Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 281813

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1113 AM PDT Fri Oct 28 2016

.SYNOPSIS...Rain will rapidly taper down through the remainder of
the morning. Then, wet and unsettled conditions will persist
through the upcoming weekend as a series of storm systems impact
the region. Temperatures will remain near or slightly below
seasonal averages through the forecast period.

&& of 09:45 AM PDT Friday... Water vapor imagery
shows the core of the storm system responsible for bringing
widespread, mostly uniform, rainfall across the area has now moved
inland and is presently over the Bay Area. The frontal boundary
ahead of this upper level system interacted with a very moist
tropical air mass advected northward by former Hurricane Seymour
to bring predominately light to moderate rain showers to the area
yesterday, and more steady moderate to locally heavy rainfall to
the area overnight. In general, it appears the North Bay mountains
of Sonoma county picked up the most precipitation, with up to
2.67" around Guerneville, Venado, and Monte Rio. Elsewhere,
rainfall amounts were unusually uniform, with almost 2 inches in
the Santa Cruz mountains, Hollister, and Bodega Bay and anywhere
from 1-1.5" for most other locations, even those typically cloaked
in a rain shadow.

In general, precipitation amounts ran on par for most populated
areas, but less than anticipated for higher elevations which
typically see more orographically induced rainfall. This likely
comes as winds which could have contributed to orographic
precipitation were incredibly light ahead of the nearly stalled
surface front. Despite the incredibly moist air mass associated
with this system (more than 200% of normal), the lifting
mechanisms were just not there to support the previously modeled
large precipitation accumulations. Interestingly enough, it was
evident by yesterday morning that nearly every single model had
initialized very poorly and were overanalyzing the storm system.
We shouldn`t be too terribly surprised as the models were dealing
with an atypical pattern given the presence of Seymour all the
while being fed very sparse and poor initialization data due to
the lack of observations over the ocean.

For today, KMUX radar is already showing precipitation now
clearing out from west to east as the core of the upper low shifts
inland. As such, except rain shower activity to rapidly come to a
close by late morning. A few lingerings showers will develop into
the afternoon, but they will dissipate rapidly as the bulk of the
moisture and energy will have then exited the area.

Looking ahead, another low pressure storm system will descend into
the region later tomorrow. This storm system will bring breezy
southwest winds ahead of the front and more typical stratiform
frontal precipitation that we are used to seeing with our
fall/winter weather systems. Rain will begin to fall in the north by
Saturday afternoon/evening before spreading southward across the
entire area by Saturday night into Sunday morning. Most
precipitation is anticipated to fall over the North Bay once again,
with up to 1.5-2" locally expected over the wettest areas.
The Santa Cruz mountains could see 1-1.5". Elsewhere, most populated
locations will see lesser amounts ranging from a quarter of an inch
to half an inch.

A weaker system arrives later Monday where it will primarily clip
the north bay later Monday into Monday night. Then, shortwave
ridging will bring drier conditions the area through midweek.
Model solutions diverge beyond midweek next week.

.PREVIOUS of 03:32 AM PDT Friday...Widespread
precipitation continues across the region this morning as an
upper level low remains about 200 miles to the west-southwest of
the Monterey Peninsula. Rainfall rates generally remain light but
have been as high as 1/2" or so per hour at times within heavier
showers. Overall, precipitation amounts have been much less than
previously thought and rain rates are also not as impressive.
Thus, have cancelled the Flash Flood Watch for the Soberanes and
Chimney Fire burn scar areas given the lessening threat for high
rainfall rates that would result in debris flows. With that said,
widespread precipitation will continue through the morning hours
as the upper level low lifts northeast toward the San Francisco
Bay Area. While the best chance for thunderstorms remains to our
south, cannot rule out the possibility of a few thunderstorms
developing over the Central Coast as the upper level low nears the
coast this morning. Widespread precipitation will tapper off to
lingering showers by this afternoon as the main mid/upper level
system pushes inland.

Given the unsettled pattern, can expect lingering showers to
persist into Saturday, yet widespread rainfall is not expected.
Our next weather system will approach from the west/northwest late
Saturday night into Sunday morning as a mid/upper level through
approaches the Pacific Northwest. This will bring another around
of widespread precipitation to the region, likely beginning over
the North Bay Saturday night and spreading south and inland
through Sunday morning. Lingering showers will again be possible
Sunday night into Monday in wake of the passing frontal boundary.
As another mid/upper level trough approaches the Pacific Northwest
and northern California Monday night into Tuesday, another round
of precipitation is expected across the region. However, the
forecast models are not showing much precipitation with this third
system as it rapidly pushes inland to our north.

Mostly dry conditions return to the much of the region by midweek
as short-wave ridging builds in aloft. Meanwhile, cannot rule out
a slight chance of precipitation over the northern portion of the
region late in the week as forecast models show a few weak systems
pushing inland to our north. With the persistent unsettled patter,
expect mostly cloudy conditions to continue along with daytime
temperatures below seasonal averages.

&& of 11:10 AM PDT Friday...For 18z TAFs. The
widespread rain will taper off through early afternoon. Visible
satellite currently shows a mixture of high and low clouds over
the area, with the general consensus of MVFR cigs being reported.
That said, another difficult cig forecast in store today through
this evening. Models are still having trouble with this system,
even as it is exiting the region. Overall, a mixture of VFR/MVFR
cigs expected this afternoon. Generally light westerly/southwesterly
winds through this evening, around 10 kt. MVFR/IFR cigs return
overnight. Would not be surprised if fog develops during the early
morning hours on Sat for some locations, due to saturated soils.
Low confidence on cig timing.

Vicinity of KSFO...Mixture of MVFR and VFR through today. Cigs
will drop tonight, possibly by 08-09z. Low confidence on exact
timing. Winds will be on the light side, with onshore winds around
10 kts. GFS lamp is hinting at some fog developing at KSTS early
tomorrow morning.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Mainly MVFR cigs today, with IFR possible
at times. IFR cigs return early this evening. Fog possible around
sunrise. Rain chances will significantly decrease by 21z for the
Monterey Bay area. Light to moderate westerly winds this
afternoon, 10-12 kt.

&& of 10:32 AM PDT Friday...Light to moderate showers
will eventually taper off this afternoon across the coastal waters
as a low pressure system exits the area. Winds will be light out
of the west to northwest today with winds switching back out of
the south tonight. Light winds and seas will persist through much
of the period. The next in a series of storms will arrive from
the northwest on Saturday morning. Rain develops Saturday night
into Sunday with a return of gusty west to southwest winds by
Sunday afternoon ahead of another cold front, along with rapidly
building seas.





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