Winter Storm Warning 12, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PST Mon Nov 27 2017

.SYNOPSIS...A weak ridge over the area today will give mostly dry
conditions. A front will bring rain Tuesday, along with very windy
conditions in the north interior and snow in the mountains. Things
will dry out Wednesday, then another front will arrive Thursday.

&&

.SHORT TERM...Radar shows scattered showers in the mountains and
on the coast but not much in the interior lowlands. This should be
the pattern for the rest of the day. Highs will be near 50.

A weather system will approach the area tonight and move inland
Tuesday. There are high pops on the coast late tonight, and these
spread to all areas Tuesday. High wind associated with this system
is likely over the north interior late tonight and Tuesday morning,
and advisory level winds are likely on the coast. Will issue a
warning and advisory later this morning.

The snow level will be between 3000 and 4000 feet. At least advisory
amounts of snow appear likely in the mountains late tonight and
Tuesday. Will coordinate with NWAC forecasters and issue a statement
later today.

Wednesday appears similar to today, with scattered showers in the
mountains and mostly dry conditions in the lowlands. Highs Tuesday
will be 45-50; on Wednesday they will be a few degrees higher. Burke

.LONG TERM...Previous discussion...Models remain pretty progressive
with the system for late Wed night/Thursday but still in
disagreement with whether or not the thing holds together. GFS does
not really allow it to deteriorate until after it has moved through
the area while the ECMWF has it disintegrating as it moves over the
area. GFS is also slightly quicker with the timing. Opted not to
stray too much from inherited POPs at this time given continued
model uncertainty.

That is followed pretty quickly by yet another frontal system and
associated trough...kicking off a wet start to the weekend as both
Friday and Saturday fall prey to the soggies. Fortunately this
system proves to be fairly progressive as well with conditions
starting to dry up late Saturday night/Sunday morning.

Models remain optimistic about next week...kind of. An upper level
ridge builds in for Sunday and while each model shows a shortwave
disturbance for Monday...the precip associated with this features
varies from model to model. The GFS brings in a pretty hearty slug
of moisture...a solution very different from 24 hours ago...while
the ECMWF remains consistent with previous solutions in that the
moisture/activity associated with this feature seems pretty minimal
at best. In terms of the forecast...have opted to go with the more
optimistic ECMWF at this time.  SMR

&&

.AVIATION...Westerly flow aloft over Western Washington this
morning will continue today as a minor upper ridge progresses from
the offshore waters across the forecast area. The upper flow will
back to southwest tonight and the warm front of a Pacific frontal
system will reach the area. The air mass will remain moist today
mainly at lower levels, then moisture will increase aloft tonight.
Stronger southeast pressure gradients will accompany the warm
front tonight with winds locally 25 kt at the coast and in the
north interior.

There was a wide range of conditions across Western Washington at
8 am, with patchy stratus giving local IFR ceilings in parts of
the Puget Sound region. There were also scattered showers, but
most stations are reporting VFR conditions. The overall trend
will be for some improvement today as the patchy stratus
dissipates, the air mass dries a bit, and showers generally taper
off. The warm frontal rains will bring deteriorating conditions
tonight especially at the coast and in the north interior.

KSEA...Southwest wind 7-12 kt today, backing to southeast tonight.
We should have VFR conditions today with the ceiling mainly aob
060. Ceilings will probably remain above 060 tonight too, with the
warm front rain beginning late tonight. McDonnal

&&

.MARINE...A weak surface ridge over Western Washington today will
shift east of the Cascades tonight, as a fairly vigorous Pacific
frontal system reaches the area. Southeast gales are likely Southerly
flow will increase as the next frontal system approaches tonight
then moves inland on Tuesday. Gales are likely over most waters
late tonight into Tuesday morning. Combined seas will build to 18
to 22 feet Tuesday night as a swell train reaches the coast.

Winds will be lighter on Wednesday with high pressure over the
area. Weaker fronts will move across the forecast area Wednesday
night/Thursday and Friday night/Saturday. McDonnal

&&

.HYDROLOGY...Minor flooding continues on the White River at the R
Street bridge. No other river flooding is expected. Following the
recent heavy rains, the risk of landslides remains elevated.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...High Wind Watch from 7 PM PST this evening through Tuesday
     morning for Admiralty Inlet Area-Central Coast-Everett and
     Vicinity-North Coast-San Juan County-Western Skagit County-
     Western Whatcom County.

PZ...Small Craft Advisory for rough bar until 4 AM PST Tuesday for
     Grays Harbor Bar.

     Small Craft Advisory until 6 PM PST this evening for 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 6 PM this evening to 9 AM PST Tuesday for
     Admiralty Inlet-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-East Entrance U.S.
     Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

     Gale Warning from 6 PM this evening to 4 AM PST Tuesday for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 6 PM this evening to 4 PM PST Tuesday
     for Puget Sound and Hood Canal.

&&

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html