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Area forecast discussion 
National Weather Service San Francisco Bay area
448 PM PST Monday Dec 22 2014

Synopsis...aside from early morning coastal and valley fog...dry
weather conditions and mild temperatures will persist through
midweek as high pressure remains over the region. A weak storm
system will then move through northern California and bring a
chance of light rain and cooler temperatures back to the region
late Wednesday into Thursday.

&&

Discussion...as of 01:34 PM PST Monday...high pressure will
continue to dominate our weather pattern through midweek with
above average daytime and overnight temperatures. Given the
recent rains across much of the region...as well as abundant low
level moisture...early morning fog will once again be likely near
the coast and across valley locations the next few mornings. The
warmest of the days will likely be Tuesday afternoon...with
temperatures 5 to 15 degrees above seasonal averages.

The ridge is then forecast to weaken by midweek as a middle/upper
level trough digs southeast toward the Pacific northwest. This
system will drop into northern California and across Nevada late
Wednesday into Christmas day. As a result...light rain will be
possible over a portion of the region late Wednesday and
Thursday. However...the fast moving system will keep rainfall
amounts at a minimum south of the San Francisco Bay area.
Meanwhile...the North Bay may pick up a few tenths of an inch
from this passing system. Offshore winds are also forecast to
increase in wake of the frontal passage with breezy and gusty
conditions possible in the higher elevations. Will have to
continue to monitor wind speeds for Thursday and
Friday...especially given the saturated soils and possibility of
downed trees.

In addition...cooler temperatures will return Thursday to the
region in wake of the frontal passage. By Thursday
night...temperatures will likely dip into the 30s and 40s with
the coolest locations being over the North Bay valleys. Near
normal temperatures will then persist through late week and into
the upcoming weekend as the middle/upper level ridge remains off the
West Coast and another reinforcing shot of cooler air slides down
the western portion of the country. Will keep the forecast dry at
this time for late in the forecast period as the secondary system
that drops across the region appears to be lacking sufficient
moisture.

&&

Aviation...as of 4:45 PM PST Monday...it's a challenging forecast
just ahead for the overnight and Tuesday time-frame. An upper level
high will remain parked over the area. The wmc-sfo pressure gradient
is presently neutral then forecast to strengthen up to 11 mb by 12z
Tuesday then weaken to 6 mb by 00z Wednesday. Most of the gradient is along
the Sierra Nevada but the NAM model does show the surface trough
temporarily migrating offshore by 09z-12z Tuesday as well indicative
of the potential for the fog to clear earlier in the Bay area tomorrow
morning. The thought is that the 11 mb of offshore and the briefly
westward migration of the surface trough will help keep the areal
extent of the fog smaller tonight and Tuesday morning compared to
this morning. Pressure gradients from Point Arena buoy to sts and
uki-sts are presently directed from the north at 3.8 mb and 1.8 mb
respectively. This supports drier north-northwest wind flow recently observed
at ksts. In summary it's a moderate to high confidence VFR forecast
for the evening but there's still lowering confidence regarding fog
coverage very late tonight. For Tuesday...think the morning fog
patches will scatter out and VFR returns a bit faster compared to
this morning.

Vicinity of ksfo...additional aviation challenges just developed
as a 10 knot west wind is helping to nudge in a low cloud patch
from the San Bruno gap. Scattered-bkn007 tempo not in 00z taf but will
do additional coordination with the cwsu Oakland and figure a soon
to be issued amendment.

Ksfo bridge approach...similar to ksfo.

Monterey Bay area terminals...MVFR forecast to return by 04z.
Local IFR forecast after 08z. Light winds trending toward southeast 5-9
knots very late.

&&

Marine...as of 9:05 am PST Monday...a building ridge of high
pressure along the coast will keep moderate northwesterly winds
going across most of our waters into Tuesday. Winds will decrease
by Tuesday night as the high moves into the Great Basin. Another
round of moderate northwesterly winds are expected on Thursday as
another high strengthens over the eastern Pacific.

&&

Mtr watches/warnings/advisories...
... Flood Advisory...coastal and Bay Shore areas
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm until 3 am
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm until 3 am
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm until 3 am
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm until 3 am
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm until 3 am
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm
Small Craft Advisory...Pigeon pt to pt pinos 10-60 nm

&&

$$

Public forecast: rgass
aviation: canepa
marine: Bell

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