Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
539 PM PDT Monday Jul 6 2015
Synopsis...a deep marine layer will remain in place through
midweek and result in widespread night and morning clouds...patchy
fog and coastal drizzle. Temperatures will generally be on the
cool side of average through the week as an upper level system
approaches the California coast. This system may also trigger
isolated showers and thunderstorms from Wednesday afternoon
through Thursday evening.
Discussion...as of 01:50 PM PDT Monday...low clouds over the
region this morning mixed out well this afternoon with only a
small patch of stratus south of Big Sur along the coast.
Elsewhere...mostly clear skies with a few/scattered high clouds
streaming in from the middle/upper level low pressure system about
385 miles west-northwest of San Francisco. Very little change is
expected through Tuesday as the upper level system is forecast to
remain well west of the region. Given this and the deep marine
layer in place...low clouds will likely redevelop quickly this
evening and make a push inland overnight. While drizzle is once
again possible early Tuesday morning...the models show it being
more patchy than this morning. Overall weather conditions on
Tuesday will be similar to today with low clouds mixing out during
the late morning hours.
The upper level low will then begin to move toward the central
California coast late Tuesday into Wednesday. As it does...mid-
level moisture is expected to wrap around the low and advect
across the region. This will cool temperatures aloft and help to
deepen or at least maintain the deep marine layer already in place
over the region. Thus...good inland penetration of low clouds and
patchy fog will continue through middle/late week. In
addition...surface temperatures will be on a cooling trend through
the remainder of the week with a cooler air mass moving over the
The main forecast challenge continues to be the potential for
elevated showers and/or thunderstorms. As mentioned earlier...mid-
level moisture will advect over the region Wednesday and Thursday
as the low approaches. This combined with increased instability
will result in a potential for isolated showers and thunderstorms.
However...the timing and potential locations where this may occur
remains in question given the differences in the forecast models
and eventual track of the system. With that said...will keep a
mention of showers and thunderstorms in the forecast from
Wednesday afternoon through Thursday evening as a heads-up to fire
agencies and any others that may be concerned of convection. Middle-
level moisture may linger into Friday morning with a slight chance
of showers over the North Bay.
The middle/upper level system will then lift to the northeast and
become absorbed in the mean flow Friday into Saturday. This should
put an end to precipitation chances region-wide Friday night into
the upcoming weekend. The forecast models generally keep a broad
middle/upper level trough along much of the West Coast late in the
period and will likely result in dry weather conditions and
temperatures near seasonal averages.
Aviation...as of 5:12 PM PDT Monday...the upper low is presently
located about 350 miles west of Point Reyes and prognosticated to drift
slowly south through Tuesday. Despite weak lower to middle level
cooling and subsequently a gradual decrease in the middle level
heights...the marine layer that once deepened since Saturday
afternoon has decreased in depth this afternoon...approx 1400-2100
feet deep. Many places reported light drizzle again last night.
In light of continued higher surface humidity over the area
interacting with an approaching weakened surface occluded frontal
boundary just west of the coastal waters...once night-time cooling
returns expect stratus and fog to redevelop and move inland.
Patches of light drizzle are possible very late.
Diurnal mixing was sufficient to clear out the stratus and fog
earlier today. There's much less areal coverage compared to Sat
and sun afternoons. Looks like we have a better probability of VFR
continuing into the evening than in recent days. Modeled forecasts
are ambitious however with returning the stratus and fog back to
the area tonight. 00z tafs indicate a general trend toward IFR
later this evening and overnight.
Vicinity of ksfo...moderate to high confidence VFR continues this
evening...at least while daylight lingers. 00z taf indicates
tempo MVFR ceiling 04z-08z...may be a bit early...but also don't want
to go too late given sfo-SAC onshore gradient is at healthy 4 mb
at the moment. If the marine layer stays compressed overnight then
ceilings may be closer to IFR by Tuesday morning...not indicated in the taf
yet but something to keep an eye on.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR will hold into the early evening
hours...but confidence lowering a bit as to when the stratus and
fog redevelop. A weak frontal boundary located out over the coastal
waters...a reduced depth of the marine layer and nighttime cooling
with dry air above the marine inversion all support the
redevelopment of IFR due to fog and stratus. Tempo IFR forecast
for ksns and kmry 03z-08z. Patchy light drizzle is possible again
very late tonight.
Marine...as of 10:43 am PDT Monday...light winds and seas are
expected through the forecast period as a weak low pressure
... Bay until 9 PM
Public forecast: rgass
Visit US at www.Weather.Gov/sanfrancisco
Follow US on facebook and twitter at: