Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 161721

Area Forecast Discussion
National Weather Service Seattle WA
921 AM PST Sat Dec 16 2017

.SYNOPSIS...A weak warm front will clip the region today. Rain
will increase on Sunday as a cold front sags southward into
Western Washington. A strengthening low center will pass across
southern Vancouver Island on Tuesday, and a strong trailing cold
front will bring wind and heavy mountain snow. Lingering showers
will continue Wednesday, then an upper ridge will bring dry
weather on Thursday and Friday.


.SHORT TERM...A warm front will clip the coast today with a chance
of rain in the interior. Precipitation amounts will be generally
light with 0.10" on the coast and a trace to few hundredths in the
interior. Cloudy skies will keep temperatures in the mid to upper
40s today.

Showers will increase on Sunday as a cold front slides SE into
Western Washington. Snow levels will hover around 4,000 feet and
will likely need a Winter Weather Advisory for snow in the
Cascades. Showers will continue to impact the region Sunday night
and Monday as the front stalls west-east over the area. Snow
levels will drop further, around 3,000 feet in the north and
central Cascades. 33

.LONG TERM...Previous discussion...Models agree in cyclogenesis
on Monday between 130W and 140W as a vigorous upper trough
preceded by lots of divergence aloft interacts with a leftover
surface front. All models show the low center moving across
southern or central Vancouver Island on Tuesday. The low center
will still be in the process of deepening as it moves by to the
north. Meanwhile, a strong trailing cold front will sweep across
Western Washington on Tuesday. The abundance of large-scale lift
will lead to a period of heavy precip in advance of the cold
front. Heavy snow should occur over the mountains. Still watching
wind potential on Tuesday, partly because winds tend to pick up
more with the passage of low centers that are still in the process
of strengthening.

A compact upper low with very cold air aloft will spread across
the forecast area early Wednesday, leading to plenty of post-
frontal convective showers.

An upper ridge axis will strengthen and amplify near 135W on
Wednesday, then move east to near 130W on Thursday. This will dry
out the weather quite readily on Wednesday night. With the upper
ridge axis continuing near 130W on Thursday and Friday, cool and
dry weather is expected at that time. Haner


.AVIATION...Light westerly flow aloft becoming moderate southwesterly
tonight as a warm front reaches the area. The air mass is stable
and moist at all levels. Spotty rain may reach the coast and some
other interior spots this afternoon. Steady rainfall will arrive
at the coast by early evening with the warm front and spread
inland. A mix of MVFR and VFR this morning. IFR appears confined
to to PWT along the west side of Puget Sound and OLM, which should
improve to MVFR by afternoon. Widespread MVFR expected tonight
with rainfall, and local IFR conditions possible.

KSEA...Mostly VFR cigs this morning around 4K ft, but brief periods
of MVFR are possible. Sprinkles possible but the bulk of rain will
arrive by mid evening with MVFR cigs. Southerly wind 4-8 knots,
become south-southwest 10-13 kt late tonight. DTM


.MARINE...A warm front will move through the waters this evening
through Sunday with small craft winds most waters, including Puget
Sound by later tonight.

Monday will be a lull in between weather systems. A vigorous
frontal system will move through the area Monday night and Tuesday
with gale force winds possible for the Coastal Waters. Offshore
flow will develop Wednesday. DTM


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


PZ...Small Craft Advisory all waters except the west and central
     Strait of Juan De Fuca.


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