Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 072333
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
335 PM PST Wed Dec 7 2016

.SYNOPSIS...Cold dry air is over Western Washington tonight.
Easterly winds will increase through Thursday with high pressure
inland and low pressure approaching the coast. A front will reach
the area late Thursday afternoon and evening and move north through
Western Washington by daybreak Friday. Cool rainy weather is likely
Friday through the weekend with a low snow level.

&&

.SHORT TERM...Skies are clear over most Western Washington this
afternoon with the exception of a little patch of stratus on the
Washing half of the Strait of Juan de Fuca banked up against the
Olympic foothills. High clouds will arrive overnight. There is
a west-east oriented front offshore and the eastern portion of that
front is moving into Northern California tonight. The front will
move north through Oregon Thursday morning. The precip should be
into Lewis county in the afternoon and it will spread north through
the metro area late in the afternoon--the 12z gfs and euro agree
on that and the 18z gfs is not any different.

The front moves north through Washington Thursday night. Snow will
fall with 1-3" for most of the lowlands, 4-6" for the Hood Canal
area, and 4-8" in the mountains. The snow should turn to rain for
most of the lowlands around daybreak, but the Fraser outflow will
help keep a wintry mix going around Bellingham so the advisory
there ends at noon. The east side of the Olympics may also take
awhile to mix out with those easterly pressure gradients. With
strong east gradients there will be windy conditions in the typical
gap wind areas Thursday--which is west of the passes for the most
part and also around Enumclaw. There is a wind advisory for that
area.

Friday afternoon through the weekend should mostly be rainy with a
fairly low snow level. There is a chance of wet snow depending on
how a surface low pressure system tracks through the area, but with
850mb temps warming to the -2c to -5c range, it will be much more
marginal for snow except for perhaps the mountain valleys--maybe
places like Darrington and Skykomish will continue to get snow
through the weekend. 19

.LONG TERM...A cool and showery weather pattern will
continue on Sunday as another upper level disturbance tracks through
Western Washington. At the same time, models show another Fraser
river outflow that may bring sub-zero temps and a chance of
rain/snow showers in the north interior. Showers will taper off on
Monday as this wave exits east. Cool but dry weather is in store
Monday night and Tuesday as the next system passes to the south. A
warm front will lift north through Western Washington Wednesday
night but temperatures should remain above freezing. An upper level
trough will maintain showers across the region on Thursday. 33

&&

.AVIATION...Light northerly flow aloft over western Washington
this evening, becoming light late tonight, then moderate
southwest aloft developing on Thursday. Strong low-level offshore
wind flow will maintain a dry and stable air mass through much of
Thursday, though some freezing fog is possible around sunrise
Thursday in favored sheltered river valleys such as the
Snoqualmie and Chehalis. A warm front will start to spread
northward through western Washington on Thursday afternoon. Precip
will spread up from the south, reaching Lewis and Grays Harbor
Counties around 21z-24z, then spreading north and beginning in the
Puget Sound Region from 00z-04z. Look for a few hours of snowfall,
then a transition to rain expected on Thursday night.

KSEA...Dry low-level offshore flow will strengthen until mid-day
Thursday, by which time could see some easterly gusts near 25
knots at the terminal. High clouds will increase slowly tonight
and Thursday morning, then thicken more quickly Thursday afternoon
in advance of a warm front coming up from the south. Precip is
most likely to begin around 01z-03z on Thursday PM. Precip should
begin as snow at the terminal but then transition to rain about
2-5 hours after beginning. Accumulation could range from a trace
to 3 inches on Thursday evening, with 1-2 inches most likely.
Haner

&&

.MARINE...Easterly offshore flow will continue to strengthen until
mid-day Thursday, by which time the coastal waters and the western
half of the Strait should be experiencing mid- to high-end
easterly gales. A warm front will lift northward through the
waters on Thursday night, cutting off the easterly winds and
ushering in a shift to south-southeast winds over the waters later
Thursday night.

A rapidly weakening 1005 mb low will pass eastward across the
coastal waters on Friday night. A broad and weak east-west aligned
surface trough will then linger over western Washington this
weekend. Northeast and east gradients will increase around Monday,
bringing another round of Fraser outflow winds and offshore winds
from Monday through next Wednesday.  Haner

&&

.HYDROLOGY...no flooding is expected over the next week.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from 4 PM Thursday to 8 AM PST Friday
     for Admiralty Inlet Area-Bellevue and Vicinity-Bremerton
     and Vicinity-East Puget Sound Lowlands-Eastern Strait of
     Juan de Fuca-Everett and Vicinity-Lower Chehalis Valley
     Area-San Juan County-Seattle and Vicinity-Southwest
     Interior-Tacoma Area-Western Skagit County-Western Strait
     of Juan De Fuca.

     Winter Weather Advisory from 4 PM Thursday to 3 AM PST Saturday
     for Cascades of Pierce and Lewis Counties-Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit
     Counties-Olympics.

     Winter Storm Warning from 4 PM Thursday to 8 AM PST Friday for
     Hood Canal Area.

     Winter Weather Advisory from 4 PM Thursday to noon PST Friday
     for Western Whatcom County.

     Wind Advisory from 9 PM this evening to 4 PM PST Thursday for
     East Puget Sound Lowlands.

PZ...Gale Warning from 9 PM this evening to midnight PST Thursday
     night for Coastal Waters From Cape Flattery To James Island
     Out 10 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To
     Cape Shoalwater Out 10 Nm.

     Gale Warning from 9 PM this evening to 3 AM PST Friday for
     Coastal Waters From Cape Flattery To James Island 10 To 60
     Nm-Coastal Waters From James Island To Point Grenville 10
     To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm.

     Small Craft Advisory from 3 AM Thursday to 3 PM PST Friday for
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 PM PST Friday for Northern Inland
     Waters Including The San Juan Islands.

     Gale Warning until midnight PST Thursday night for Central U.S.
     Waters Strait Of Juan De Fuca-West Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$

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