Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 222153
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
300 PM PDT Sat Jul 22 2017

.SYNOPSIS...A weak weather system moving into Western Canada will
increase low level onshore flow into Western Washington tonight for
an increase in morning clouds Sunday. Then higher pressure aloft is
forecast to build over the region Sunday night through Tuesday with
dry and warming conditions. Low level onshore flow is then expected
to increase again resulting in moderating high temperatures closer
to late July seasonal conditions for the rest of next week.

&&

.SHORT TERM...The upper level low off the southeast Alaskan coast
is expected to continue moving inland through Western Canada through
Sunday night. This pattern should encourage stronger low level
onshore flow tonight resulting in marine cloudiness blanketing much
of Western Washington Sunday morning. The clouds will dissipate in
the afternoon in many areas but produce cooler high temperatures
than those of today, particularly in the interior.

Progs agree on rising heights aloft for a warming and clearing trend
Monday and Tuesday with high temperatures rising into the 70s along
the coast and 80s inland. Buehner

.LONG TERM...The overall long-wave pattern with the upper ridge over
the intermountain west and the trough in the vicinity of 140-150W
looks to continue through the rest of next week. This pattern leaves
Western Washington primarily in a SW flow aloft. The progs concur on
a sequence of weak shortwaves rotating out of the offshore long-wave
trough with the first arriving Tuesday night and Wednesday. Heights
aloft decline and low level onshore flow increases for another surge
inland of marine clouds with moderating high temperatures.

Guidance continues to hint at the possibility of convection near the
Cascade crest Wednesday night. Upon collaboration with neighboring
forecast offices, will increase a threat of thunderstorms there.

Progs differ on timing, but a few more ejecting shortwaves should
maintain low level onshore flow into Thursday and Friday for more of
the morning cloud/afternoon sun regime with close to seasonal
temperatures. The coming weekend may offer the start of a warming
trend as heights aloft rise. No rain is in sight, so the dry streak
in Seattle on day 35 today looks to extend through at least the
coming week. Buehner

&&

.AVIATION...A broad upper ridge over the area will shift inland with
a weak upper trough moving over Western Washington tonight and
Sunday. The flow aloft will be westerly. At the surface, onshore
flow will continue with high pressure offshore and lower pressure
inland. The air mass is stable. Low and mid level moisture near the
coast will spread inland tonight. The low levels will remain moist
for most of the day on Sunday.

KSEA...Mid clouds or VFR low clouds should develop this evening,
followed by MVFR low clouds late tonight. The low clouds will lift
to VFR Sunday afternoon and eventually scatter late in the day.
Southwest wind 4-8 knots should switch to northwest in the next few
hours and remain northerly into Sunday. Schneider

&&

.MARINE...Varying amount of onshore flow will prevail for the next
several days with high pressure offshore and lower pressure inland.
The Central and Eastern Strait of Juan De Fuca should have small
craft advisory west winds most evenings. Gales force winds are
possible Tuesday evening. The Coastal Waters will also have small
craft advisory northwest winds at times for the next few days,
especially the outer waters. Schneider

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 6 PM this evening to 3 AM PDT Sunday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 6 AM Sunday to 6 AM PDT Monday 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.

&&

$$

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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