Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service San Francisco Bay area
435 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.

&& of 3:20 am PDT 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

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.
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.

&& of 4:30 am PDT upper level low off the
central coast is moving slowly east. The marine layer has deepened
to over 2000 feet which will result in a late burnoff from the
sfo Bay area.

Vicinity of ksfo...MVFR ceilings through 18-19z. West winds gusting to
25 knots after 20z.

Ksfo bridge approach...MVFR ceilings through 19z.

Monterey Bay area terminals...MVFR ceilings clearing sns after 18z.
Partial clearing at mry after 20z.

&& of 3:12 am PDT Monday...low pressure off the
California coast will keep light winds through the week. The low
will move across the coastal waters Thursday bringing a slight
chance of thunderstorms.


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



Public forecast: dykema
aviation: west pi
marine: west pi

Visit US at www.Weather.Gov/sanfrancisco

Follow US on facebook and twitter at:

National Weather Service Glossary of Abbreviations