Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
544 PM PDT Wednesday Sep 28 2016
..light rain possibly returns for northern parts of our area
starting on Sunday...
Synopsis...cooling trend will continue into the weekend as the
marine layer and onshore flow become better established with a
trough along the West Coast. In addition, a dry cold front will go
through on Friday which will lead to breezy conditions. A system
will drop down from the north over the weekend which will bring
rain to our area especially over the North Bay.
Discussion...as of 2:30 PM PDT Wednesday...as expected
temperatures have taken a tumble today for many inland locations
due to a return of onshore flow, the ridge heading to the east,
and a building marine layer. A few spots are sitting more than 15
degrees cooler than Tuesday at this time.
This is the start of a substantial change in our synoptic pattern
which will feature the ridge continuing to shift farther to the
east as a system associated with a longwave trof dives down the
West Coast. Out ahead of that an associated dry cold front will
move through our area late on Friday. As the front approaches
winds will steady increase from the west and northwest through the
day with locally gusty winds to over 30 miles per hour possible. This will
produce an elevated concern for fire weather. Winds will abate on
Saturday. Along with the stronger winds the shift in the pattern
will lead to cooler temperatures. By Saturday all locations should
be just in the 60s to mid 70s.
On Sunday a system that will be dropping down from the north will
move into northern California. Rainfall associated with a frontal
system now appears likely to move into the North Bay on Sunday
and continue to progress to the south through the day. Models are
now in good agreement that the rain will make it at least as far
as San Francisco with less agreement to Monterey Bay. Rainfall
amounts will range from 1/3" in parts of the North Bay to a tenth
or less for San Francisco and southward. Instability will also be
on the increase with the frontal passage which could trigger a few
thunderstorms. Will take a closer look at that tonight before
adding it to the grids. Although totals will be light, due to the
amount of oil that has collected on roads, diving conditions could
be quite tricky with the onset of rainfall. Worth noting that the
Gem has up to 1/2" down to Monterey Bay. At this time, that is an
outlier and appears to be an unlikely solution.
Behind the front a secondary piece of energy may go through Monday
and Monday night. Models are in less agreement with this feature
for both coverage and amounts. That piece will likely produce less
than 1/10" in most spots. Temperatures will continue to be cooler
than normal on Monday.
A ridge of high pressure will start to rebuild back to our region
by the middle of next week. This will lead to a warming trend
along with dry conditions.
Aviation...as of 5:15 PM PDT Wednesday...a deepening marine layer
and increasing onshore gradient will allow low clouds to move
farther inland this evening and tonight. Low clouds will fill in
over most area terminals overnight with clearing anticipated
around 17z-18z Thursday. Moderate winds will ease this evening
becoming light overnight.
Vicinity of ksfo...VFR conditions will prevail this evening with
borderline VFR/MVFR cigs developing after 08z.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...IFR cigs expected this evening. Cigs and
vsbys will lower to LIFR after 06z tonight. Clearing expected
between 18z-19z Thursday.
Fire weather...as of 2:45 PM PDT Wednesday...marine layer will
continue to deep over the Bay area the next few days leading to
better recoveries and cooler temperatures. The most notable
weather concern over the next 48-72 hours will be increasing
winds on Friday afternoon. A passing dry cold front will bring
gusty northwest winds to the coast and higher terrain. Models
continue to show gusts in the 20-30 mph will be possible Friday
evening into Saturday.
Marine...as of 2:17 PM PDT Wednesday...high pressure building
off the northern California coast will bring increasing northwest
winds to the northern waters through tonight. The high will
weaken Thursday as an upper level trough deepens off the West
... Arena to Pigeon pt 10-60 nm
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm until 3 am
Small Craft Advisory...sf Bay until 10 PM
Public forecast: Bell
fire weather: mm
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