Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 122246

Area Forecast Discussion
National Weather Service Seattle WA
245 PM PST Tue Dec 12 2017

.SYNOPSIS...A strong upper level ridge will maintain dry and
stagnant conditions across most of Western Washington through
Thursday afternoon. The ridge will break down on Thursday allowing a
cold front to bring rain late Thursday night and Friday. Another
frontal system will continue the wet weather pattern into the


.SHORT TERM...Some areas have cleared out nicely this afternoon but
it appears there will still be cloudy periods this evening. The
forecast calls for decreasing clouds. Guidance suggests clouds will
not dissipate until after midnight, when the upper ridge sharpens a
bit. As the clouds fade, and winds remain light, fog is likely to
appear, especially around Puget Sound. Overnight lows will be mostly
in the 30s.

Wednesday and Thursday will be mostly sunny with areas of night and
morning fog. Highs will be in the mid 40s to lower 50s, but low
temperatures will fall into the mid 20s to mid 30s. An approaching
front will spread clouds over the area Thursday night and eventually
bring a little rain late Thursday night and Friday. Models have
slowed this system a bit and now it appears the interior will not
get rain until after 12Z. Pops on Friday are likely or higher, but
QPF is low with this weakening system. Increasing clouds Thursday
night will keep the overnight lows mainly in the 30s. Highs on
Friday will be 45-50.

With the upper level ridge in place and light flow at the lower
levels through the short term, the air stagnation advisory will
remain in effect through Thursday afternoon. Burke

.LONG TERM...The pattern will be different from Friday on. Instead
of a big upper ridge, flow aloft will be westerly with upper troughs
passing to the north and clipping Washington. Have used a blend of
the GFS and Euro models, which means chance pops each day, with
higher pops around Tuesday when a stronger system appears in the
models. Highs will be 45-50 with lows in the 40s. Burke


.AVIATION...Moderate west flow aloft will become northwesterly on
Wednesday as an upper level ridge builds over the coastal waters.
At the surface, high pressure extending from south-central British
Columbia into Eastern Washington will give light offshore flow.
The air mass is stable with decreasing mid and high level

The combination of clearing skies this evening, light offshore
pressure gradients, and a strengthening low level inversion will
result in fog developing late tonight in area valleys. The fog
will become more widespread Wednesday morning and many terminals
from KPWT-KPAE-KAWO southward will see LIFR conditions 12Z- 21Z
Wed. Albrecht

KSEA...Skies are now clearing. Expect VFR conditions for much of
the night to give way to fog 13Z-21Z Wednesday. The fog will be
accompanied by north to northeast winds 5-7 knots Wednesday
morning. Albrecht


.MARINE...A ridge of high pressure extending from south-central
Columbia into Eastern Washington will give light offshore flow to
the waters tonight through Thursday. The light pressure gradients
and a strengthening temperature inversion aloft will result in
areas of fog over the southern portions of the inland waters
during the late night and morning hours.

A cold front will move east through the waters late Thursday
night into Friday morning.

A warm front will brush the coastal waters on Sunday. Albrecht


WA...Air Stagnation Advisory until 2 PM PST Thursday for Admiralty
     Inlet Area-Bellevue and Vicinity-Bremerton and Vicinity-
     East Puget Sound Lowlands-Eastern Strait of Juan de Fuca-
     Everett and Vicinity-Hood Canal Area-Seattle and Vicinity-
     Southwest Interior-Tacoma Area-Western Skagit County-
     Western Whatcom County.


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