Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 101133
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
335 AM PST Sat Dec 10 2016

.SYNOPSIS...Showers are expected again today through Sunday. High
pressure building over British Columbia will spread cold air
southward across the area on Monday. A wintery mix of
precipitation is possible as the cold air mass spreads south.
Dry northerly flow aloft next week will maintain a cold dry
weather pattern for much of next week.

&&

.SHORT TERM...Looks like activity is calming down out
there...according to current radar imagery. Still some showers out
and about though...and what looks to be possibly a convergence zone
on the Snohomish/Skagit county line. Temps up that way look to be in
the mid to upper 30s though...so likely either all rain or mostly
rain with a few flakes or icy bits mixed in for shock
value...nothing headline worthy. Speaking of headlines...current set
looked to either expire or come close to expiration at the time of
this writing. Felt it best to take an ax to them all...given current
radar coverage...or lack thereof.

Moving into forecast mode...a series of shortwave disturbances look
to bring showers to the area again today...most likely in the
afternoon as models hint at a bit of a break in precip this
morning...which...given current radar...seems legit. As the upper
level low draws closer late this afternoon and evening shower
activity looks to increase area-wide and linger into Sunday. Temps
look to remain in the upper 30s to low 40s...both highs and lows as
there will not be much difference between the two...and thus looking
at mostly rain for this particular time frame.

A cold front looks to pass through Western WA in the wee hours of
Sunday night and into early Monday morning. Due to a series of
unfortunate events...this time period looks to fall under heavy
scrutiny for the prospect of lowland snow. Models cannot agree on
moisture and moisture amounts and the track of the surface low...all
three of these factors prove to be critical factors in determining
risk. GFS and NAM suggests bulk of moisture will be aimed more
toward OR as the surface low takes a bit more of a southerly track
while the ECMWF brings the low right onto the Olympic peninsula and
though OR is still favored in moisture amounts...W WA sees its fair
share as well. Combining this uncertainty with the one feature that
they all seem to agree on...that being the onset of Fraser outflow
conditions thanks to the pairing of the aforementioned low and high
pressure over BC thus bringing arctic air back over the CWA...and
yes...the prospect for lowland snow does exist. Thankfully even
though the ECMWF has better moisture and therefore more likely
higher precip chances...it is just about as progressive as the other
models and as such...still only provides a narrow window as to when
any potential snowfall could occur before conditions dry out.
Ultimately...models are still having a hard time getting things to
line up in such a way that would produce confidence in the prospect
of lowland snow. As such...current forecast puts chances of a
rain/snow mix Monday at around 40-ish percent...in other words...low-
end chance...and that seems a fair assessment at this time until
some model cohesion starts to develop.  SMR

.LONG TERM...The long term forecast looks fairly simple...especially
given the above paragraph. A couple of deep upper level troughs over
the center of the country will keep any precip either south or east
of the CWA for the duration of next week. The good news is no rain
or snow and might even see the sun a few days. The bad news is its
going to be cold with daytime highs in the mid 30s and overnight
lows in the mid to lower 20s.  SMR

&&

.AVIATION...Weak high pres aloft will prevail today. Contd wly
flow aloft. Low level sly flow will strengthen some this afternoon
in response to an approaching surface trof, which will move across
the area today.

Meanwhile...Areas of MVFR CIGs will become widespread during the
day. There will likely be pockets of -sn or -fzra at the lower
elevations of Whatcom County and (especially) in the Cascade valleys
this morning. The mtns will be obscd.

KSEA...VFR conditions should not last long. MVFR CIGs are expected
to return by 1800 UTC (10 AM PST, this morning). Winds will remain
sly. Anticipate ocnl gusts near 20 knots tonight.

&&

.MARINE...
A surface trough extending from a 999 MB Low west of northern
Vancouver Island was well offshore early this morning. This trough
will move across the area today. Expect increased onshore or
westerly flow behind it. Onshore flow will weaken on Sunday.
Anticipate a weak modified arctic front to move south across the
Northern Inland Waters Sunday night.

&&

.HYDROLOGY...No flooding is expected over the next week.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.

PZ...Gale Warning in effect until 10 PM PST tonight for the coastal
  waters from Cape Flattery to Cape Shoalwater from 10 NM out
  to 60 NM.

 Small Craft Advisories are in effect for the remaining waters.

  Small Craft Advisory for rough bar in effect from 4 PM today
  until 10 AM PST Sunday for the Grays Harbor Bar.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html


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