Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 241013
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 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...Skies remain mostly clear over W WA this early morning
and are expected to continue through the day and into tonight. Some
clouds may be present along the coast mid to late this
morning...with the usual thinning in the afternoon and redevelopment
at night. As the upper level ridge makes its way eastward...cannot
imagine said clouds would encompass much in the way of
coverage...likely remaining within partly cloudy wording thresholds.

Today will kick off a warming trend...thanks to the aforementioned
approaching upper level ridge...as widespread 80-plus degree temps
are expected in the interior lowlands. Coastal locations look to get
in on the action as well with temps expected to be warmer by 2 degs
in HQM to 8 degs warmer in UIL. Tuesday notches up the heat a little
more for the interior while the coast remains pretty static. Part of
that looks to be due to a thermal pressure trough developing over
the west Sound allowing for some weak offshore surface flow.

The ridge ejects Tuesday night allowing for a good marine
push...bringing a return to clouds and cooler temperatures. The
upper low behind this ejection follows a similar path as one that
went well north of the area in northern BC over the past weekend. As
such...a cooling trend kicks in for both Wed and Thu...with Thu
proving to be the cooler of the two days.  SMR

.LONG TERM...Also similarly...once that Low and its associated
troughiness moves eastward...W WA will once again welcome another
shortwave ridge for Friday that moves eastward pretty quickly as by
late Friday afternoon it has already headed for the hills...or
Cascades in this case. This means a bit of a warm up...but interior
lowland temps will remain right around 80. Models want to suggest
another upper level low for the upcoming weekend...this time
drooping a little further south than its two predecessors...but both
GFS and ECMWF hesitate to bring it so far south as to bring any real
chance of precip to the area. That said...looks like current dry
streak is showing no signs of abating any time soon.  SMR

&&

.AVIATION...Northwest flow aloft over Western Washington early
this morning will continue today and weaken tonight, ahead of an
upper ridge offshore. Weak northerly onshore flow will continue
at low levels. The air mass has become fairly dry overnight and it
will dry further today and tonight.

There is some low stratus along the coast from around Grays Harbor
southward, which will probably move into the KHQM terminal at
times this morning. Otherwise skies are mainly clear across
Western Washington. Patchy low stratus could form around daybreak
in the central Puget Sound region and last for a couple hours, but
it is not expected to produce ceilings. Tonight looks similar,
with a bit of coastal stratus and generally clear skies elsewhere.

KSEA...Mostly clear skies with a northerly breeze will be the rule
through tonight. Patchy low stratus could form around day break;
hence the TEMPO SCT004 group in the TAF. McDonnal

&&

.MARINE...Varying degrees of onshore flow will continue today
through Friday with high pressure over the offshore waters and
lower pressure east of the Cascades. Small craft advisory
northwest winds should continue over the outer coastal waters
through Tuesday, with small craft advisory winds over the inner
coastal waters this evening. Small craft advisory westerlies
should develop this evening in the central and eastern Strait of
Juan de Fuca this evening. McDonnal
&&

.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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