Small Craft Advisory 123, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1035 PM PDT Fri Jun 26 2020

.SYNOPSIS...Westerly winds increase Saturday afternoon/evening as
a dry cold front approaches the region. Windy and cooler inland
Sunday behind the front. High pressure rebuilds Monday through
much of next week with seasonably warm and dry weather forecast
through the 4th of July.

&&

.DISCUSSION...as of 9:30 PM PDT Friday...Wedge-shaped marine
layer along with an upper-level ridge off the California coast
resulting in classic summertime conditions for the CWA,with wide
temp variations between the coast and inland locations. Max temps
ranged from 106 at Lake Berryessa to 63 in Half Moon Bay as is to
be expected during these sorts of setups. Warm conditions the
interior will taper off this weekend as the upper-level trough off
the coast of the Pacific Northwest will follow an inside slider
pattern as it centers itself over idaho and Montana this weekend.
As a result, heights will begin falling across the CWA by Saturday
afternoon as the cold front moves in from the NW. Onshore winds
will gradually pick up across the Bay Area, especially over
coastal gaps and eventually higher terrain during the FROPA.
Moreover, NAM-12km runs have constently suggested that some areas
along the coast may measure some drizzle, primarily along the San
Francisco Penninsula and some areas along coastal Santa Cruz and
Moneterey counties.

Expect for significant cooling across the interior, with some
locations in San Benito, Santa Clara, and Alameda counties
experiencing a 15-20 F temp decrease. Primary concern with this
system will be the uptick in surface winds across portions of the
Bay Area this weekend.

NAM-12km and local WRF model are both suggesting that along with
strong onshore flow Saturday evening thru Monday, conditions Sunday
night also potentially will result in strong low-level jet with
northerly flow (25-40 mph)over Napa and Sonoma counties. This flow
and the fairly low RH values associated with it may heighten fire
concerns in the North Bay and parts of the East Bay Hills Sunday
night and going into the overnight hours.

Following the weekend FROPA and looking ahead into next week, both
the ECMWF and GFS are suggesting that temps will rebound across the
interior and that the CWA will gradually return to a more seasonable
pattern as the high pressure offshore restablishes itself.

From Previous Discusion...For Sunday night, the marine layer will
be scoured out as the trough descends into the vicinity due to
significant cooling aloft and powerful winds that will turbulently
mix out the marine inversion from the top down. More importantly
for the North Bay, the pressure gradient will shift to favor
northeasterly flow Sunday night and into Monday morning. Northeast
winds will ramp up across the North Bay hills and mountains,
where they too could experience sustained offshore winds of
20-30mph with gusts 35 to 45mph. These winds speeds will taper off
through Monday morning but result in poor overnight recoveries
there, temporarily increasing fire weather risk overnight Sunday
and during the day on Monday as minimum humidities are forecast to
drop to around 20% in the driest spots after the overnight
offshore push. See fire weather discussion for more details.

For next week, a seasonable weather pattern is expected from late
June into early July with high pressure lingering offshore.

&&

.AVIATION...As of 10:35 PM PDT Friday...Similar stratus pattern to
yesterday as cigs are spreading into the East Bay and the North
Bay valleys. Southwesterly flow has persisted along the coast
which will keep the stratus out of SFO and the approach through
early morning. Upper level trough will be approaching Friday night
which will deepen the marine layer and bring stratus in a little
earlier.

Vicinity of KSFO...MVFR cigs should again stay out until after
12Z. Clearing after 17Z.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...IFR cigs with bases 500-600 ft.
Possible reduced vsbys after 09Z. Clearing after 18Z.

&&

.FIRE WEATHER...As of 3:15 AM PDT Friday...After several days of
above normal temperatures latest fuel charts show much of the
inland areas with abnormally dry fuels. Areas influenced by the
marine layer remain closer to normal in terms of fuel dryness.
Continued seasonably warm to hot inland today through Saturday
afternoon. Onshore winds increase late Saturday afternoon and
evening with gusty west winds blowing most of the night around SF
Bay. Even though temperatures will be much cooler inland on Sunday
conditions will be near critical Sunday afternoon across the
interior areas, especially the far East Bay transition zone out
into the Central Valley. Gusts from 35-45 mph for favored
locations such as around Mt Tam, and the Altamont while breezy
conditions are likely for much of Sunday in the East Bay hills.
Winds will be light for the Central Coast but locally strong near
Pacheco Pass and down the Salinas Valley.

A brief period of dry north winds may develop Sunday night into
early Sunday for the North Bay hills. This wind reversal will
need to be monitored closely for Sunday night should any ignitions
occur Sunday afternoon. Drying and warming next week leading up
to the 4th of July holiday.

&&

.MARINE...as of 10:29 PM PDT Friday...Northwest winds will
increase over the northern and outer waters Saturday afternoon
through Sunday as a dry cold front moves through the district.
Wind gusts could reach near gale force  Saturday night and Sunday
over the northern outer waters. Mixed  seas will persist with a
shorter period northwest swell and a  longer period southwest
swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm

&&

PUBLIC FORECAST: Diaz/Dykema/DRP
AVIATION: W Pi
MARINE: W Pi
FIRE WEATHER: RWW

Visit us at www.weather.gov/sanfrancisco

Follow us on Facebook, Twitter, and YouTube at:
www.facebook.com/nwsbayarea
www.twitter.com/nwsbayarea
www.youtube.com/nwsbayarea