Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 120457

Area Forecast Discussion
National Weather Service Seattle WA
857 PM PST Mon Dec 11 2017

.SYNOPSIS...A strong upper level ridge will maintain dry and
stagnant conditions across most of Western Washington through
Thursday afternoon. A very weak front will bring a chance of rain
to the coast tonight before dissipating Tuesday morning. The
ridge will begin breaking down on Thursday allowing a cold front
to bring rain Thursday night and Friday. Another frontal system
will continue the wet weather pattern next weekend.


.SHORT TERM...Water vapor imagery shows a small upper level low
moving northward offshore along roughly 130W through this
evening, as it is steered up and eventually over the large upper
level ridge over the region. The associated weak front has been
moving steadily toward the WA coast this afternoon. At 04Z/8 PM
RADAR was picking up some weak echoes about 25 NM W of the N
coast, but that`s it. The front doesn`t look very strong on IR
imagery and it may struggle to even bring measurable rain to the
coast and Olympics. Still the the N coast and Olympics have the
best chance of rain tonight.

The front will spread clouds across the interior by sunrise
Tuesday morning. However, it may take a little longer for the
clouds to reach the area tonight. There was some expectation that
the overnight cloud cover would keep temperatures up a bit
tonight, but as of 04Z/8 PM quite a few mesowest stations have
already dropped into the lower to mid 30s. There still isn`t a
whole lot of moisture in the boundary layer, but enough for some
areas of fog and patchy fog overnight. With clouds arriving late
tonight at least they may limit the expansion of fog late early
Tuesday morning.

The small upper level low cruising over the top of the ridge will
weaken and flatten it on Tuesday, and the clouds leftover from
the front should hang around for most of Tuesday. High temperatures
will be about the same and pretty close to normal.

The large upper level ridge still sitting over the W coast will
re-strengthen a little Tuesday night and Wednesday which should
allow sunny skies to return Wednesday and Thursday. The models
stay on target...stay on target... with weakening the ridge
rapidly on Thursday, thereby allowing an approaching cold front to
bring rain to the area late Thursday night and Friday.

As long as the strong upper level ridge remains over the area, the
strong surface based inversion will maintain the stagnant low
level air mass presently over W WA. Weak offshore flow will
probably not supply much in the way of mixing. The 00Z GFS shows
the inversion breaking by 12Z Friday morning. For now, the air
stagnation advisory for the interior lowlands of W WA will remain
in effect through 2 PM Thursday. Kam

.LONG TERM...Previous discussion from the 3 PM AFD...The upper
ridge will sag into zonal flow beginning Thursday night. A series
of weather systems will reach the area Friday through Monday for a
wet weekend. The models all show a dry period Saturday, to some
extent. Have played it that way in the zones and grids.
Temperatures will still be a bit above normal, with highs near 50
and lows in the 40s. Burke


.HYDROLOGY...River Flooding is not expected for the next 7 days.


.AVIATION...The ridge has weakened a bit as a disturbance passes
just offshore. The HRRR shows some marine stratus making it to the
coast tonight, but otherwise just some mid and high clouds will pass
by the area. Some fog patches will again set up in the foggy bottoms
and river valleys and near Puget Sound overnight.

KSEA...Some mid and high clouds ought to show up at some point,
but the weak weather system just off the coast wont amount to much.


.MARINE...A weak system is passing by just offshore, and the
flow over the coastal waters has turned to the southeast 10-
20kt. The flow will turn back to light northerly flow again by
midweek. A cold front will reach the area late Thursday night
into Friday. A warm front will approach the coastal waters on


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...Small Craft Advisory until 3 AM PST Tuesday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville Out 10
     Nm-Coastal Waters From Point Grenville To Cape Shoalwater
     10 To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater Out 10 Nm.


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