Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
909 PM PDT sun Aug 2 2015
Synopsis...a subtle cooling trend is forecast to continue through
Tuesday as onshore flow and the marine influence increases. Modest
warming is then forecast for Wednesday and Thursday. Aside from
patchy coastal drizzle and fog during the overnight and early
morning hours...dry weather conditions are expected to persist
through the forecast period.
Discussion...as of 09:00 PM PDT Sunday... afternoon temperatures
peaked a few to up to five degrees cooler across most locations
today. This aided in dropping most locations within range of their
normal afternoon temperatures for this time of the year... which
comes after several days of above average temperatures for the
region. The area has also seen a slight drying of the atmosphere
over the course of the day... especially so at the low to middle
levels. Precipitable water values have fallen twenty percent from
1.0" to 0.8" over the previous 24 hours. Model data and soundings
show a rather dry layer situated directly atop the marine layer
this evening which is inhibiting typical Summer evening coastal
stratus development. Visible satellite shows a stratus free
coastline as far north as Point Arena so far this evening.
However... coastal stratus is expected to develop later tonight
and push inland by morning.
As high pressure shifts downstream... a closed low situated
around 500 miles west of northern California is gradually trending
atmospheric heights and temperatures lower across the region.
Temperatures are expected to dwindle further Monday and Tuesday as
onshore flow and marine influence increases. Temperatures will
be at or below normal on Monday and Tuesday afternoons with
Tuesday being the coolest day of the week. Futhermore... this
feature is sending pulses of energy and moisture towards California
from the southwest. Models hint convection may be possible further
north and east of our forecast area given the right combination
of energy and moisture...but slim to unlikely here. Drizzle along
the coast tomorrow morning looks like our best chance for any
precipitation... but can not completely rule out a stray shower or
two advancing inland from the west.
Temperatures rebound Wednesday and Thursday as the weather pattern
retrogrades upstream... forcing the closed low further west and
paving the way for the Desert Southwest ridge to encroach into the
Previous discussion...as of 01:30 PM PDT Sunday...after a cloudy start to
the today...much of the Bay area clear out...even some coastal
locations. Despite the clearing temperatures today are running a
few cooler than yesterday. This is most likely due to the filtered
sunshine from high clouds overhead and slightly cooler airmass.
The long wave pattern continues to show an upper level ridge
covering much of the west. However...a closed upper low and broad
trough remain parked west of 130w. The circulation associated with
the trough/low is helping to stream the upper level clouds over
the Bay area. Latest guidance shows the ridge weakening with the
low remaining nearly stationary through middle week. Therefore...day
to day change of sensible weather will be small. Night and morning
clouds will be common with afternoon sunshine. Temperatures will
remain seasonably cool with highs in the 60s/70s coast and 80s/low
-mid 90s inland.
Temperatures will warm up briefly for Wednesday and Thursday as
shortwave ridging develops over the region thanks to a deepening
upper low to the west. Coastal clouds will keep temperatures in
check...but inland areas will be more seasonable with more
locations in the 90s.
Temperatures moderate on Thursday but cool down Friday and over
the weekend. Another upper low is forecast to develop off the
coast late in the week and gradually move inland...the reason for
the cool down. In addition...some of the model guidance suggests
possible precipitation for portions of California. Moisture and
instability look marginal at this point for the Bay area. Better
chance north of the County Warning Area. Given that time period is still several
days away and guidance is not the best...will not include any
mention of precipitation. However...a big however...will have to watch
this closely given recent fire weather concerns around the state.
Especially from a possible convection standpoint impacting current
fire fighting efforts.
Aviation...as of 5:21 PM PDT Sunday...a closed upper level low
remains located 600 miles west of Cape Mendocino. Much drier and
somewhat cooler air in the lower to middle levels well east of the
low center has helped erode the coastal stratus clouds today.
Mainly westerly winds will continue at the surface while SW winds
continue aloft. Stratus will redevelop tonight...but will be
slower than last evening's redevelopment as modest levels of
mixing continue at the interface between the top of the marine
layer inversion and the lower level cooler/drier air.
Vicinity of ksfo...VFR. Westerly wind gusts to 20-22 knots
forecast until 04z. Coordinate with cwsu Oakland and agree stratus likely
to redevelop but not until very late...00z ksfo taf advertises tempo
MVFR ceiling 09z-13z followed by prevailing MVFR until 18z Monday.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR for most of the evening followed
by tempo IFR ceilings 06z-10z. IFR ceilings late tonight. Clearing returns
by late Monday morning.
Marine...as of 07:56 PM PDT Sunday...weak low pressure off the
Southern California coast will generally result in light winds
across the coastal waters through Tuesday. However...locally
stronger winds will exist through this evening off of the Big Sur
coast. By midweek...high pressure will start to build off the
northern California coast bringing increasing northwest winds.
Seas will be mixed...northwest swell and long period south to
southwest swell into next week.
... Pinos to pt Piedras Blancas 0-10 nm until 9 PM
Small Craft Advisory...sf Bay until 10 PM
Public forecast: drp
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