Weather Service Report


850 
FXUS66 KSEW 241642
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
942 AM PDT Mon Jul 24 2017

.SYNOPSIS...An upper level ridge will bring sunny and warmer weather 
to Western Washington through Tuesday, except for some isolated 
morning low clouds. An upper level trough will approach the region 
Tuesday night producing onshore flow and allowing more clouds and 
cooler weather to return Wednesday and Thursday. Another upper level 
ridge will build over the area next weekend.

&&

.SHORT TERM...Visible satellite imagery shows clear skies over W WA 
this morning, except for a little bit of stratus and/or fog on the 
central coast near Grays Harbor. Water vapor imagery shows the 
approaching low amplitude upper level shortwave ridge out near 135W-
140W this morning. Models remain on track with the approaching 
ridge. Northerly low level offshore flow has developed over W WA and 
will continue through Tuesday as the ridge traverses the region. 
Temperatures will be a little warmer today with highs in the 70s to 
lower 80s. Warming from the ridge and weak low level offshore flow 
on Tuesday should push highs up into the lower to mid 80s in the 
warmest spots.

The ridge axis should be over the coast Tuesday afternoon then shift 
over the ID Panhandle by 12Z/5 AM Wednesday morning. A weak upper 
level trough will be right behind the ridge. This eastward 
transition of the ridge and trailing trough are good indicators of a 
marine push. Onshore flow will increase Tuesday evening, but the 
weak to moderate pressure gradients indicated by the models suggest 
only a shallow marine layer will reach Puget Sound Wednesday morning 
before dissipating before midday. This may yield about 5 degrees of 
cooling on Wednesday. Kam

.LONG TERM...On Thursday, the SE part of a large upper level trough 
will have settled over W WA with SW flow aloft. More important is 
that a surface ridge just offshore will maintain a moderate onshore 
flow pattern. A couple more nights of onshore flow Wednesday night 
and Thursday night should produce a gradually deeper marine layer 
over the interior. Skies should clear by afternoon once again on 
Thursday, but the low clouds might last a little longer and cover a 
bit more of the lowlands on Friday. 

The models remain in flux for this weekend. Run-to-run consistency 
has not been very good. The GFS had been advertising the SW U.S. 
ridge building northward over the area but has now migrated to the 
ECMWF solution. The consensus now is that the large upper level 
trough offshore will eject NE across B.C. this weekend, in the 
process keeping W WA under SW flow aloft, onshore flow, and a cooler 
air mass. This should keep temperatures pretty close to normal. The 
models now postpone the strengthening SW U.S. ridge until next 
Monday. Kam

&&

.AVIATION...A broad upper ridge will be over the area today and 
tonight with northwest flow aloft. At the surface, onshore flow
will continue with high pressure offshore and lower pressure
inland. The air mass is dry and stable. Patchy valley low clouds and 
fog should burn off later this morning.

KSEA...Clear. Northerly winds 5-10 knots, strongest this
afternoon and early evening. Schneider

&&

.MARINE...Varying degrees of onshore flow will continue through
Friday with high pressure offshore and lower pressure inland.
Small craft advisory strength northwest winds will continue over
the outer Coastal Waters through Tuesday. There will also be small
craft advisory northwest winds for the inner Coastal Waters the 
next couple of evenings. 

The Central and Eastern Strait of Juan De Fuca should have small 
craft advisory west winds each evening for the next several days. 
Gales are possible each evening starting Tuesday. For now, the
best chance of gale force winds looks like Wednesday evening. 
Schneider

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 6 PM PDT Tuesday 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.

     Small Craft Advisory from 6 PM this evening to midnight PDT 
     tonight for Coastal Waters From Cape Flattery To James 
     Island Out 10 Nm-Coastal Waters From James Island To Point 
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To 
     Cape Shoalwater Out 10 Nm.

     Small Craft Advisory from 6 PM this evening to 3 AM PDT Tuesday 
     for Central U.S. Waters Strait Of Juan De Fuca-East 
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html

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Western Regional Climate Center, wrcc@dri.edu