Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
855 PM PDT Monday Apr 27 2015
Synopsis...a dry cold front will approach the coast resulting in
increasing onshore flow and a cooling trend Tuesday and Wednesday.
Dry and seasonable weather is then forecast from Wednesday through
the upcoming weekend.
Discussion...as of 8:55 PM PDT Monday...after a warm day that
produced a record high at our downtown Oakland site of 82 degrees
there is an impressive marine push under way. Late this afternoon
the marine layer started to envelop the coast well ahead of a
fairly strong surface cold front and upper trough. Earlier this
afternoon the Sonoma County Airport reported a high of 89 degrees
and its currently 55 degrees or 34 degrees of cooling in just a
few hours! Even the city warmed to 76 degrees and is now down to a
blustery 51 degrees with 24 miles per hour winds at the Golden Gate and up to
30 miles per hour at ksfo. Its under these shallow marine layers (currently
at about 1000 feet but deepening) and during the initial push of
marine air that we often see some gusty winds along the coast and
through the coastal gaps such as San Bruno. Cant help but notice
with our SST temperatures back down in the upper 40s and lower 50s these
marine pushes are behaving a little more like we used to observe
and have some gusto as they push onshore.
Anyway...this obviously signals the beginning of a cooling trend
for Tuesday as a potent but dry front moves through northern
California and ramps up the onshore gradients and associated winds
off the ocean for Tuesday. Will have to see how deep the marine
layer gets but there should be enough cooling aloft and mixing in
the low-levels to preclude any all day gray inside the Bay for Tuesday.
Conditions should moderate a little bit on Wednesday but still keep
gusty onshore winds blowing Wednesday near the coast and inside
The surface pattern will push a thermal trough towards the coast
late Wednesday night into Thursday morning. This will allow dry
northeast winds to develop in the hills. Expect a sharp upward
climb in temperatures for Thursday with offshore flow and drying
winds. The European model (ecmwf) MOS is picking up on this and shows a 90 for
Santa Rosa on Thursday and 93 at Concord. Offshore pattern may
persist into Friday but not as strong.
An upper trough may try and develop over northern California this
weekend keeping temperatures near normal with dry weather.
Previous discussion...as of 3:00 PM PDT Monday...sunny across almost all
of our County Warning Area today with temperatures running 8 to 15 degrees ahead of
yesterday at this time. Look at the latest readings show San
Francisco at 73...San Rafael 79...Livermore 84...and San Jose plus
Salinas at 80. The westerly surface gradient which had been nearly
neutral has been slowly increasing through the day to weak
westerly while at the same time the northerly gradient has been
cut in half. Along with the inland heating...those factors have
allowed clouds to move to many coastal spots along San
Mateo...Marin...and Sonoma counties. Also received a report of
less than 1 mile visibility at the farallones due to fog.
For tonight...with the westerly gradient expected to increase and
a large area of clouds upstream expect clouds along the coast plus
adjacent valleys. Will likely get reports of fog during the
morning commute particularly along Highway 1. At the same time an
upper level trough will advance to the pacnw/British Columbia coast by tomorrow.
Although all associated moisture with this feature will stay well
to our north...it will help to drop temperatures locally by more than 10
Quiet weather will continue for the remainder of the week as the
flow becomes weakly zonal with the main storm track well to the
north. Temperatures will warm back close to the values we saw
today by Friday. Still no chance of rain at least into the first
part of next week.
Aviation...as of 5:17 PM PDT Monday...a strong upper ridge is
compressing the marine layer to 600-1000 feet deep. A large area of
stratus can be seen from Eureka to the Big Sur coast and the stratus
has filled in quickly around the Monterey Bay region in the last
1-2 hours. Think the key to ceiling/visibility forecasts for this period
hinge on the shallow marine layer holding in place overnight into
Tuesday. Overall the marine layer may fluctuate a little in depth
overnight into Tuesday but should hold fairly close to 1000 feet.
The 04/27 18z NAM model run initialized too low on the north-south
gradients...but did well initializing the sfo-SAC gradient. Models are
trending stronger with the west-northwest gradients and winds...especially over
the Bay area...by this time tomorrow and again Wednesday afternoon
Vicinity of ksfo...it's a high confidence VFR forecast for the
evening due to the shallow marine cloud layer. Decent late PM
ocean-land temperature differences...local onshore gradients and a shallow
marine layer will combine to produce more westerly wind gusts in
the 20 to 30 knot range until 04z. Not entirely confident the stratus
will reach ksfo Tuesday morning if the marine layer does stay
compressed. Therefore decided to scale back the duration of
forecasted MVFR ceilings to a tempo group 13z-17z. Overall the bigger
story is likely to be the strength of the westerly winds over next
24-36 hours especially in the latter forecast period. Coordinate and in
agreement with the cwsu office.
Sfo bridge approach...VFR except a few brief patches of stratus are
possible Tuesday morning.
Monterey Bay area terminals...IFR ceilings are likely to develop this
evening as the stratus continues to move in from the northwest.
Expecting the stratus clouds to mix out by late Tuesday morning
but most likely to be poised to move back in Wednesday evening.
Marine...as of 08:33 PM PDT Monday...high pressure will result in
moderate northwesterly winds across the coastal waters through
tonight. Wind speeds will increase midweek and persist through the
end of the week as a weak frontal system passes to the north. In
addition...northwest swell will build through midweek.
... Bay north of the Bay Bridge until 9 PM
Small Craft Advisory...sf Bay south of the Bay Bridge until 9 PM
Small Craft Advisory...Monterey Bay until 9 PM
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm until 9 PM
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm from 9 PM
Public forecast: rww
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