Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 132250
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
300 PM PST Wed Dec 13 2017

.SYNOPSIS...A strong upper level ridge will maintain dry conditions
across Western Washington for one more day. The ridge will break
down on Thursday allowing a cold front to bring a little rain late
Thursday night into Friday morning. A couple of weak weather systems
will brush the area over the weekend keeping a chance of showers in
the forecast.

&&

.SHORT TERM...Skies are mostly clear this afternoon except for
occasional high clouds. Temperatures are 45-50. Tonight will also be
mostly clear with high clouds at times and lows from the upper 20s
to near 40. Thursday will be the last mostly sunny day for awhile,
with highs 45-50 again. There will be patchy fog in the morning.

The temperature inversion is not quite as strong as it was on
Monday, but it is still present. Light surface winds will keep
the air stagnation advisory going through Thursday afternoon.

Models have been slowly weakening the weather system due Thursday
night and Friday. The general idea of increasing clouds Thursday
night, with rain developing on the coast and spreading inland
Friday, is still intact. Model consensus is that everywhere will get
a little rain on Friday. Increasing cloud cover will keep most
places above freezing Friday morning but a few locations from Tacoma
south could fall a couple of degrees below freezing before clouds
arrive. Snow levels will drop quickly with the front, to 3000-4000
feet. Showers are likely in the mountains Friday evening; at this
point it looks like Stevens Pass might get a few inches of snow
while in Snoqualmie Pass the precipitation type will be rain or
possibly rain mixed with snow. Highs on Friday will be 45-50.

What had earlier looked like a significant weather system on
Saturday now appears pretty weak. The forecast now has chance pops
for the whole area. Saturday will be a bit cooler with highs near
45 or so. Burke

.LONG TERM...Model details continue to change in the extended. In
the GFS, Sunday now has a system slightly stronger than the weak one
Saturday, followed by showers Monday. A system stays mainly to the
north on Tuesday, then moves south through the area on Wednesday.
There are similar themes in the Euro, with timing differences. In
the forecast the highest pops--mostly likely pops--are Sunday night
and Monday, and again Wednesday. Despite the lack of major weather
producing systems, there will have been a pattern change. There will
be lots of clouds around starting Friday and continuing all next
week. Highs will still be 45-50, but lows will not fall much below
40. The temperature inversion will be gone, so it will actually get
colder as you go up in the mountains. Burke

&&

.AVIATION...High pressure aloft over the coastal waters will shift
inland on Thursday. Light westerly flow aloft will become
southwesterly on Thursday. A surface ridge of high pressure
extending from south-central British Columbia into Eastern
Washington will persist through Thursday. The air mass is dry and
stable. Areas of moisture will develop below a low level inversion
during the late night and morning hours late tonight into Thursday
morning.

Observations, cameras, and visible satellite imagery show only
high clouds across the area this afternoon with good VFR
conditions prevailing. Only one small patch of low clouds and fog
persists, near KUIL on the north coast.

Clear skies tonight will giving good radiational cooling. The
surface cooling combined with the development of a strong
inversion with a base around 500-800 feet will likely give
increased fog across the area 09Z tonight into Thursday morning.
The fog that does form will likely persist into the early
afternoon hours. Albrecht

KSEA...Expect good VFR conditions through late this evening with
NE winds around 10 KT decreasing to 4-6 kt after dark. There is
about a 60 percent chance that a fog deck will form around the
terminal about 12Z Thursday and persist until about 21Z. The fog
will be rather shallow with tops 800 ft or so. The fog will be
accompanied by a NE wind around 5-6 kt. Albrecht

&&

.MARINE...A ridge of high pressure extending from south-central
British Columbia into Eastern Washington will give light offshore
flow to the waters through Thursday. Areas of fog will develop
over the waters late tonight and continue into the early afternoon
hours on Thursday.

Easterly outflow through the West Entrance to the Strait of Juan
de Fuca and west of Grays Harbor is expected to be near 20 knots
late tonight through Thursday morning. There is a chance that a
short-fused small craft advisory may need to be issued if winds
increase any more than currently forecast.


A cold front will move east through the waters late Thursday
night into Friday morning. Small craft advisory northwest winds
are expected over the coastal waters and through portions of the
Strait of Juan de Fuca behind this front.

A warm front will brush the coastal waters late Saturday night or
Sunday. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
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.

PZ...None.
&&

$$

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