Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 151008
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
308 AM PDT Sat Jul 15 2017

.SYNOPSIS...An upper level trough and low level onshore flow will
bring mild weather to Western Washington today with mostly cloudy
skies over the coast and sunny skies elsewhere. Onshore flow will
increase tonight as the upper level trough crosses the area,
resulting in partly to mostly cloudy skies Sunday and possibly
some isolated convergence zone showers over north Puget Sound in
the morning. Onshore flow will persist next week allowing this
stretch of typically mild summer weather to continue.

&&

.SHORT TERM...Low level onshore flow has increased this morning,
ahead of an approaching upper level trough. So far though, this
hasn`t resulted in stratus forming over W WA. However, IR fog
imagery shows the leading edge of low clouds from an old front
spreading eastward across the coastal waters, which should bring
stratus to the coast probably around sunrise. This is too late for
the approaching stratus to reach inland areas this morning so at
best only isolated patches of stratus may develop in situ over the
interior. So at least for today, the main impact of the
approaching upper level trough and the increased onshore flow will
be cooler temperatures today with inland highs in the lower to
mid 70s. There could be some isolated light showers over the
coastal waters and N coast today from clouds approaching the
coast.

Models remain on track for tonight, with a moderate marine push
developing as the upper level trough crosses the area. The
stronger onshore flow should result in gale force winds in the
Strait of Juan De Fuca. Low clouds should increase over W WA
tonight, becoming widespread Sunday morning. It also looks like
there will be a convergence zone developing over N Puget Sound
late tonight and Sunday morning which could bring some isolated
light showers. The marine layer will deepen rapidly on Sunday but
the GFS shows the low level flow becoming more northwesterly in
the afternoon which could allow enough sun breaks to call it
partly sunny. It looks like convergence will keep the N Puget
Sound area mostly cloudy. Further cooling will probably knock back
highs to around 70.

Drier northerly low level flow will develop Sunday night and
Monday as a flat upper level shortwave ridge moves across the
area. This should allow the cloud cover to diminish for more
sunshine on Monday. GFS 500 MB heights remain around 5720 meters
which would suggest cooler temperatures than what MOS guidance is
indicating. I would favor the cooler model output over MOS but for
now I will stick with current forecast of lower to mid 70s. Kam

.LONG TERM...Typically mild summer weather will continue through
next week. A large warm upper level ridge will continue over the
Western U.S. and an upper level trough will remain centered
offshore around roughly 48N/135W. The key feature will be the
surface ridge just offshore, which will maintain weak to moderate
low level onshore flow. All models eventually eject the upper
level trough NE across mainly B.C., but the faster GFS does it on
Wednesday while the slower ECMWF and Canadian models hold off
until Thursday or Friday. Bottom line is that areas of morning
clouds with afternoon sunshine will be the rule, with highs in
the 70s. There may or may not be a few showers around when the
trough ejects. Kam

&&

.AVIATION...Dry southwesterly flow aloft will continue through the
day today before turning westerly Sunday. Low level flow will remain
onshore. The air mass is stable with very few clouds over Western
Washington. Models maintain some hints at low level clouds for the
early morning hours...but current obs and satellite imagery show
sufficient evidence to the contrary. Thus 12Z TAFs are likely to
need some cleaning up as most terminals will remain SKC or
SCT250...save for HQM...as they might see some stratus sneak in.
Things look a bit better for stratus development come early Sunday
morning.

KSEA...VFR with mostly clear skies with clouds increasing late
tonight and into Sunday morning. Light variable winds less than 5
kts will become southwest late this morning at 5-10 kts and will
remain there for the remainder of the TAF period. SMR

&&

.MARINE...High pressure offshore with lower pressure east of the
Cascades will keep the flow onshore for the next several days.
Current winds in the strait remain in upper end SCA range. With
onshore flow forecast to strengthen today...would expect an increase
in wind speeds in the strait and...as such...have upgraded gale
watch into a gale warning for tonight and into early Sunday morning.
Onshore flow will continue next week, with small craft advisories or
low end gale warnings likely each evening in the
strait...occasionally spilling into other waters. SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Watch from this afternoon through late tonight for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 5 PM PDT this afternoon 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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