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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 172231

Area Forecast Discussion
National Weather Service Seattle WA
330 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...Temperatures over the interior this afternoon have 
been running 6 to 8 degrees cooler than this time yesterday with 
highs still on track to reach the mid to upper 80s with a few 
locations well away from the water hitting the low 90s. Conversely 
temperatures along the coast will be hard pressed to hit 70 today. 
Strengthening onshore flow tonight will bring a more developed push 
of marine air into the interior overnight into Wednesday morning. 
This will return the interior temperatures to normal levels 
Wednesday with highs remaining in the mid 70s over the interior and 
upper 60s along the coast.

An upper level expected to dig into into Southern British Columbia 
on Friday 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...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...Light to moderate westerly flow aloft will continue 
tonight and Wednesday. At the surface, moderate to strong onshore 
flow tonight will weaken Wednesday. The air mass is dry and stable 
with VFR clear conditions except for low level marine stratus along 
the coast.

Increasing low level onshore flow this afternoon and evening will 
bring stratus inland tonight and Wednesday morning. IFR ceilings 
will develop around the Puget Sound terminals 09Z-12Z Wednesday 

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


.MARINE...Onshore flow will increase tonight with gales likely 
through the Strait of Juan de Fuca. Small craft advisory strength 
winds are also likely over the outer coastal waters, Admiralty Inlet 
and northern inland Waters. A similar pattern is likely Wednesday 

High pressure over the northeast Pacific will maintain moderate 
onshore flow across the region through the rest of this week. 
Highest wind and waves will be through the strait, mainly during the 
late afternoon and evening hours each day. CHB


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.