Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 110533

Area Forecast Discussion
National Weather Service Seattle WA
933 PM PST Sun Dec 10 2017

.SYNOPSIS...A strong upper ridge will maintain dry and stagnant
conditions across Western Washington through Wednesday. The ridge
will break down by Thursday with rain returning Thursday night
and Friday. Wet weather will continue through the weekend with
strong moist flow over the area.


.SHORT TERM...An upper ridge will be over the Pacific Northwest
tonight and Monday for dry weather. An upper short wave will move
over the top of the ridge Monday night, temporarily flattening
the ridge. An associated weak front will reach the coast Monday
night then dissipate Tuesday morning. The main affect from the
Monday night system will be to bring some high clouds to the area
but there might be some sprinkles near the coast. The upper ridge
will rebound over the area Tuesday and Wednesday.

There are less high clouds than last night which should allow some
fog to form over the interior lowlands tonight. High clouds with
the next weak weather system will probably limit fog Monday night
but that will depend on how thick they are. Tuesday night should
see a big more fog again with less clouds cover. Stagnant air will
continue through Wednesday. A decision on whether to extend the
current air stagnation advisory will be made Monday. Schneider

.LONG TERM...It looks like Thursday will be our last dry day of
this stretch as the ridge will start to break down. We should see
rain returning by Thursday night or Friday as a trough moves in
from the west. The models have been waffling on the exact timing
of this system and it`s now slightly slower compared to
yesterday`s (faster) solutions. Moist west to northwest flow will
continue trough the weekend for more wet weather with snow in the
mountains. 33


.AVIATION...A strong upper level ridge axis will persist over
Eastern Washington through Monday evening, but it will flatten
some on Monday. A dissipating front will brush the coast on Monday
night. Light southerly flow aloft through Monday morning, becoming
moderate southwest on Monday afternoon and Monday night. The air
mass will be dry and very stable. Shallow dense fog will be the
main forecast challenge through Monday morning over the Puget
Sound lowlands. It will mainly favor sheltered valleys. In
contrast, dry east wind off the Cascades will minimize fog over
King County. Looking ahead to Monday night, conditions for fog
development will be less favorable due to thicker cirrus and a
little more mixing in the boundary layer.

KSEA...Fog may settle into the valleys and over the waters near
the terminal later tonight, but the airfield is most likely to be
above it all, causing SEA to remain free from fog with the help
of a light but drying E-SE breeze later tonight. Monday will
feature clear skies and light south wind. Monday night appears
unfavorable for fog development at SEA due to thicker cirrus and
better dry air entrainment into the surface layer. Haner


.MARINE...A 1042 mb high over Eastern Washington and lower
pres offshore will result in moderate offshore flow through Monday
evening. A dissipating front will brush the north coast on Monday
night, causing easterly gradients to weaken. Light northerly
offshore flow will prevail on Tue and Wed. Expect a cold or
occluded front to move across the area late Thursday night or
early Friday. This will cause the flow to become onshore and


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


WA...Air Stagnation Advisory until noon PST Monday 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 9 PM PST Monday 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 until 3 PM PST Monday for Coastal Waters
     From Cape Flattery To James Island Out 10 Nm-West Entrance
     U.S. Waters Strait Of Juan De Fuca.


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