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Area forecast discussion 
National Weather Service San Francisco Bay area
1050 am PDT Monday Jul 6 2015

Synopsis...a rather deep marine layer will remain in place and
result in widespread night and morning clouds...patchy fog and
coastal drizzle through midweek. Temperatures will also trend
cooler through much of the week as an upper level low pressure
system approaches the California coast. The low may trigger
isolated showers and thunderstorms from Wednesday afternoon
through Thursday evening.

&&

Discussion...as of 09:01 am PDT Monday...a 2000 foot marine layer
remains over the region this morning and resulted in low clouds
pushing well inland overnight. In addition...patchy drizzle was
reported along many coastal locations with some sites picking up a
few hundredths of an inch of rainfall. Given the depth of the
marine layer...do expect clouds to mix out well by early
afternoon. Temperatures will warm into the 70s to 80s inland this
afternoon while coastal locations remain in the upper 60s to
lower 70s. No changes needed to the forecast at this time. Please
read previous discussion for more details.

&&

Previous discussion...as of 3:20 am PDT Monday...an upper low is
centered about 350 miles west of San Francisco early this
morning. The relatively close proximity of this low to our area is
maintaining a rather deep marine layer which has allowed low
clouds to develop well inland once again overnight. 3 am
temperatures are warmer than typical for this time of the
year...mostly in the lower 60s...which is due in part to warmer-
than-normal sea surface temperatures along our coast.

No significant changes are expected in the short-term. The models
agree that the low will wobble about out near 130w through midday
Tuesday and then track east-southeast towards Point Conception from Tuesday
afternoon through late Wednesday night before lifting northeast
across central California on Thursday and Thursday evening. The
low's approach during the first half of this week will likely
result in further deepening of the marine layer...more widespread
low clouds cover...and additional daytime cooling...especially
inland.

The primary forecast challenge for the week ahead concerns the
possibility for convective precipitation in our forecast area as the low
draws nearer and then moves inland. Satellite currently shows
there is moisture wrapping around the low center offshore...and
models indicate the low may entrain some of the monsoon moisture
to our east by midweek as the low approaches the coast. Thus...it
appears there may be sufficient middle/upper level moisture for
convective precipitation. The models also indicate increasing middle-level
instability as the cooler pool of air aloft associated with the
upper low arrives by midweek. Both the GFS and European model (ecmwf) forecast
scattered precipitation across at least a portion of our area sometime
between Wednesday afternoon and Thursday evening and so confidence
is growing that our area will see at least isolated convective
precipitation. But when and where that precipitation might develop is the
most difficult part of the forecast. The 00z GFS forecasts
scattered precipitation out ahead of the approaching low on Wednesday
afternoon and evening...mainly across inland portions of Monterey
County and San Benito County. The 00z European model (ecmwf)...however...does not
develop precipitation until Thursday and then mainly across the North Bay
and sf peninsula as moisture wraps around the northern and western
sides of the low. Due to these model differences...have decided to
blanket most of our forecast area with a slight chance of showers
and thunderstorms from Wednesday afternoon through Thursday
evening. If isolated thunderstorms do develop...there is a
potential for dry lightning given that most of the moisture with
this system will be in the middle and upper levels. Will need to
consider a Fire Weather Watch if confidence in thunderstorm
development increases. For now will mention the possibility of
midweek thunderstorms as a heads-up to fire agencies.

Shower/thunderstorm chances will end by Thursday night as the
upper low moves well off to our northeast. Expect dry and slightly
warmer weather for the upcoming weekend.

&&

Aviation...as of 10:55 am PDT Monday... a weak upper low
lingering offshore will lead to predominately onshore flow and VFR
conds today. Marine layer is forecast to deepen tonight and spread
inland similar to yesterdays early arrival and persist as long as
or slightly later for tomorrow morning as upper low shifts inland.

Vicinity of ksfo... VFR today with onshore flow. Sust winds 15 knots
with gusts to around 20 knots this afternoon. MVFR returns at or above 05z
Tuesday.

Ksfo bridge approach... similar to ksfo.

Monterey Bay area terminals...VFR north and South Monterey Bay...
with MVFR through 19z at kwvi. Onshore flow sust 10 knots with gusts to
15 knots this afternoon. Early return of low ceilings by 01-02z Tuesday.

&&

Marine...as of 10:43 am PDT Monday...light winds and seas are
expected through the forecast period as a weak low pressure
lingers offshore.

&&

Mtr watches/warnings/advisories...
... Bay from 2 PM

&&

$$

Public forecast: rgass/dykema
aviation: drp
marine: drp



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