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Area forecast discussion 
National Weather Service Seattle Washington 
900 am PST Mon Dec 11 2017 


Synopsis...a strong upper ridge will maintain dry and stagnant 
conditions across most of western Washington through Wednesday, 
although a weak system will dissipate over the coast Tuesday 
morning. 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...there is a patch of fog over the Puget Sound area, but 
otherwise skies are clear this morning. The fog will dissipate 
giving sunny skies this afternoon. Gradients are generally light and 
so are winds, though the cross-Cascades gradient is around -12 mb 
and there are gusty east winds near the Cascades. Air quality 
remains in the moderate category at most locations. The air 
stagnation advisory that is in effect and will be extended to 2 PM 
Thursday. 


There is little change in the forecast today with the upper level 
ridge firmly in place over the area. The inversion is still intact, 
with temperatures at Hurricane Ridge, Mount Baker, and Paradise on 
Mount Rainier all in the 50s. The freezing level at kuil this 
morning was 12,300 feet. Camp Muir, at 10,000 feet on Mount Rainier, 
was 41 degrees at 8 am, which is considerably warmer than everywhere 
in The Lowlands. Temperatures are well below freezing in the passes, 
where cold air is flowing through from the east side. Highs today 
will be in the 40s to lower 50s, except along the north coast where 
downslope easterlies off the Olympics will push temperatures into 
the upper 50s. 


A weak system will reach the coast tonight and dissipate. Will boost 
pops a bit, into the chance category for the coast and possibly the 
Olympics tonight and early Tuesday morning. This weak system will 
not have enough punch to bring rain to the Puget Sound area. There 
will be an increase in clouds Tuesday, but skies will clear Tuesday 
night as the upper level ridge strengthens again. Lows tonight 
and Tuesday night will be in the mid 20s to mid 30s. Highs Tuesday 
will be in the mid to upper 40s, rising to the upper 40s to mid 50s 
Wednesday. Burke 


Long term...previous discussion...extended models remaining 
consistent with the ridge breaking down and a series of systems 
moving through western Washington. Timing on the last few runs for 
the first system has been a little slower than the current forecast. 
Will go ahead and dry the forecast out for Thursday. With the models 
showing good consistency have raised the pops into the likely or 
higher category for Friday through Sunday. Rainfall amounts in the 
central Puget Sound might not amount to much for the middle of 
December with model solutions showing the area somewhat rain 
shadowed over the weekend. Felton 


&& 


Aviation...a strong ridge at the surface and aloft over eastern 
Washington will slowly weaken through Tuesday morning. A weak 
frontal system over the offshore waters and its associated upper 
level trough will dissipate as it moves onshore tonight. Light 
southerly flow aloft will become moderate southwesterly tonight 
into Tuesday. The air mass is stable and dry with patchy low level 
moisture in a surface inversion this morning. Mid and high level 
moisture will spread onshore tonight with some low level moisture 
over the coastal zones. 


Observations show patchy shallow dense freezing fog around kbli 
and around the Seattle Metro area. Expect whatever fog that is 
around to scatter out by 19z leaving clear skies around the 
region. While pressure gradients will relax somewhat tonight into 
Tuesday morning, mid and high level moisture associated with the 
offshore weakening front will spread inland tonight and southerly 
gradients and mixing will increase. Hence expect less fog Tuesday 
morning. MVFR and IFR ceilings with low level moisture from the 
dissipating frontal system Tuesday morning are expected to be 
confined to the northern coastal zones. Albrecht 


Ksea...cameras and observations around the terminal show fog over 
the water to the west and patchy fog around kbfi. Will keep the 
tempo 1/2sm fzfg group in the terminal forecast until about 18z 
then expect clear skies to prevail. Light southeast winds will 
become more easterly tonight. Expect less chance of fog 
development tonight and Tuesday morning with an increase in mid 
and high level moisture and better mixing downward of dry air from 
aloft. Albrecht 


&& 


Marine...a 1040 mb high over eastern Washington and lower pres 
offshore will result in moderate offshore flow through this 
evening. East to southeast flow will decrease later tonight as a 
weak frontal system now over the offshore waters dissipates as it 
moves onshore. Small craft advisories continue through this 
evening for the outer portions of the coastal waters, the West 
Entrance to the Strait of Juan de Fuca, and the inner coastal 
waters adjacent to the West Entrance to the Strait. 


Light northerly offshore flow will prevail Tuesday and Wednesday. 
A cold front will sweep through the waters late Thursday night 
into Friday. Albrecht 


&& 


Sew watches/warnings/advisories... 
Washington...air stagnation advisory until noon PST today 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 this evening 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 this afternoon 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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