Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 111104
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
304 AM PST Thu Jan 11 2018

.SYNOPSIS...A storm will move across Western Washington from the
offshore waters today, bringing rain, mountain snow, and windy
weather. An upper ridge will build and move across the Pacific
Northwest for a drying trend Friday through the weekend. Sunday
should be dry with some sunshine. Wet weather will return next
week as frontal systems move through in quick succession.

&&

.SHORT TERM...An upper ridge is moving inland across Western
Washington early this morning. A vigorous frontal system is
immediately upstream of the ridge, centered over the offshore
waters near 46N/135W. Light rain is spreading into the forecast
area ahead of the system`s warm front at 3 am, with temperatures
ranging from the upper 30s to upper 40s.

The core of the warm front will move northeast across the area
later this morning, followed quickly by the occluded/cold front
and upper trough late this afternoon. The last several model runs
have been in fairly good agreement with regard to the depth and
track of the surface low, although they show a trend toward a less
threatening wind situation. The low should bottom out around 996
mb over the offshore waters in the next few hours, then
fill/weaken rather quickly as it moves onto the coast tonight.

A wind advisory for the coast looks good, with more garden variety
breezy to windy conditions inland.

The system will bring 2 to 4 inches of water equivalent
precipitation to the mountains, with the heaviest amounts over the
Cascades south of Snoqualmie Pass. The snow level will rise to
around 3500 feet in the north and 6000 ft in the south today, then
fall to 3000 to 4000 ft behind the cold front tonight. There is a
winter storm warning in effect for the Cascades, mainly for
elevations above 3500 feet, where totals of 10 to 20 inches are
likely. Given the snow level forecast, Stevens Pass (at around
4000 feet) should receive heavy snow while Snoqualmie Pass (at
around 3000 feet) should receive little if any snow.

Showers will linger over Western Washington on Friday, as an upper
ridge builds and approaches the Pacific Northwest from the
offshore waters. The ridge will move inland Friday night and
Saturday morning, while a warm front immediately upstream and
along the north side of the ridge brings a chance of generally
light precipitation to the area -- especially the coast,
mountains, and lowland areas north of Seattle. By Saturday
afternoon and night, the area should be mainly precipitation-free.
McDonnal

.LONG TERM...The upper ridge discussed just above will build and
move further inland on Sunday. It should be a day of dry weather
across the forecast area, with some sunshine and highs in the
upper 40s to mid 50s. Wet weather should return Monday and
continue through Wednesday, as systems move through the area in
quick succession. McDonnal

&&

.HYDROLOGY...The system that will move through Western Washington
today and tonight should bring 2 to 4 inches of liquid equivalent
precipitation to the mountains. The snow level will rise to around
3500 feet in the north and 6000 feet in the south today, then it
will fall tonight. This system should pose a threat of flooding
only on the uniquely flood prone Skokomish River. Model guidance
also suggests that the White River at R Street Bridge, which is
downstream of Mud Mountain Dam, could also get close to flood
stage. If flooding does occur at these spots, it should be
relatively minor. McDonnal

&&

.AVIATION...An occluded frontal system will continue to bring rain
across Western Washington today. The flow aloft is westerly. The
air mass is moist and stable. Expect IFR to MVFR conditions
across most areas. It will be windy along the coast with gusts to
35 kt. This system will weaken as it moves inland tonight with
showers mainly impacting the coast and mountains. 33

KSEA...Rain and IFR to MVFR conditions expected throughout much
of the day. E/SE to 10 kt becoming S tonight. 33

&&

.MARINE...A strong Pacific system will impact the region today
with gales likely over the Coastal Waters. This system will weaken
as it slowly moves inland tonight. Meanwhile, seas will build to
13 to 15 feet as a swell train reaches the area tonight.

Weak high pressure will develop over the region on Friday with
generally light winds expected. A warm front will clip the coast
Friday night into Saturday. A weak cold front will approach from
the west on Sunday then dissipate offshore. A stronger frontal
system is slated for Monday. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 10 PM PST this evening for Cascades
     of Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties.

     Wind Advisory from noon today to 7 PM PST this evening for
     Central Coast-North Coast.

     Winter Weather Advisory until 10 PM PST this evening for
     Olympics.

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

     Gale Warning until 10 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.

     Small Craft Advisory until 4 AM PST Friday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca-Northern Inland Waters Including The
     San Juan Islands-Puget Sound and Hood Canal-West Entrance
     U.S. Waters Strait Of Juan De Fuca.

&&

$$

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