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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 190211

Area Forecast Discussion
National Weather Service Seattle WA
711 PM PDT Tue Sep 18 2018

.SYNOPSIS...Unsettled pattern returns Wednesday with cool and
showery weather into the weekend. Drying and warming trend next
week as high pressure returns. 


Pleasant evening across the area with some mid to high level
clouds filtering in ahead of troughing pattern taking place across
the Pacific Ocean. Some patchy fog may form again overnight but
coverage is expected to be less than this morning. First in a 
series of troughs will swing southeast across the area later 
tomorrow into Thursday. Moisture associated with this initial 
system is not impressive so not expecting widespread shower 
activity with light amounts where showers do occur as the cold
front moves through. The trough will serve to increase clouds 
across the entire area with perhaps a degree or two drop in 
temperatures. Stronger, wetter system approaches the coast by 
Thursday night into Friday morning though timing differences still
exist between GFS/NAM on faster side of the envelope and slower 

A more substantial system will dive into the southwest BC coast 
by Friday and into the weekend. This system will have more 
moisture and be stronger so there will be a greater chance of rain
by the weekend. Temperatures will trend a few degrees below 
normal this weekend as well, owing to the increased cloud cover, 
rain, and onshore flow. Troughing will ease Sunday and then 
quickly transition by Monday. A strong mid level ridge of high 
pressure will build in across the eastern Pacific. 500mb heights 
will climb quickly early next week and the pattern will feature a 
drying and warming trend. Heights will exceed 580dm by late Monday
and into Tuesday, which would mean highs well into the 70s and 
possibly even some low 80s south if the current forecast of 
ridging and rising heights holds true!

Key Messages in the long term:
No significant impacts in the long term.
Warming and drying trend next week some 70s/80s?



.AVIATION...The air mass across Western WA is dry and stable with
onshore flow. The flow aloft is westerly. Expect VFR conditions
this evening. Low level clouds will likely reform in the interior
early Wednesday morning for a return of MVFR conditions. A weak
cold front will bring scattered showers to the Olympic Peninsula
Wednesday afternoon. There is a chance of showers in the interior
Wednesday night into early Thursday as the cold front continues 
to push through Western WA. 33

KSEA...VFR conditions this evening. Possibility remains for some
low stratus into KSEA after 12z with a temporary reduction to MVFR
cigs through 18Z though confidence remains low on occurrence.  
N/NE winds to 10 kt becoming S/SW by 12z. 33/CEO


.MARINE...Onshore flow will increase on Wednesday as a weak system
moves through. Small craft advisory winds are likely in the
central and eastern portions of the Strait of Juan de Fuca. A
stronger system will arrive Thursday night into Friday with small
craft advisory winds over the coastal waters and Strait. Onshore
flow will continue Friday night and Saturday. CEO