the day.

Those spending time outside this weekend are urged to be vigilant
with sources of ignition. Any new fire starts, especially in the
grassy fuels will spread rapidly.

&&

.MARINE...as of 8:29 AM PDT Friday...A strong area of high
pressure over the Eastern Pacific will maintain moderate to strong
northerly winds across the coastal waters through late Saturday
night. The strongest winds will be located over the northern outer
waters, north of Pigeon Point. These sustained strong winds will
produce relatively large and steep winds waves on top of a
building northwesterly swell with moderate periods. Winds will
decrease Sunday as a southerly surge moves up the coast, but
remain breezy along the inner coastal waters.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pigeon Pt to Pt Pinos 0-10 nm from 12 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm from 4 PM

&&

$$

PUBLIC FORECAST: Rowe/RGass
AVIATION: Sims
MARINE: Sims

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