Scientific Forecaster Discussion
fxus66 kmtr 142053
Area forecast discussion
National Weather Service San Francisco Bay area
153 PM PDT Mon Oct 14 2019
Synopsis...slight warming is expected Tuesday ahead of a weak
frontal system that will move through the region late on Wednesday.
This may produce light rain over the northern portions of the
North Bay late Wednesday and Wednesday night. Post-frontal breezy
conditions and cooler temperatures are expected from late
Wednesday through Thursday and may persist into the upcoming
Discussion...as of 01:53 PM PDT Monday...satellite imagery shows
low clouds just offshore with mostly clear skies over land.
Temperatures across the region are in the 60s and 70s a few
degrees cooler than yesterday. Some additional warming can be
expected the rest of the afternoon with highs in the mid 60s to
low 70s at the coast to 70s and low 80s inland. Synoptically, the
upper trough that has been over much of the West Coast has begun
to shift eastward while a weak short-wave ridge starts to move in
to take its place. Behind this ridging is an upper low in the Gulf
of Alaska with a cold front approaching California and the
These features will drive our weather pattern for much of this
week. The weak short wave ridge will move over California by
tomorrow allowing for a slight increase in temperatures across
the region. Coastal areas will remain in the upper 60s to low 70s
with 70s to low 80s across the interior. Isolated inland locations
will reach the mid 80s. This ridging will be quickly overcome by
an upper low approaching western Canada with a trough extending
southward along the West Coast of the U.S. This will allow
temperatures to cool some on Wednesday ahead of the system.
A weak cold front will then move into northern California late
Wednesday and Wednesday night bringing a slight chance of rain to
the North Bay. Not more than a few hundredths is expected out of
this system with measurable rainfall south of Sonoma County still
unlikely. The upper trough and surface cold front will also act
to cool temperatures further across the region with afternoon
highs across the interior in the 70s by Thursday. Locally gusty
northwesterly winds are expected behind the front into Thursday,
mainly along the coast. These winds will accompany higher relative
humidity values lessening fire concerns. Breezy afternoon
conditions may continue through late this week and into the coming
weekend with temperatures just below seasonal averages.
Looking into this weekend, models show a shortwave trough moving
in to our north on Saturday. Both the 12z GFS and the European model (ecmwf) show
some showers over the far northern portions of the region, but
differ on timing. Regardless, the best shower chances will be
outside of our area. As the trough continues to shift inland into
the Great Basin over the weekend, models do show northerly winds
developing by Sunday and Monday as an upper ridge builds over the
eastern Pacific. These winds do not look to be strong enough to raise
significant fire weather concerns at this point. This set up will
also bring some warming and drying to the region into early next
Aviation...as of 11:00 am PDT Monday...for 18z tafs. Satellite
imagery indicates stratus clearing to the coast this morning with
all taf sites now VFR. VFR conditions will persist through the
afternoon hours. Weak pressure gradients will lead to light winds
today, becoming onshore this afternoon. A ridge of high pressure
will move over the area today through tonight. This may compress
the marine layer somewhat so any cigs tonight may be lower in some
places than this morning. Cigs will redevelop in some locations
overnight tonight, especially near the coast, with fog likely once
again in the North Bay valleys and ksts.
Vicinity of ksfo...VFR conditions are forecast to prevail.
Light northerly winds will become onshore this afternoon around
10-15 kt. Models suggest patchy stratus will develop over the Bay
late tonight through Tuesday morning. At this time not expecting
cigs to develop at the terminal through the taf period.
Sfo bridge approach...VFR through the afternoon and evening.
Patchy cigs may develop at times over the approach on Tuesday
morning, but low confidence on extent.
Monterey Bay terminals...cigs have cleared and VFR will continue
through the afternoon. Light onshore winds through the afternoon
becoming light southeasterly overnight. Patchy cigs developing
this evening and will continue overnight through Tuesday morning.
Marine...as of 2:35 am PDT Monday...generally light northwest
winds will prevail over the coastal waters through Columbus day.
Mixed seas will continue over the next few days with a light
shorter period northwest swell and a light southerly swell. A
larger northwest swell is forecast to arrive later this week with
forerunners expected in the next couple days.
Public forecast: as
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