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FXUS66 KMTR 171739

Area Forecast Discussion
National Weather Service San Francisco Bay Area
939 AM PST Tue Jan 17 2017

.SYNOPSIS...Increasing clouds but dry with seasonable temperatures
today. Rain develops over the North Bay by midday Wednesday with
increasing southerly winds through the day and evening as rain
spreads from north to south into Weds night. Some lingering
showers Thursday but mainly dry. Next front quickly brings the
next round of rain late Thursday night into Friday morning before
tapering to showers by Friday evening. Mainly dry Saturday except
for isolated showers. Third and final storm in the series arrives
by early Sunday with periods of rain through the day turning to
showers Sunday night into Monday. 

&& of 08:54 AM PST Tuesday...High clouds continue to
spread inland across the region this morning in advance of
Wednesday's storm system. Meanwhile, low clouds also impacting
portions of the North Bay and East Bay this morning with
temperatures region-wide in the 30s to lower 40s. Expecting
breaks in the cloud cover through the day with ongoing dry weather
conditions and temperatures warming into the 50s to lower 60s. The
ongoing forecast remains on track for today and no changes are
needed at this time. Please see the previous discussion below for
details on the upcoming storm systems.


.PREV of 4:41 AM PST Tuesday...Its the calm
before the storm today. Satellite and surface obs show some dense
fog in the North Bay with both Santa Rosa and Napa reporting 1/4
mile fog as a band of high clouds passes over the region. The band
of high clouds is out ahead of the next weather system. However at
this time of year just the existence of mid and high clouds will
be enough to keep temperatures seasonably cool over the Bay Area
and slow the burn off of morning fog thats entrenched deep in the
North Bay valleys with limited sunshine to promote mixing.
Afternoon highs will struggle into the low and mid 50s over the
Bay Area with some warmer readings near 60 around the central
coast with more sunshine.

All attention then turns to our next cold front. Models have been
pretty consistent with timing and intensity of the next few
systems. First off the storms this week are very different than
last weeks storms. They will be much faster moving, have less
available moisture and be much cooler with lower freezing levels.
The first system due to bring rain to the North Bay by late Weds
morning is currently tied into a plume of 1.8 inch tpw air.
However the models keep the main tongue of moisture offshore.
Expect a quick burst of moderate to heavy rainfall as the front
moves from north to south through the region Weds afternoon and
evening. Have posted a short duration wind advisory to account for
strong and gusty winds that will precede the front as well as
occur during the frontal passage when gusts of 40-50 mph are
forecast. Current model timing suggests the Weds afternoon commute
will be most adversely impacted when short duration heavy rain
rates will likely lead to urban and small stream flooding. Not
expecting the main stem rivers to be impacted like we saw last
week as the fast moving nature of this system will preclude
excessive rainfall totals. On average it looks like a quick inch
or so for the valleys with 2-3 inches for the coastal hills from
Sonoma southward to Monterey county including the Santa Cruz mtns.

Post frontal instability showers will continue on Thursday but
much of the day will be dry with the showers most likely confined
to the coast and inland hills.

Second storm will quickly arrive late Thursday night into Friday
morning with another round of steady rainfall. The structure and
evolution of this system looks very similar to the Weds storm.
Again this will be a fast moving frontal passage with a few hours
of stratiform warm advection rain before the cold front brings a
few hours of heavy rain and gusty winds. The saturated soils
remain a concern for another round of localized urban and small
stream flood issues with possible wind advisories needed as well.
There are many areas in the cwa that are still cleaning up
from last weeks storms in terms of damaged roads and mudslides.
These storms will obviously not help those situations. The Thu/Fri
system will again bring about 1 inch of rain to the lower
elevations with 2 to 3 inches on average for the hills. Snow
levels will drop as low as 3500 feet but for the most part not
impact any populated areas. The low freezing levels will of course
coincide with some chilly overnight lows in the upper 30s and 40s
with the cold rainfall to impact homeless and other vulnerable

The forecast for Saturday is similar to what we expect on Thursday
with isolated/lingering showers but for the most part day.

Then the final in a series of storms should arrive by early
Sunday morning. This system may have the potential to bring the
most rainfall but the jury is still out on the exact qpf numbers.
High confidence though for at least another round of moderate rain
for much of Sunday before turning to showers Sunday night into

GFS is trending drier next week while the Euro doesnt show any
significant storms but keeps a cool trough with possible lingering
precip through Monday and another weak shortwave by the middle of
next week. Will be interesting to see if we finally get a pattern
shift or not as January remains very wet and active.


.AVIATION...As of 9:40 AM PST Tuesday...Areas of low clouds and
fog around the North Bay and also edging eastward from the Central
Valley into the East Bay. Hazy conditions are occurring around the
San Francisco Bay with some slant visibilities to 5 miles.
Otherwise, high clouds are increasing over the area and will
gradually lower tonight. Southeast winds will ramp up tomorrow
ahead of an approaching weather system.

Vicinity of KSFO....Mainly VFR today with some slant vsby down to
5 miles in haze. Light east winds becoming southeast by Wednesday
morning and increasing.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR. 

&& of 08:54 AM PST Tuesday...High pressure off the
central California coast will weaken today as a low pressure
system approaches the Pacific Northwest. Southerly winds will
increase Wednesday ahead of a frontal system. Winds will decrease
late Wednesday night as the low lifts northeast but will increase
again by Friday as another storm approaches. Very large west
swells will arrive later this week with swells building as high as
20 to 25 feet Friday and Saturday.






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