Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 222141
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
245 PM PDT SUN MAY 22 2016

.SYNOPSIS...An weakening upper trough will result in a mix of
scattered light showers and sunbreaks through Monday. A weak upper
ridge will shift through the area Tuesday and Wednesday. Another
upper trough will move down over the Pacific Northwest for Thursday
through Saturday.

&&

.SHORT TERM...There are isolated showers on the radar this
afternoon--and there are some sunnier areas that can be seen on the
satellite imagery. The upper trough that has been over the western
U.S. over the past several days is weakening as the main center
kicked out into southern Alberta today. Through Tuesday there will
be a little weak upper ridging nosing over the area, but it is
pretty subtle. The weather over the next several days will be
determined by marine layer cloudiness sloshing around, building up
into afternoon cumulus, and then possibly spreading out late in the
day. That said, the UW column integrated cloud water forecast
suggests that large areas over the lowlands of Western Washington
could see mostly clear skies Monday evening and again Tuesday
evening--with a tendency for there to be more cloudiness in the late
night and morning hours.


.LONG TERM...Another upper trough will drop into the region after
the middle of the week--for a return of some shower activity. The
GFS and ECMWF are in pretty good agreement through Thursday but
diverge a great deal over the weekend on how the upper low
evolves and where it drifts.

&&

.AVIATION...A large upper level trough will remain over the Pacific
Northwest through Monday. The main upper level low center in the
trough was over southern Alberta at 21Z and will be continuing E,
allowing the large trough to gradually weaken through Monday.

The lower air mass over W WA is still somewhat moist. VFR conditions
predominate with a widespread BKN060 layer. From KPAE northward
there is a lower layer SCT-BKN around 3000 feet. There will probably
be a minor decrease in cloud coverage through this evening and the
lower layer will likely become more scattered.

After 07Z a weak upper level shortwave trough over southern B.C.
which is rotating around the main low, is expected to spread clouds
and some isolated light showers southward over W WA. The best
estimate is that the cloud layers will be mainly 3000-10000 feet,
spreading over the N interior late tonight and north Puget sound in
the morning, then over southern parts in the afternoon. There will
probably be some areas of lower MVFR cigs in the morning independent
of the approaching cloud layers.

KSEA...VFR conditions are expected to continue through 06Z with a
predominant SCT-BKN050-070 layer. Surface winds will be SW 8-16 kt
through 00Z then diminish to 5-12 kt. Kam

&&

.MARINE...A weak surface trough just W of the WA coast this
afternoon will drift S over the NW Oregon coastal waters Monday
morning then dissipate. The presence of the surface trough just
offshore will weaken the onshore flow tonight, making SCA winds in
the central/eastern strait of Juan de Fuca less likely. The forecast
will hold the W winds in the 10-20 kt range tonight...but confidence
is only moderate.

Once the surface trough dissipates Monday afternoon the large
surface ridge centered offshore along 140W will become the dominant
feature. The offshore surface ridge will build closer to the WA
coast...increasing the W-SW onshore flow Monday night. This onshore
flow pattern will continue the rest of the week, with SCA W winds
expected to return to the strait during the evenings the rest of
this week. Kam

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html



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