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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 181101

Area Forecast Discussion
National Weather Service Seattle WA
401 AM PDT Mon Jun 18 2018

.SYNOPSIS...An upper level ridge offshore will build into British 
Columbia today then shift inland over the Pacific Northwest Tuesday.
The ridge will shift east Tuesday night and Wednesday. High 
temperatures over interior will be within a few degrees of records
through Wednesday. A trough will bring cooler marine air into the
area Thursday and Friday. High pressure will try and rebuild over
the region next weekend. 


.SHORT TERM...Satellite imagery shows just some cirrus embedded
in the northeasterly flow aloft over the area moving over the
northwest portion early this morning. Clear skies for the 
remainder of the area. It's a mild early morning at 3 am/10z with 
temperatures in the 50s to mid 60s.

Upper level ridge offshore building into British Columbia today
while the upper level low to the southeast remains over Idaho. 
Little change in the pattern over western Washington for most of 
the day with northeasterly flow aloft and flat gradients at the 
surface. Surface gradients turn onshore slightly in the afternoon 
along the coast and through the Strait which will put a cap on the
high temperatures in those locations. For the interior Sandpoint 
profiler shows the temperatures aloft are similar to slightly 
warmer that Sunday morning at least in the Seattle area. Even 
without any offshore flow at the surface will little change in the
pattern will go with highs similar to Sunday with 80s and lower 
90s for the interior. Along the coast and Strait with the onshore 
flow kicking in expect highs in the upper 60s to lower 80s, 5 to 
10 degrees cooler than Sunday.

Upper level ridge moving over the area tonight into Tuesday. All
of the models have some degree of onshore gradients through
Tuesday. Having the upper level ridge axis overhead does not bode
well for much of a marine push Monday night and Tuesday for the
interior. Will keep the cloud cover Tuesday morning west of the
Puget Sound and only less than 5 degrees cooler on the high 
temperatures for the interior on Tuesday, 70s to mid 80s. Highs 
along the coast and Strait in the 60s to lower 70s. Lows tonight 
will be mild again, in the 50s to lower 60s.
Upper level ridge shifting east Tuesday night into Wednesday with
the flow aloft over western Washington becoming south
southwesterly as an upper level trough offshore digs south. 
Onshore gradient go flat again allowing high temperatures in the 
interior on Wednesday to warm up a couple of degrees. The air mass
over the mountains will become somewhat unstable in the afternoon
and with the southerly flow aloft introducing some mid level 
moisture there is a chance of afternoon and evening showers or 
possibly thunderstorms in the Cascades and Olympics. Highs for the
interior on Wednesday in the mid 70s to near 90. Highs for the 
coast and Strait remaining in the 60s to lower 70s.

.LONG TERM...Extended models in good agreement with an upper level
trough moving through the area Thursday morning. Increasing
onshore gradients behind the front with the resulting marine push
cooling high temperatures 10 to 20 degrees over the interior.
Showers chances with the trough will be confined to the higher
terrain. Models solutions become inconsistent on Friday with the
GFS moving the trough through the area faster than the ECMWF.
Onshore flow continuing at the lower levels keeping highs near
normal, in the 60s to mid 70s. Some differences int he models 
over the weekend with the GFS a little deeper keeping an upper
level trough offshore versus the ECMWF which is weaker but more
progressive with the trough. Will stay with a dry forecast for now
which is more in line with the GFS and keep high temperatures in
the 60s to mid 70s. Felton


.AVIATION...Northeast flow aloft continues over Western 
Washington this morning as an upper ridge remains centered over the 
offshore waters and an upper level low makes slow eastward progress 
over southern Idaho. The air mass remains somewhat moist in the mid 
levels and will become unstable over the Cascades with daytime 
heating. This may lead to a few mountain showers and thunderstorms. 
Light low level onshore flow will increase this afternoon with some 
stratus reaching the coast this evening and pushing locally inland 
tonight. VFR conditions expected across the region with the 
exception of some IFR ceilings and visibilities in fog/stratus 
coastal areas tonight and locally into the interior Tuesday AM.

KSEA...VFR through tonight. There is a possibility of some stratus 
briefly reaching Sea-Tac on Tuesday morning, but confidence is low 
in that occurring. Light S-SE wind this morning becoming W-NW 4 to 8 
knots this afternoon. Wind becoming light S-SW tonight.    27


.MARINE...Generally light surface flow over the waters today. 
Onshore gradients increase tonight with small craft advisory 
westerly winds likely in the central and eastern portions of the 
Strait. Higher pressure offshore and lower pressure inland will 
maintain weak onshore flow through Wednesday. Stronger onshore flow 
will develop Wednesday night into Thursday. Gale force westerlies 
are possible in the central and eastern Strait, especially Wednesday 
evening. Onshore flow will persist Friday into the weekend. DTM


.CLIMATE...Even though summer is right around the corner it is
still hard to get a stretch of really warm weather in western
Washington in June. Since records started in 1945 at Sea-Tac
airport there have only been 3 streaks of 4 or more days in a row
with highs 85 degrees plus in Seattle in June. Two 5 days streaks,
June 17-21, 2004 and June 21-25, 1992 and one 4 day streak June
2-5, 1978. Felton