Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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969
FXUS66 KSEW 212322
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
322 PM PST Sun Jan 21 2018

.SYNOPSIS...A cool upper trough will maintain showers tonight with
heavy snow possible in the mountains. After a lull on Monday,
another front will cross Western Washington Tuesday night and
Wednesday. A cool showery upper trough will reach the area
Thursday.

&&

.SHORT TERM...Winds have eased from a strong cold front that moved
inland this morning. The strongest winds were over the north
interior, especially western Whatcom county and the San Juans with
peak winds near 60 MPH. The front pushed into the Cascades midday
and now we are in post-frontal onshore flow. Showers are enhanced
by a cool upper level trough and we`ve seen a few lightning
strikes near the coast and Olympics. The cool trough is will also
keep snow levels down in the mountains. Heavy snow showers are
possible in the Olympics and Cascades and the Winter Storm Warning
and Advisories will continue into early Monday morning. Showers
will taper down Monday afternoon and evening as the trough exits
east and a weak ridge starts to build over the Pac NW. Temperatures
will be near to few degrees cooler than normal.

The next front will arrive on Tuesday but looks like it will
split and weaken as it moves inland. However, it still looks windy
along the coast and north interior with gusts to 40-45 MPH possible.
Snow levels will remain low in the mountains with a few more
inches of snow expected.

Western WA will be under moist SW flow on Wednesday with a deep
upper low spinning offshore. Models are showing showers from a
trough axis as it lifts NE through the region. 33

.LONG TERM...A cool upper level trough will cross the Pac NW
Wednesday night into Thursday. This system is deep with 500 mb
heights in the 520s and temps -30C. The cooler air mass will knock
snow levels down to around 1,000-1,500 feet with a chance of snow
showers on the higher hills. The surface flow is southerly,
though, which should moderate temps. Heavy snow is expected in the
mountains. Toward the weekend, a strong warm front may bring heavy
rain to the coast and mountains. However, this front is wavering
between southern B.C. and Western WA in the models for a wide
range in QPF. 33

&&

.AVIATION...A broad upper trough will cross Western Washington
tonight, then a flat upper ridge axis will pass by on Monday
afternoon and evening. Moderate to strong westerly flow aloft
through Monday evening. The air mass is moist and somewhat
unstable, becoming more stable on Monday evening. Waves of
convective showers will pass across the area, with an upper
feature enhancing showers early Monday morning. Then showers will
become more confined to windward mountain on Monday afternoon.
Cigs will most commonly be in the 025-040 range, except lower
during the early Monday round of showers. Breaks between
convective cell will lead to temporarily higher cigs.

KSEA...One wave of showers will pass by from now through about
02z or 03z. Another wave of showers will cross the terminal
between 10z and 16z. Cigs mainly in the 025-040 range through
Monday, except a few hours below 020 during the early Monday
shower enhancement. Southerly flow continuing for next 30 hours
and beyond. Haner

&&

.MARINE...Moderate southerly onshore flow will persist
tonight, then ease on Monday morning. A warm front on Tuesday
will give strong east to southeast gradients with gales likely
over the coastal waters and eastern Strait. A weakening low center
will reach the waters off Vancouver Island on Thursday. A lull
between systems is possible on Friday, before another strong warm
front increases southeast wind on Friday night and Saturday.
Haner

&&

.HYDROLOGY...The Skokomish river in Mason county rose above flood
stage around midday today with minor flooding expected. The river
will crest tonight then slowly recede on Monday.

Rainfall through tonight will probably push the USGS landslide
indices close to thresholds for shallow landslides.

Models are showing heavy rain possible in the Olympics and
Cascades next weekend from a strong warm front. Exact impacts are
unclear as the front may waver between southern B.C. and Western
WA.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 9 AM PST Monday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties.

     Winter Storm Warning until 9 AM PST Monday for Cascades of
     Whatcom and Skagit Counties-Olympics.

PZ...Small Craft Advisory until 9 PM PST Monday 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.

     Small Craft Advisory for rough bar until 9 PM PST Monday for
     Grays Harbor Bar.

     Small Craft Advisory until 3 AM PST Monday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 9 AM PST Monday for Admiralty Inlet-
     Central U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands-Puget Sound and Hood
     Canal.

&&

$$

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