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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 180317

Area Forecast Discussion
National Weather Service Seattle WA
817 PM PDT Tue Jul 17 2018

.SYNOPSIS...Strengthening onshore flow tonight will set up 
a typical pattern of morning clouds and near normal temperatures 
that will persist into Saturday. Offshore flow will return Sunday 
bringing warmer conditions into at least the early part of next week.


.SHORT TERM...The onshore push is on its way. Stratus clouds are
up to Shelton already with solid gales through the Strait of Juan
de Fuca. Most lowland areas will be covered under the stratus
early Wednesday morning. The stratus will be shallow and will 
burn back toward the coast during the late morning and early 
afternoon hours. The cooler marine air will knock temperatures 
down into the 70s, closer to normal. 33

Previous discussion...An upper level low is expected to dig into 
into Southern British Columbia on Friday and will brush 
Washington and Oregon as it ejects eastward Saturday. Model 
guidance continues to keep precipitation with this system well to 
to the north of the area Thursday and Friday. The primary impact 
for the passing upper level trough will most likely be a boost to 
the onshore flow Thursday and a deeper marine layer into Friday. 

.LONG TERM...Previous discussion...The upper level low over 
British Columbia will shift eastward Friday into Alberta. Medium 
range models continue to show the trough shifting slowly to the 
east Friday night into Saturday morning with a shortwave trough 
rotating around the back side of the low as it slides eastward. 
This has the effect of keeping a bit of troughiness over the area 
into early Sunday morning. Expect varying degrees of night and 
morning low clouds Saturday and again Sunday with highs creeping 
back up into the low 80s in the interior from Seattle southward on
Saturday and a bit warmer on Sunday. High pressure building over 
central and southern of British Columbia will drive a shift back 
to offshore flow Sunday into Monday over Western Washington. The 
shift in the pattern will help limit morning clouds over the 
interior Monday and boost high temperatures into the mid to upper 
80s. Tuesday will trend warmer with highs near 80 along the coast 
and into the low 90s over the interior. 


.AVIATION...A strong onshore push will bring low level stratus
clouds inland tonight with MVFR to IFR conditions expected. The
flow aloft is westerly. Low clouds will burn back toward the coast
19-21z Wednesday afternoon with VFR conditions expected in the
interior. 33

KSEA...VFR clear this evening. IFR ceilings in stratus expected to 
return by 09-12Z Wednesday morning. Surface winds southwest 6-12 
kt tonight, easing to 5-10 kt Wednesday.


.MARINE...Strong onshore will continue tonight with gales through
the Strait of Juan de Fuca. Winds will ease late tonight. Expect
another onshore surge on Wednesday as a weak front passes through.
Gales are possible through the strait again and a Gale Watch is in
effect. High pressure over the NE Pacific will maintain moderate 
onshore flow across the region through the rest of the week. 33


PZ...Small Craft Advisory until 11 AM PDT Wednesday 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.

     Gale Watch from Wednesday afternoon through late Wednesday night 
     for Central U.S. Waters Strait Of Juan De Fuca-East 
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 5 AM PDT Wednesday for Central U.S. Waters 
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of 
     Juan De Fuca.

     Small Craft Advisory until 5 AM PDT Wednesday for Admiralty 
     Inlet-Northern Inland Waters Including The San Juan Islands.