Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 181815

Area Forecast Discussion
National Weather Service Seattle WA
1000 AM PST Mon Dec 18 2017...updated Aviation/Marine

.SYNOPSIS...A nearly stationary frontal system will remain over
Southwest Washington through this afternoon. A strengthening low
center will track across Northwest Washington on Tuesday, with a
sharp cold front trailing the low on Tuesday afternoon. This will
bring heavy mountain snow and generally windy weather on Tuesday.
Showers will linger, especially around convergence zones, Tuesday
night and Wednesday morning with snow levels falling to below 1000
feet. A cold and dry air mass will end the showers on Wednesday.
Around Thursday night, a wave diving southeast through B.C. will
increase clouds with a low chance of some light lowland snow. Cool
and dry weather expected under high pressure from later Friday
through next weekend.


.SHORT TERM...a frontal system extends from the north Cascades to
around Seattle then to Astoria and westward into the Pacific. This
front will pull up stationary across southwestern portions of
Washington later this afternoon and tonight. Light rain and
drizzle is ending to the north of the front - so a lull in
precipitation can be expected through this afternoon or early
evening from Seattle or so northward.

Short term models are consistent in showing good cyclogenesis
offshore starting this afternoon as a difficult to find 1016 mb
wave of low pressure now near 43N 136W deepens to just below 1000
mb as it moves onshore between Forks and Hoquiam on the Washington
coast Tuesday morning. The low will continue deepening as it moves
into the North Cascades of Washington early Tuesday afternoon.
Concerning wind: Tight pressure gradients are generally expected
to remain south of a Hoquiam to Seattle line. The best chance of
Advisory level winds are south of Seattle or Tacoma in the
interior and Hoquiam southward on the coast.
Concerning mountain snow: Snow will begin this evening and get
heavy late tonight through Tuesday with very heavy amounts above
about 4000 feet. Snoqualmie Pass will likely stay rain or mixed
precipitation for the heaviest portion of the event Tuesday

Behind the low, snow showers will continue in the mountains with a
convergence zone developing from north of Seattle to about
Arlington in the interior. 1000-850 mb thicknesses fall to below
1300 meters and 850 mb temperatures fall from -4C Tuesday evening
to -6C Wednesday morning as the convergence zone weakens. An
update was issued to lower snow levels Tuesday night and Wednesday
morning with precipitation amounts moved toward the meso model
consensus. This brings some mixed precipitation or maybe even some
wet snow to higher hills as precipitation ends Wednesday morning.
At this time it appears that accumulations will be limited to
higher hills and will not be significant. We will be keeping a
close eye on this as the event comes more into focus.

Cool and dry air will move southward into the area on Wednesday
with highs in the 30s and lows in the mid 20s to lower 30s.

.LONG TERM...From the previous long term discussion: The latest
wrinkle in the forecast is a shortwave that will slide southeast
through interior B.C. late Thu and early Fri. The 00z ECMWF is
sharper with this feature and blossoms some light precip over the
interior Western Washington on Thursday night and Friday morning.
Meanwhile, the 06z GFS is weaker and brings no precip. Only a few
of the GEFS 20 ensemble members shows measurable precip for this
time period. Still, with a cold air mass and a weak precip signal,
have gone with low chance PoPs in a rain/snow mix over the
lowlands Thu night and Fri morning.

Models then agree in showing an upper ridge axis with cold and dry
weather from late Friday through the end of Day 7.  Haner


.AVIATION...A warm front will meander across southern WA today, then
lift north across the area this evening ahead of the next low. Rain
at times with persistent widespread MVFR cigs, and areas of IFR
(especially KPWT/KOLM/KHQM). Light westerly flow aloft becoming
strong southwesterly flow later tonight and Tuesday associated with
a deepening 995 mb low tracking into central Washington. Moist at
all levels and stable.

KSEA...The convergence zone is stalled around KSEA/KBFI, and this
could continue for several hours wind some variable wind shifts at
KSEA. KBFI should mostly stay northeast 5 kt or less then go south
early this evening. The NE wind at KSEA is around 13-16 kt w/ gusts
to 20 kt at 10 am, but these speeds are expected to weaken
significantly over the next 1-2 hours. By 20z easterly winds may be
around 5-8 kt then going south early this evening. Rain today with
IFR cigs, possible improving to low MFR this afternoon.

Colder air arrives late Tuesday night and Wednesday. Snow levels
falling to 1500 feet after midnight Tuesday night and down to 1000
feet Wednesday morning. Snow accumulation is not expected at KSEA
but period of wet mixed precipitation cannot be ruled out Wednesday
morning. Cold rain is most likely.


.MARINE...Small craft westerlies continue to linger in the
central/east Strait and North part of Admiralty Inlet, so the
advisory was extended into the afternoon for those areas. Parts of
Puget Sound could reach 20 kt at times with northerlies today north
of Tacoma and south winds Tacoma southward. S flow returns to the
Sound tonight as the flow lifts north.

A deepening 995 mb low will track into Wrn WA Tuesday morning. Gales
area expected in parts of the coastal waters and possibly Puget
Sound and Admiralty. Will examine the new guidance with some changes
to headlines possible with the 3 pm PST forecast.


.HYDROLOGY...Rises on area rivers are expected on Tuesday due to
heavy precipitation, but snow levels 3500 to 4000 feet will limit
runoff. Therefore, river flooding is not expected the next 7


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

     Winter Storm Warning from 6 PM this evening to 10 PM PST Tuesday
     for Cascades of Pierce and Lewis Counties-Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit

     Winter Storm Warning from 4 PM this afternoon to 3 PM PST
     Tuesday for Olympics.

PZ...Gale warnings 4 southern coastal waters zones.

     Small craft advisory 2 north coastal zones and the
     central/west Strait.


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