Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
310 am 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
the middle of the week...along with temperatures near to slightly
below normal. An upper level low is then forecast to develop over
California by Thursday. This low will mix out the marine
layer...but also maintain relatively cool weather conditions into
the second half of the week. There is also a slight chance of
showers with this weather system...mainly across the southern
portion of our area from late Thursday night through Friday.
Discussion...as of 3:10 am PDT Sunday...the marine layer has
deepened slightly since early yesterday morning and is currently
at a depth of just over 2000 feet at Fort Ord. Onshore pressure
gradients are similar to 24 hours ago...although the Delta breeze
at Travis AFB is stronger early this morning with the 2 am
observation indicating onshore winds of 25g33 knots. Widespread
low clouds have developed in coastal areas and also into many
valleys. Clearing will likely occur a little later today given the
increased depth of the marine layer. All of these factors suggest
that the cooling trend that began on Friday will continue
today...although the magnitude of today's cooling is expected to
be less than what occurred during the past two days.
A weak upper trough currently off the West Coast is forecast to
shift eastward over the California coast by Monday morning. Both the NAM
and WRF models indicate this will result in an even greater inland
surge of marine layer clouds tonight...as well as continued cool
conditions into Monday.
After the weak upper trough moves inland on Monday...surface high
pressure will strengthen off the northern California coast...resulting in
increasing northwest winds along the coast on Monday night and
Tuesday. These winds should produce sufficient mixing to clear out
much of the low cloudiness by Tuesday afternoon and thus bring
about slight warming on Tuesday.
An upper low is forecast to develop over the Pacific northwest by midweek.
The GFS and European model (ecmwf) agree reasonably well in dropping that low due
south across northern and central California from late Wednesday
night through Thursday night...and then ejecting that low
eastward on Friday. Both models also forecast scattered light
precipitation across portions of our area from Thursday afternoon
through late Friday...with the most widespread shower activity
projected to occur across the southern part of our area late
Thursday night and Friday morning as shortwave rotates inland over
the central coast.
The airmass over our area will cool several degrees from late
Wednesday into Thursday as the upper low drops in from the north.
This cooling will be sufficient to mix out the marine layer across
the northern part of our area by late Wednesday and in all areas
by Thursday. Temperatures will cool below normal in all areas by
Thursday and remain cool through the end of the week.
The upper low is forecast to move well off to our east by late
Friday...and shortwave ridging is expected to produce dry and
slightly warmer weather next weekend. But the longer range models
maintain an unsettled pattern...with another weak weather system
projected to approach California early next week.
Aviation...as of 10:40 PM PDT Saturday...forecast on track.
Stratus has spread into the mry Bay area and the North Bay valley
and will spread through the Golden Gate into the East Bay first
reaching Oak after 06z. Stratus will wrap around the Bay reaching
sfo after 10z.
Vicinity of ksfo...MVFR after 10z. West to northwest winds to 18
knots through 04z.
Sfo bridge approach...MVFR spreading south along the East Bay
then wrapping around the Bay and up the peninsula.
Monterey Bay area terminals...ceilings a little higher than expected.
Now expecting MVFR conditions through 17z.
Marine...as of 2:18 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 Sunday and continue through
Beaches...as of 2:40 PM PDT Saturday...a series of southern
hemisphere storms have generated a pair of long period southerly
swell trains which are expected to begin to arrive at the coast
overnight tonight peaking on Sunday. Although the wave heights
associated with these long period waves will be relatively
small...4 to 6 feet...the powerful nature of 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
... Arena to pt Reyes 10-60 nm
Small Craft Advisory...sf Bay north of the Bay Bridge from 3 PM
Public forecast: dykema
aviation: west pi
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