Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 131707
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PST Wed Dec 13 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 into Friday morning.
A couple of weather systems will brush the area over the weekend
keeping a chance of showers in the forecast.

&&

.SHORT TERM...Satellite imagery shows skies are mainly clear with
only patchy fog, and a few high clouds on the coast. Today will be
sunny with light winds and highs 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.

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. If the NAM is to be believed, it may not even rain
in Seattle on Friday. Other models have generally lower pops--chance
rather than likely. Still, the general idea of increasing clouds
Thursday night, with rain developing on the coast and spreading
inland Friday, is still intact. 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 but at
this point it looks like in Stevens and Snoqualmie passes the
precipitation type will be rain or possibly rain mixed with snow.
Highs on Friday will be 45-50. Burke

.LONG TERM...Previous discussion...Confidence not real high in the
extended forecast. Models waffling around on the position and
intensity of a warm front on Saturday. The ECMWF keeps the chance of
rain over the northern portion of the area while the GFS has spotty
light rain over most of the area. On Sunday the ECMWF has the
trailing cold front reaching the northern portions while the GFS has
this feature further north. The models have another warm front for
the first part of next week with little consistency in the timing and
intensity of the system. With the uncertainty in the models will
go with just chance pops for the extended period for most of the
area. Temperatures will be near normal with highs in the mid 40s
to lower 50s and lows in the mid 30s to lower 40s. Felton

&&

.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 with patchy moisture trapped below a low- level inversion
in the interior of Western Washington.

Observations and cameras this morning show only a few thin cirrus
overhead with patches of low stratus and fog. The low level
moisture will decrease midday leaving good VFR conditions across
the region. Clear skies giving good radiational cooling tonight
combined with a strong inversion with a base around 500-800 feet
will likely give increased fog across the area 09Z tonight into
Thursday morning.

KSEA...Fog patches have stayed away from the terminal this
morning. Expect good VFR conditions through late this evening with
light NE winds 4-6 kt. Expect a fog deck to form around the
terminal about 09Z-12Z Thursday and persist until about 21Z. The
fog will be accompanied by a NE wind around 5-6 kt.

&&

.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. Patchy fog over the waters
this morning will dissipate midday. The light pressure gradients
and a strengthening temperature inversion aloft will result in
areas of fog over the inland waters tonight through Thursday
morning. Easterly outflow through the West Entrance to the Strait
of Juan de Fuca and west of Grays Harbor is expected 20 knots or
less through Thursday.

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.

&&

.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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