Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 071134
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 AM PST Wed Dec 7 2016

.SYNOPSIS...Cold dry air will be over Western Washington again
today. Easterly gap winds will increase tonight. A weather system
will arrive from the south Thursday afternoon and heavy lowland snow
is possible from then through Friday morning. Cool wet weather is
likely Friday through the weekend.

&&

.SHORT TERM...Conditions remain cold and dry this early morning with
sub-freezing temperatures area-wide. Fraser outflow looks to keep
bringing cold air into the area...as well as breezy conditions to
northern portions of the CWA. Pressure gradients look to slowly
increase through the day today resulting in increasing easterly
winds...particularly in the Cascade foothills and passes.

So...Thursday. An weather system from the south will start to make
its way into the area right around noon on Thursday and continue
northward as the day progresses. That is the easy part. What that
system will bring with it and how it will impact Western
Washington...that is the hard part. The warmer air associated with
this system is expected to override the arctic air that is currently
in place...resulting in precip starting off as snow then gradually
changing over to rain...eventually. Unfortunately...it is in these
fine details...such as when the transition will occur...where the
models have difficulty agreeing...even with themselves. For
example...looking at GFS ensemble members this morning shows the
operational GFS...the one typically looked at...to be a serious
outlier when it comes to potential snowfall amounts while the
ensemble mean looks to be far more reasonable and...very
fortunately...in line with inherited forecast amounts. All that
being said...given that models have not changed too much from progs
that previous shift briefed from...will opt to leave inherited
forecast alone for the bulk of this system and Winter Storm Watch
looks on track with its amounts. Slightly staggered WSW for Cascades
and Olympics also looks to be on track.

Friday should see the transition from snow to rain completed during
the morning hours...with mixed precip likely rearing its ugly head
at some point before the full transition to rain during the
afternoon and continuing for the remainder of the day.  SMR

.LONG TERM...GFS and ECMWF both hint at a bit of a break in the
action for Saturday although their timings are not in
agreement...the ECMWF being about 12 hours slower than the GFS
solution. A return to wet conditions comes Sunday before...dare it
be typed...an actually dry start to the week ahead with
Monday...Tuesday and possibly even into Wednesday being mostly
precip free...with lowland high temps right around 40 each day.  SMR

&&

.AVIATION...Northerly flow aloft ever Western Washington early
this morning will continue today, switching to southeast late
today and tonight as a Pacific frontal system starts to approach
the region. At low levels the flow will remain easterly. The air
mass will be dry today and tonight, with no significant cloud
cover below 12000 ft. There will be patchy freezing fog this
morning in the most favored locations, mainly in the south Puget
Sound region and southwest interior.

KSEA.... Wind north to northeast 4-10 kt. Mostly clear skies today
and tonight. Snow is possible Thursday afternoon and night.
McDonnal

&&

.MARINE...Easterly flow will increase today as a strong Pacific
frontal systems slowly approaches the region, with gale force
easterlies expected over the coastal waters, west entrance Strait
of Juan de Fuca and central Strait beginning tonight and
continuing into Thursday night. The gale watch is being upgraded
to a warning, and confidence in this event is high. For the
remainder of the inland waters, the wind forecast is less certain
but it looks like the winds will probably remain just below gale
force.

The winds will probably shift to southwest over the coastal waters
on Friday, in the wake of the cold front or occluded front associated
with the system discussed above. There is not much confidence in
forecast particulars for Friday (or beyond) though, as model
solutions continue to vary widely. Specifically, most GFS
solutions bring a fairly deep low into the offshore waters Friday
or Friday night; however, even the GFS is not very consistent from
run to run. The ECMWF has a weaker low much further offshore. In
intentionally vague and general terms, the back side of the
frontal system will probably move inland Friday or Friday night,
with a trough remaining over the area this weekend. McDonnal

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Watch from Thursday morning through Friday morning
     for Admiralty Inlet Area-Bellevue and Vicinity-Bremerton
     and Vicinity-East Puget Sound Lowlands-Eastern Strait of
     Juan de Fuca-Everett and Vicinity-Hood Canal Area-Lower
     Chehalis Valley Area-San Juan County-Seattle and Vicinity-
     Southwest Interior-Tacoma Area-Western Skagit County-
     Western Whatcom County.

     Winter Storm Watch from Thursday afternoon through Friday
     afternoon for Cascades of Pierce and Lewis Counties-
     Cascades of Snohomish and King Counties-Cascades of Whatcom
     and Skagit Counties-Olympics.

PZ...Small Craft Advisory from noon today to 9 PM PST this evening
     for Central U.S. Waters Strait Of Juan De Fuca-Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From James Island To Point Grenville
     Out 10 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
     To Cape Shoalwater Out 10 Nm-West Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Gale Warning from 9 PM this evening to midnight PST Thursday
     night for Central U.S. Waters Strait Of Juan De Fuca-
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     Coastal Waters From James Island To Point Grenville Out 10
     Nm-Coastal Waters From Point Grenville To Cape Shoalwater
     Out 10 Nm-West Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 9 PM this evening to 3 AM PST Friday for
     Coastal Waters From Cape Flattery To James Island 10 To 60
     Nm-Coastal Waters From James Island To Point Grenville 10
     To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm.

     Small Craft Advisory until 10 AM PST this morning for Northern
     Inland Waters Including The San Juan Islands.

&&

$$

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