Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 212159
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
300 PM PDT SAT MAY 21 2016

.SYNOPSIS...An upper trough will keep the weather showery and cool
through Sunday--especially in the mountains. Weak high pressure will
move through the area Monday through Wednesday. Another upper trough
will likely drop down over the region toward the end of next week.

&&

.SHORT TERM...Scattered showers over Western Washington this
afternoon should, over time, be increasingly pushed up into the
Cascades. Sunday looks like a cloudy day, with plenty of low level
onshore flow, but the showers may not do much more than dribs and
drabs in the lowlands. The Cascades could see another wet cold day
however on Sunday. The chance of showers will taper off Monday and
by Tuesday and there is a good chance for sunnier weather with high
temps warming a bit.

.LONG TERM...There will probably be a period of dry weather for two
or three days centered around midweek. Night and morning marine
layer cloudiness should tend to break up for the afternoon hours.
Another upper trough will likely drop down over the Pacific
Northwest toward the end of next week for another round of showers.
The 12z gfs is possibly too dry, keeping the showers with low
pressure offshore on Friday and not kicking off any rain til
Saturday in the southeast flow aloft as the pretty deep upper level
low drops south and stalls just off Oregon through next weekend. The
ECMWF solution is different with an weaker open trough dropping
further inland and then developing into a cut off upper low over
Idaho instead of off the Oregon coast.

&&

.AVIATION...A large upper level trough will remain over the
Pacific Northwest through Sunday. The main low center over eastern
Oregon at 21Z will move over western MT by 12Z. E flow aloft over W
WA tonight will become weak northerly on Sunday. Moderate W to SW
onshore flow will develop tonight which will increase the low level
moisture over the interior by Sunday morning. The air mass over W WA
remains moist and stable at 21z with mainly IFR/MVFR cigs from
roughly Puget Sound southward. VFR cigs predominate over the N
interior. Cigs are expected to lower overnight, aided by onshore
flow, especially over Puget Sound with widespread MVFR and areas of
IFR expected Sunday morning. Areas of MVFR will likely develop over
the N interior. Cigs should lift a little Sunday afternoon.

KSEA...IFR cigs are expected to lift to MVFR BKN-OVC015-025 from 00Z-
06Z the lower back down to BKN-OVC006-015 through Sunday morning.
Lowest conditions are likely Sunday morning through 18Z. Developing
SW onshore flow will contribute to the lowering conditions
overnight. Surface winds will be SW 5-15 kt. Kam

&&

.MARINE...A weak surface low over Vancouver island will maintain
weak to moderate W-SW onshore flow through this weekend. The onshore
flow will rise to SCA levels in the central and east strait this
evening peaking in the 20-30 kt range then decreasing late tonight.
The SCA winds in the strait will leak over the portion of the N
inland waters over the southern San Juan Islands. Mesoscale models
also show borderline SW winds rising to SCA levels across Puget
sound early this evening. Recent models are slightly weaker, but
still show some winds towards the lower end of the SCA range over
Puget Sound.

The surface low over B.C. is expected to shift slightly NE early
next week, but this will not really impact the onshore flow pattern
very much. Onshore flow should continue through mid week with
varying strength, probably weaker on Monday then stronger on Tuesday
and Wednesday. SCA winds will occur mainly in the evenings. Kam

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small craft advisory central and east strait, north inland
waters and Puget Sound tonight.

&&

$$

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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