Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 161030
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 AM PDT Wed Aug 16 2017

.SYNOPSIS...An upper level ridge moving over the Pacific Northwest
coupled with low level onshore flow will maintain seasonably mild
summer weather into Friday. A weak upper level trough is expected to
bring some cooling and possibly some drizzle Friday night into
Saturday. Another upper level ridge will rebuild over the region
Sunday through Tuesday.

&&

.SHORT TERM...Current satellite imagery shows marine stratus working
its way onto the coast now and will likely work a little further
eastward as the morning progresses. Current forecast does not have
stratus passing SHN...and given their plodding natures...this seems
to be on track.

A passing front to the north in BC may have enough southward extend
to possibly trigger a slight chance for showers over the far
northwestern tip of the Olympic peninsula today...while passing
north of all other portions of the CWA. Sustained upper level
ridging and dry northwesterly flow aloft will keep conditions dry
and seasonable into Thursday morning. Then the upper level flow
turns more westerly as the ridge axis preps to cross the area
throughout the day Thursday. Increasing upper level heights may
nudge temperatures a degree or two Thu and again on Fri...but models
maintain that there is a prospect for partly cloudy conditions on
both days...which would serve to offset any significant heating that
one would normally expect. A weak trough will traverse the area
Friday...but any precip associated looks to remain to the north in
current model solutions. Given that solutions 24 hours ago certainly
were not convincing with regards to precip prospects down here
during that time frame...have no problem buying into the
current drier solution.

Overall...not much in the way of changes to inherited forecast and
appears to be a benign weather pattern ahead for the near term.  SMR

.LONG TERM...Upper level troughiness looks to linger over the area
for the weekend but again any precip chances look to remain to the
north of the CWA. In fact...impact looks to be fairly minimal as
temperatures may cool a degree or two...but are generally expected
to remain in the mid 70s for the interior lowlands. With all eyes on
Monday...and hopefully not directly into the sun...models remain
consistent regarding bringing another upper level ridge over the
area. ECMWF and GFS are still a little fuzzy on the details...such
as the amplitude of said ridge...but current solutions continue to
show that clouds generally will not be much of a problem come
showtime...although locations along the coast may have to contend
with some lingering morning stratus. Beyond that...the trough looks
to move east of the Cascades Tuesday potentially setting up for an
upper level low over the Pacific to start influencing local weather
for the second half of the week. Should this solution hold...next
Thu and Fri might look good for some precip.  SMR

&&

.AVIATION...A weak upper level system will pass north of Washington
today. Light northwesterly flow aloft with low level onshore flow
continuing. Air mass stable and mostly dry except for passing high
clouds and patchy near-surface moisture this morning along the coast
and Strait. VFR skies today except for patchy IFR fog/stratus near
the coast/Strait and possibly around KOLM this morning. Onshore flow
will bring a thicker stratus cloudy layer around 5K into most of
Western Washington late this afternoon or evening, lowering into
areas of MVFR stratus late tonight/Thursday morning.

KSEA...VFR conditions. A few high clouds, then a 5K ft BKN stratus
deck will develop after 00z late in the day. Stratus will likely
lower into an MVFR ceiling late tonight/Thursday morning. NE wind 5-
8 kt becoming S after 14Z, then northerly 5-8 kt by late this
afternoon.

&&

.MARINE...Varying degrees of onshore flow will prevail through the
weekend. Westerly small craft strength winds in the central and
eastern Strait will peak during the evening and overnight hours. Low-
end gales are possible Friday evening. dtm

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory through tonight for the central and
     eastern Strait of Juan De Fuca.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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