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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 161054

Area Forecast Discussion
National Weather Service Seattle WA
354 AM PDT Mon Jul 16 2018

.SYNOPSIS...Upper level ridge remaining over Western Washington
today for another round of much above normal temperatures for the
interior. A weak upper level trough will move through the area on
Tuesday increasing the low level onshore flow resulting in cooler
high temperatures. The low level onshore flow pattern will remain
intact into the weekend with high temperatures near normal. 


.SHORT TERM...Satellite imagery shows stratus forming right along
the coastline this morning with the stratus as far north as Point
Grenville at 3 am/10z. The stratus covers Grays Harbor and has
made it about as far east as Elma. For the remainder of the area
just some high clouds over the Cascades from Mount Rainier south
otherwise skies were clear. Pretty mild early morning in some 
locations with Sea-Tac airport still at 70 degrees at 3 am while 
Forks has dropped down to 50 degrees. 

One more hot day for the interior today with the upper level 
ridge remaining over the area. Surface gradients are flat early
this morning so do not expect the stratus to make it much further
east in the lower Chehalis Valley. Temperatures aloft continue to
warm today but only by a couple of degrees. This will be offset 
by the lack of any low level offshore flow. Sandpoint profiler 
has temperatures 3 or 4 degrees warmer in the lower levels this 
morning versus yesterday but also has north northwesterly winds up
to 2000 feet versus the northeasterly winds Sunday morning. End 
result is high temperatures within a couple of degrees of Sunday's
highs for the interior, 80s and lower 90s. With the lack of any 
offshore flow today the coast will be cooler by 10 to 15 degrees 
with the wind coming off the ocean all day.

Changes in the pattern show up on Tuesday with a weak upper level
trough moving into the area late in the day. No precipitation
associated with the trough but it will increase the low level
onshore flow across the area. This weak marine push will drop high
temperatures over the interior by 5 to 10 degrees with 70s to mid
80s forecast. Along the coast with the onshore flow all day high
temperatures will have trouble getting to 70.

Trough east of the area Tuesday night into Wednesday with the low
level onshore flow pattern remaining intact. Stratus deck will 
make a run at the southern and central Puget Sound areas Wednesday
morning but with the marine layer being as shallow as it is 
forecasted for now will keep morning cloud cover west of Puget 
Sound. Another 5 to 10 degrees of cooling for the interior on 
Wednesday with highs back down to near normal, in the 70s. Highs 
on the coast will mostly be in the 60s. 

.LONG TERM...Extended models in good agreement on Thursday with
zonal flow aloft and low level onshore flow for another day of
high temperatures near normal, 70s for the interior and 60s on 
the coast. Another upper level trough moving into the area on 
Friday which will keep the low level onshore flow pattern going 
into Friday night. The ECMWF is a little deeper with the trough 
versus the GFS resulting in a deeper marine layer. Have used a 
model blend keeping high temperatures near normal for Friday. 
Trough shifting east over the weekend with the low level onshore 
pattern weakening adding a few degrees to the highs for the 
weekend with the warmer locations getting into the lower 80s. 


.AVIATION...Upper ridge axis over the region today with light flow 
aloft. Low level flow is weakly onshore. VFR across the area today 
except for patchy stratus near the immediate coast. Onshore flow 
increases this evening for some IFR in stratus near the coast, but 
remaining VFR over the interior of Western Washington.

KSEA...VFR. Surface winds N-NE 5 to 10 knots becoming NW in the 
afternoon hours. 27


.MARINE...High pressure over the NE Pacific will maintain
onshore flow across the region this week. Highest winds today are
over the outer coastal waters where Small Craft Advisory winds 
are likely. Onshore flow will increase on Tuesday. Gales are 
possible through the Strait of Juan de Fuca on Wednesday with a 
stronger onshore push. Moderate onshore flow will continue into 
the weekend. 33


.CLIMATE...Today is the day of the somewhat forgotten first 100 
degree day in Seattle. Since 1945 the official Seattle weather 
records have been recorded at Sea-Tac airport. Before the airport 
the official Seattle weather site was the Federal Building 
downtown. On this day in 1941 the high temperature at the Federal 
Building was 100 degrees. The first triple digit reading in 
Seattle recorded weather history. Since the records have been 
taken at Sea-Tac there have been two 100 degree plus days, July 
20th, 1994 100 degrees and 103 degrees on July 29th, 2009. Felton 


PZ...Small Craft Advisory until noon PDT today 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.