Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 171740

Area Forecast Discussion
National Weather Service Seattle WA
940 AM PST Sun Dec 17 2017

.SYNOPSIS...Rain will increase today as a warm front spreads
inland from the Pacific. Then a cold front will stall over the
region on Monday. A strengthening low center will pass across the
area on Tuesday morning, and a strong cold front will bring wind
and heavy mountain snow. Lingering showers will continue Wednesday
morning, then an upper ridge will bring dry weather on Thursday
and Friday.


.SHORT TERM...Expect rain at times today with a warm front over
the area. The trailing, slow-moving cold front will then slide SE
into Western Washington tonight then stall through Monday. The
biggest impact will be snow in the mountains, especially the north
and central Cascades. Snow levels are tricky and highest snow
amounts will be over 3,500 to 4,000 feet. This mainly affects
Mount Baker and Stevens Pass. In the lowlands, temperatures will
be in the 40s today with a few low 50s possible. Same goes for
Monday. Models are showing a tight pressure gradient with the
cold front tonight. In the interior, expect a S wind of 15 to 30
MPH with gusts to 35 to 40 MPH possible.

A stronger and wetter Pacific system will reach the area on
Tuesday with another round of wet and blustery weather. Heavy snow
is expected in the Cascades again and snow levels will be lower -
around 4,000 feet lowering to around 2,000 feet Tuesday night.
Winter Storm Warning are possible in the Cascades. It`ll be windy
too as a cold front moves inland Tuesday afternoon with gust to
40 MPH possible, mainly coast and south sound. This system is a
quick mover and showers will taper off Tuesday night. 33

.LONG TERM...Previous discussion...Strong cold front will exit
southeast from the forecast area on Tuesday night. A compact cold
core aloft will follow the front, with 500 mb temps briefly
dipping to near -34C early Wed morning. Cold air aloft will
support plenty of showers. Later Wed morning, upper heights will
rise in response to an upper ridge axis near 140W bulging onto the
coast. The air mass will stabilize, and drier air will advect
into Western Washington thanks for northeast surface wind
delivering drier continental air from the interior of B.C..
Showers will quickly taper off on Wednesday morning and end late

Dry northwest flow aloft and upper ridging around 130W-135W will
keep the weather cool and precip-free for the rest of the week.


.AVIATION...A warm front will stall over the area through at least
Monday morning. Moderate southwest flow aloft becoming westerly on
Monday and becoming light. Air mass moist and stable. Widespread
MVFR cigs with light rain today through Monday. Local IFR likely
this morning and again later tonight. Southerly winds increasing
through this evening, the gradually easing. Gusts to 35 kt possible
from KSEA to KPAE/PWT mainly late this afternoon and evening.

KSEA...MVFR cigs 1200-1800 feet through Monday with persistent light
rain. S winds 9-12 kt becoming southwest later this afternoon 13-
17kt with gusts up to 30 kt this afternoon and evening, gradually
easing late tonight/Monday.


.MARINE...A warm front will bring small craft advisory strength
winds to all waters. Strongest winds up to 30 kt will likely occur
later this afternoon and evening from north Puget Sound to Admiralty
Inlet and east entrance to the Strait. There will be relative lull
in between weather systems Monday. A stronger frontal system will
move through the area Monday night and Tuesday. Small craft winds
likely or possibly gales for the Coastal Waters with this system.
Offshore flow will develop Wednesday and continue into Thursday.


.HYDROLOGY...River flooding is not expected the next 7 days.


WA...Winter Weather Advisory until 10 AM PST Monday for Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit

PZ...Small Craft Advisory until midnight PST tonight 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.

     Small Craft Advisory until 6 AM PST Monday for Coastal Waters
     From Point Grenville To Cape Shoalwater Out 10 Nm.

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

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

     Small Craft Advisory from 4 PM this afternoon to midnight PST
     tonight for West Entrance U.S. Waters Strait Of Juan De

     Small Craft Advisory from 10 PM this evening to noon PST Monday
     for Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 4 PM PST this afternoon for Admiralty

     Small Craft Advisory until 4 AM PST Monday for Puget Sound and
     Hood Canal.


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