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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