Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service San Francisco Bay area
830 am PDT Wednesday Apr 16 2014

Synopsis...dry conditions will continue through the weekend.
Patchy fog and clouds near the coast will become more extensive
Thursday night and Friday as a weak weather system moves onshore.
A stronger low pressure system will arrive late Monday and Tuesday
bringing a threat for showers...colder temperatures...and gusty
winds.

&&

Discussion...as of 8:30 am PDT Wednesday...minor forecast update
this morning with clouds mostly confined around Monterey Bay and
through the Salinas valley. The reason? Northerly surface gradient
ramped up to over 7 mb overnight helping to push the stratus deck
to the south. Mostly sunny conditions are forecast for all area by
later this morning with warm conditions inland -- upper 70s to
around 90. At the coast an weak onshore flow will continue which
will keep it closer to the 60s and lower 70s. One exception will
be for parts of Santa Cruz where the northerly flow will
downslope and help to push highs 75-80.

Quiet weather will continue through the rest of the week into
Monday before a big change hits US on Tuesday. Good model
agreement that a cold system will drop down to our area helping to
drop 700 mb temperatures to the -9 to -11c range. Moisture availability
is the one big question mark, although some showers should be
generated. If the models verify snow levels will drop to under
4000 feet and instability will greatly increase leading to some
thunderstorms as well. Winds will also become gusty from the
north. Unless there is a huge change in the models, temperatures will be
lowered for Tuesday into Wednesday especially for the hills. Still
nearly a week out, but it's very interesting for a late April
system!

Previous discussion...tonights viirs satellite imagery around
230 am showed patchy low clouds near Monterey Bay and Salinas
valley. Latest GOES fog product imagery indicating stratus has
become a little more widespread in the Salinas valley...near Half
Moon Bay...and Monterey Bay the last couple hours. Breezy
northwest winds over the open waters have kept the boundary layer
well mixed and confined stratus to convergence and upslope areas
near the coast favored by northwest winds.

Short wave ridge will progress eastward across the state
today...keeping the marine layer under 1500 feet. Surface pressure
gradients will remain strong from the northwest to southeast with
sfo-acv over 6 mb today and winds will remain at least breezy over
the open waters. Hrrr model surface relative humidity field indicates limited
stratus again today. As the ridge shifts eastward the flow will
become more onshore Thursday with a better established marine
layer...so expect extensive stratus to return to coastal areas by
Thursday morning.

A weak trough in zonal flow will approach the coast Thursday night
deepening the marine layer and increasing onshore flow. This trough
splits over the area Friday...and the lack of upper level lift
should keep conditions dry. Cooler air aloft with 850 mb
temperatures dropping to 10 degree c should disrupt the marine layer
Friday but spread cooler air inland with continued onshore flow.

After a brief break Saturday under weak high pressure and mostly clear
skies...another minor system will brush by to the north of our area
Sunday with more coastal clouds.

A stronger trough will drop over California late Monday and Tuesday
bringing a threat for showers and much cooler conditions with
gusty winds. Forecast models are consistently advertising this
stronger system...and even though the system lacks a significant
moisture tap...cold air aloft will be plentiful and showers are
certainly possible especially over the mountains.

&&

Aviation...as of 4:30 am PDT Wednesday...satellite imagery shows
patchy low clouds in mry Bay and in the Salinas valley and along
the San Mateo coast this morning. Strong north-S pressure gradient indicates
dry northerly flow that will keep clouds out of the sfo Bay area
today.

Vicinity of ksfo...VFR. West afternoon winds to 20 knots after 20z.

Ksfo bridge approach...similar to ksfo.

Monterey Bay area terminals...MVFR ceilings through 17-18z.

&&

Mtr watches/warnings/advisories...
... Pt to pt pinos 0-10 nm
glw...pt Arena to pt Reyes 10-60 nm
glw...pt Reyes to Pigeon pt 10-60 nm
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 pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm
glw...Pigeon pt to pt pinos 10-60 nm
Small Craft Advisory...Monterey Bay from 1 PM

&&

$$

Public forecast: Bell/kbb
aviation: west pi

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