Area forecast discussion
National Weather Service San Francisco Bay area
1032 PM PDT Sat may 18 2013
Discussion...as of 8:05 PM PDT Saturday...the airmass began to
warm over our region today as an upper level ridge centered
offshore along 135w began to shift eastward. But temperatures did
not warm uniformly across the region. In fact...some locations
near the coast and in the southern Santa Clara Valley were a
degree or two cooler than yesterday. But most areas warmed at
least a few degrees and some locations warmed as much as ten
degrees. The warming trend will get fully underway tomorrow and
then continue into Monday. Much cooler weather is then forecast
by the middle of next week.
A shortwave trough dropping southeast near the Oregon/California border late this
afternoon and early this evening triggered an increase in
northwest winds across our region late in the day. Winds were
gusting up to 40 miles per hour at San Francisco Airport at 7 PM PDT and
several other locations were reporting gusts close to 30 miles per hour.
Winds are probably peaking at the current time and should
gradually decrease overnight.
The Fort Ord profiler shows a marine inversion developed at around
1500 feet by late afternoon. Not long thereafter...coastal low
clouds began to develop. The brisk northwest flow interacting with
the coastal terrain resulted in an interesting phenomena in the
developing low clouds late in the day...especially when viewed on
visible satellite imagery: lines of low clouds extending from the
San Mateo and Monterey County coasts SW out over the ocean.
Although low clouds are increasing along the coast this
evening...do not expect them to reach very far inland overnight
given the increasing northerly surface pressure gradient (already
up to 6.8 mb from acv to sfo). Skies will then clear quickly on
Sunday morning and clear skies will prevail from Sunday afternoon
through Monday evening...even at the coast. Temperatures will warm
above normal in all areas tomorrow as onshore flow decreases and
the airmass warms aloft by 4-5 degree c. The upper ridge will build
directly over California by late Sunday and Sunday night and the
airmass at 850 mb will warm up to 20 degree c across the southern
part of our County Warning Area by Monday. Monday is expected to be the warmest
day of the week for a couple of reasons. First...the airmass aloft
will be warmest then. Second...an offshore flow is forecast to
develop Sunday night and continue into Monday morning. Surface
high pressure will build into northern Nevada late Sunday and
Sunday night...causing winds to veer to the north and northeast
in the hills and also increase. Local wind gusts in the north and
East Bay hills may reach 40 miles per hour on Sunday night. Winds will
rapidly decrease on Monday. These warm and dry winds will raise
fire weather concerns to some extent from late Sunday into early
Monday. But winds with this offshore flow event are not expected
to be strong enough over a widespread area...or last long enough to
warrant a red flag warning. Headlines for warm and dry conditions
and locally gusty offshore winds were added to the fire weather
forecast.
The models have been consistent in forecasting a rapid pattern
change early next week. The upper ridge on Monday will be quickly
replaced by a cool upper trough dropping southeast out of the Gulf
of Alaska. Significant cooling will occur on Tuesday as the trough
begins to plunge south into northern California and strong onshore flow
develops. High temperatures will probably cool by as much as 20
degrees f from Monday into Tuesday. Cooling will continue into
Wednesday as the trough deepens further over California. No precipitation is
currently expected with this upper trough. The primary impact will
be to cool temperatures well below normal by midweek. The models
indicate that the trough will be in no hurry to move and is
forecast to remain near the West Coast through next
weekend...producing continued cool weather...especially for inland
areas. Coastal areas will see plenty of sun for most of next week as
the airmass aloft will be too cool to support a marine layer
beyond Tuesday morning.
&&
Aviation...as of 10:30 PM PDT Saturday...increasing northerly
gradients will keep stratus to areal coverage tonight. Thus on;y
brief ceilings are expected in the sfo Bay area Sunday morning.
Vicinity of ksfo...brief MVFR ceilings possible between 12z and 16z.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...MVFR ceilings through 17z.
&&
Mtr watches/warnings/advisories...
... Pt to pt pinos 0-10 nm until 3 am
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm
&&
$$
Public forecast: dykema
aviation/marine: west pi
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