Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
517 PM PDT sun may 3 2015
Synopsis...a relatively deep marine layer and persistent onshore
flow will maintain areas of night and morning low clouds through
at least Tuesday. An upper low is forecast to develop over
California Thursday and Friday with a few showers possible
especially over the mountains. The marine layer will mix out
about midweek with near to slightly below normal temperatures
continuing through the week.
Discussion...as of 01:47 PM PDT Sunday...persistent low clouds
this afternoon in parts of the North Bay and along the coast
especially northern Monterey Bay and San Mateo coast. Boundary
layer winds remain southwesterly which contributes to the
persistent stratus in places like Santa Cruz...Big Sur...and coast
Marin and Sonoma counties. Downslope effect in southern Monterey
Bay with mostly sunny skies. Cool temperatures continue this
afternoon with Santa Rosa only in the 50s. Onshore flow remains
strong with sfo to SAC now at 2.7 mb and marine layer up to 2500 feet.
Boundary layer flow turns northwesterly by Monday with increasing
sfo to acv gradient...and shifts the main stratus areas back to the typical
locations. Expect Santa Cruz and North Bay to have a better chance
at afternoon sunshine Monday with more clouds in northwest flow favored
stratus areas like Monterey. Onshore gradient will peak on Monday
with another relatively cool day even inland. Marine layer should
remain around 2000 feet or deeper...though could reform at a
slightly lower level in the next day or so. Given propensity of
continued troffing along the coast...would not expect significant
decrease in marine layer depth.
Models in good agreement into medium range with amplifying ridge
in eastern Pacific driving trough southward along West Coast toward
midweek. Cooler 850 mb temperatures should allow marine layer to
mix out Wednesday and Thursday...dropping almost to zero degrees c by Friday.
Vorticity lobes rotating around the developing low over California will
be tricky to forecast. First piece of energy moves onto central
coast in northwest flow Wednesday night with weak dynamics and mixed low
clouds over the hills. Stronger lobes pivot around the low and
could have enough dynamics and instability for a few showers
mainly mountains and upslope northwest flow areas near the coast Thursday
afternoon into Friday.
Weak ridge returns next weekend...but should be temporary with
another trough approaching the coast late in the weekend. Main
message into middle may is near normal or cooler than normal
temperatures. No sign of a significant storm with mostly dry
Aviation...as of 4:45 PM PDT Sunday...southwest flow at the coast
will allow stratus to spread into the North Bay valley and the mry
Bay area early tonight. Clouds will spread through the Golden Gate
into the sfo Bay area but sfo and Oak should stay VFR until after
Vicinity of ksfo...southwest flow will keep MVFR ceilings out until
after 06z. Southwest winds to 15 knots through 04z.
Sfo bridge approach...similar to sfo.
Monterey Bay area terminals...MVFR ceilings after 03z.
Beaches...as of 4:36 am PDT Sunday...beach hazard statement
continues through tonight for strong rip currents and large shore
break. Buoys off the northern and central California coast show
mixed swells of 3 to 5 feet with 24 second period and 5 to 7 feet
at 11 seconds. The long period waves will result in exceptionally
strong rip currents along the coast especially on southerly facing
beaches such as Stinson Beach in Marin County and the beaches
along the Monterey Bay coast of Santa Cruz County.
Additionally...these long period waves will also result in large
shore break at Twin Lakes state beach and the Santa Cruz boardwalk
beach. Large shore break can result in neck and back injuries. If
you plan on entering the water please do so near a lifeguard and
always keep an eye on the ocean when visiting the beach.
Marine...as of 10:31 am PDT Sunday...gusty northerly winds will
continue over the northern outer waters through early next week.
Elsewhere...southerly flow will continue today. Northerly flow
will develop over most of the coastal waters tonight and Monday as
high pressure builds over the region. A long period southerly
swell will impact the coastal waters today and continue through
... Arena to pt Reyes 10-60 nm
Small Craft Advisory...sf Bay north of the Bay Bridge until 9 PM
Public forecast: kbb
aviation: west pi
marine: west pi/Sims
Visit US at weather.Gov/sanfrancisco
Follow US on facebook, twitter, and youtube at: