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Area forecast discussion 
National Weather Service San Francisco Bay area
458 am PDT Friday Sep 4 2015

Synopsis...temperatures will remain cooler than seasonal norms
today as upper level troughing continues over the West Coast...along
with onshore flow at the surface. A warming trend is then expected to
begin over the weekend and continue into next week as high
pressure builds inland and low level winds turn offshore. High
confidence that dry conditions will continue through the extended
forecast period.

&&

Discussion...as of 3:55 am PDT Friday...latest water vapor
satellite imagery shows a large-scale upper level trough axis
extending down along the West Coast from the Puget Sound sound area of
Washington state through the coast of northern California and then
southward and offshore. Although most associated cloudiness and
all showers will remain well north of our area...its approach is
resulting in both cooling aloft...and some ramping up of the
onshore flow at the surface. The former has led to the marine
layer getting largely mixed out...with stratus coverage at present
basically limited to coastal and near coastal areas from around
Half Moon Bay southward to the Monterey peninsula.

In the shorter term...mostly sunny conditions will prevail across
the district today...aside from persistence into the first part of
the morning of some areas of coastal low cloudiness. Afternoon
temperatures will generally be pretty similar to those on
Thursday...and thus remaining on the cool side for this time of
year especially inland. Do expect it to get a bit breezier though
in the latter part of the afternoon and on into the evening
hours...as the upper level trough axis progresses inland and
winds turn more northwesterly and increase aloft...and then also
couple with the northwest to southeast surface pressure gradient.

Sunshine and the beginning of a warming trend is expected for Saturday
as the upper level trough axis progresses eastward and...as an
early harbinger of the pending fall season...surface high pressure
builds inland through the Pacific northwest and into the northern
Great Basin. This will turn low level winds offshore...clearing
out the marine layer...diminishing the marine air influence
inland...and yielding some subsidence warming especially in more
susceptible places like the Santa Rosa and Santa Cruz areas.
Ultimately though the magnitude of the warm-up on Saturday will be
limited by the continuing cool air mass aloft...with latest model
output indicating that's when 850 mb temperatures will bottom out...in
the 9c to 13c range. And in the hills of the north and East
Bay the first appreciable occurrence of offshore winds of the
season...or really forthcoming season...is forecast to occur late
Friday night into Saturday morning...though with wind speeds even
over the higher ridges remaining modest and well below Wind
Advisory levels.

Sunday and into the first part of next week...offshore flow will
continue along with a warming air mass above. The net result for
US will be warm and sunny days and clear nights...even areas
closer to the coast.

In the extended...no clear indication at present of when the flow
will turn back to onshore and the marine layer re-establish.
Wouldn't be surprised if this comes about as a result of a
southerly coastal stratus surge in a midweek sort of time
frame...but this sort of mesoscale development GOES beyond what
the longer range models can capture. High confidence though that
dry conditions will continue through the end of the week as very
good agreement between models that a broad region of upper level
high pressure will develop and then remain parked off the West
Coast.

&&

Aviation...as of 04:55 am PDT Friday...low level clouds remain
confined to the Monterey Bay region this morning as a weak upper
level disturbance brought a cooler and drier airmass down from the
north. Expecting these clouds to burn-off around 17z with vff
conditions likely region-wide this afternoon. Wind speeds will
increase late this morning with locally strong and gusty winds
this afternoon and evening.

Vicinity of ksfo...VFR. Light westerly winds this morning will
increase this afternoon and continue through the evening with
gusts to around 30kt. Wind speeds then diminish late in the
evening with VFR conditions likely to persist into Saturday
morning.

Ksfo bridge approach...similar to ksfo.

Monterey Bay area terminals...MVFR ceilings likely to persist through
about 16-17z. VFR conditions then expected for the remainder of
the day along with increasing westerly winds through the
afternoon. Return of MVFR ceilings possible late tonight into Saturday
morning...yet confidence remains low.

&&

Marine...as of 04:55 am PDT Friday...moderate to strong
northwest winds will persist over the coastal waters through
Saturday morning as an upper level trough deepens along the West
Coast. Wind speeds will then gradually diminish through the
weekend. A long period southerly swell is forecast to move into
the coastal waters this weekend.

&&

Mtr watches/warnings/advisories...
... Pt to pt pinos 0-10 nm from 2 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm from 2 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 Pigeon pt 10-60 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...sf Bay from 2 PM
Small Craft Advisory...mry Bay from 2 PM

&&

$$

Public forecast: blier
aviation: rgass
marine: rgass

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