FXUS66 KMTR 141847

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1147 AM PDT Fri May 14 2021

.SYNOPSIS...A cooling trend will continue to expand inland today
and there will be an increase in morning low clouds and patchy
fog. Patchy drizzle is likely tonight into Saturday morning. Below
normal temperatures are forecast this weekend and temperatures
will likely remain near to slightly below seasonal averages well
into next week.


.DISCUSSION...as of 08:50 AM PDT Friday...Stratus has a strong
hold over our CWA and will be slow to retreat this morning with a
deep marine layer (~2000 ft) in place. GOES-Satellite imagery
shows thick stratus over the ocean and coast, reaching into
western Napa, East Bay, and down the Santa Clara and Salinas
Valleys. Some mist and light drizzle was observed this morning,
with a hundredth measured at Salinas and Watsonville airports, and
at Oakland North RAWS.

For today, expect partly to mostly cloudy and cool conditions
along the coast, with late morning/early afternoon clearing
around SF Bay, but earlier inland. Coastal Monterey Bay may remain
under cloud cover throughout the day with steady onshore flow. As
an upper trough, currently off the Oregon coast, approaches
northern CA late this afternoon, winds will become gusty along the
coast, and through typically favored coastal gaps and elevated
passes. May see gusts over 30 mph in some of these spots. Temps
will generally be in the upper 50s to 60s around the coast and
bays, with 70s inland, possibly low 80s in the warmest inland
valley locales. Overnight, an upper low forms over our area,
bringing the chance of drizzle and light showers. Forecast package
for today will try to hone in on the scope of these showers, but
any accumulations will surely be light. See below for more details
on the weekend and extended outlook.


.PREV DISCUSSION...As of 3:05 AM PDT Friday...Early morning profiler
data indicate a marine layer depth of 1500 to 1800 feet and
satellite currently shows low clouds have developed into most
valleys. It`s likely the marine layer will deepen today as an
upper trough currently centered along the Oregon Coast digs south
into northern California. The digging trough will also trigger an
increase in onshore flow today, and breezy westerly winds can be
expected by afternoon. These developments will mean a continuation
of the inland cooling trend today. High temperatures for inland
areas today have been adjusted several degrees below the NBM,
which is almost certainly much too warm. Skies will clear in most
inland areas by midday, but coastal areas will likely see little
or no afternoon clearing.

Expect further deepening of the marine tonight as an upper low
forms over the North Bay. A deep moist boundary layer and robust
onshore flow will likely mean patchy drizzle tonight and Saturday
morning. Drizzle is likely not only near the coast, but also
along west-facing slopes of the coastal ranges, and as far inland
as the East Bay. Enough drizzle or very light rain may fall to
measure in some areas.

The upper low is forecast to meander slowly to the southeast over
the weekend and be centered near Las Vegas by late Sunday. The
cooler airmass associated with the upper low will bring widespread
below normal temperatures to our region this weekend. There may be
enough cooling aloft by Saturday to mix out the marine layer and
result in less cloud cover in coastal areas. Or, more likely, low
clouds will be persistent due to weak mixing. Models agree that
the upper low will generate scattered showers and possible
thunderstorms over the higher mountains to our north and east over
the weekend, but there is less model agreement on the potential
for isolated showers in our area, or where shower activity will
occur if they do form. The NBM develops slight chance POPS across
only southern Monterey County late Saturday and Saturday night.
Will go with this for now and anticipate greater clarity as we
draw closer to the weekend and more high resolution model data
becomes available.

The upper low is forecast to move well east of our area by Monday
and a weak shortwave ridge will produce modest warming early in
the workweek, boosting temperatures up to around normal.

Longer range models, both deterministic and ensemble means, agree
that another trough will arrive along the West Coast by the
middle of next week, bringing another round of cooling, and
perhaps an slight chance of precipitation in the far northern
part of our forecast area.


.AVIATION...As of 11:45 AM PDT Friday...For the 18Z TAFs.
Approaching trough will act to deepen the marine layer over the
TAF period and bring a period of gusty winds later today and
light to locally moderate to heavy drizzle overnight. The trough
acted to deepen last night/this mornings marine layer
significantly versus the previous night and as a result, it pushed
deep inland and brought patchy DZ along the coast. For the
remainder of the day, fingers of stratus will persist through
coastal gaps given the deep upstream feed and brisk onshore winds
out of the W/SW ahead of the surface front. These feeds will be
present through the Golden Gate Gap to KAPC, San Bruno Gap towards
Alameda, and the southern Monterey Bay wrapping counter clockwise
over KMRY/KSNS/KWVI and even KCVH. Some brief midday SCT/BKN
clearing is possible at these terminals, but a return of stratus
through these same feeds is anticipated in the 02-05Z timeframe.
The arriving front will provide enough lift to force -DZ to +DZ
overnight with LIFR-IFR cigs and VISBYS 4-6SM possible. Otherwise,
gusty onshore W/SW winds today, peaking at around 30KT along the
coast and near coastal gaps.

Vicinity of KSFO...WSW Stratus feed through San Bruno Gap has a
steady upstream feed and may persist through the day, bringing
intermittent MVFR FEW-SCT-BKN cigs into the early afternoon. On
the sunny side, clear skies exist across the south Bay and improve
odds for visual approach from KSMB northward to the line of
stratus through the San Bruno Gap. Gusty onshore winds are
possible later today, peaking at 25-30KT AOA 22Z FRI. HREF
ensemble data and WRF model output suggest a return of by 02-03Z
SAT. Patchy -DZ overnight with late SAT burn off.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Steady onshore stratus feed wrapping
counterclockwise will keep Monterey Bay terminals under borderline
IFR/MVFR BKN/OVC cigs through most of the day. While models show
some brief clearing through midday, current satellite trends
provide a more pessimistic all day event, especially for
KMRY/KSNS. Otherwise, breezy to gusty onshore winds today,
15-25kt, with LIFR-IFR cigs overnight and a potential drizzle fest
with DZ/+DZ possible into sunrise SAT.


.MARINE...as of 11:35 AM PDT Friday...Moderate to gusty northwest
winds over the waters today and tonight as a low pressure system
moves through. The strongest winds are expected across the
northern outer waters where isolated gale force gusts will be
possible through late tonight. These winds will generate steep
fresh swell resulting in hazardous seas conditions, particularly
for smaller vessels. Winds will diminish through this weekend but
restrengthen early next week. Mixed  northwest and southerly swell
through the period.


     .Tday...SCA...Mry Bay from 3 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 3 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...SF Bay from 3 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 3 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm




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NWS MTR Office Area Forecast Discussion