Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 202134
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
240 PM PDT FRI MAY 20 2016

.SYNOPSIS...There are scattered showers over the mountains this
afternoon. Showers will probably increase over the weekend as a
disturbance moves into the area. An upper level trough will remain
over the region for typical May weather into next week.

&&

.SHORT TERM...An upper level low is over the Pacific Northwest,
California and Nevada. Daytime heating has triggered scattered
showers over the mountains this afternoon. A disturbance over
eastern Oregon and Idaho will probably move north into Eastern
Washington and then shift westward--this might bring an increase in
showers over Western Washington through the weekend.

.LONG TERM...Upper troughiness and low level onshore flow will keep
a chance of showers in the forecast. There will be sunny periods,
mainly in the afternoon, and most of the showers will be over the
Cascades. High temps will mostly be in the 60s for the lowlands.


&&

.AVIATION...A large upper level low will remain over the
PacNW through Sunday. The low center will remain down over N CA and
S Oregon through tonight with an upper level trough extending N over
WA. Flow aloft over W WA will be light E-SE. The air mass over W WA
is somewhat moist and unstable at lower levels, with cumulus clouds
SCT-BKN030-050 over the lowlands and mostly clear above. The cumulus
layer will dissipate this evening as the lower air mass becomes more
stable.

A weak upper level shortwave trough rotating around the NE part of
the large low at 21Z will spread moisture westward across the
Cascades after 09Z with light rain or showers possibly reaching the
Puget Sound lowlands after 12Z. Areas of MVFR cigs are expected to
redevelop after 12Z.

The upper level low will weaken Saturday night and Sunday as the
center drifts up over the northern Rockies, but W WA will still
remain under the west part of the broad low.

KSEA...Clouds have diminished this afternoon but the cumulus layer
is still present in the form of a SCT040 layer mainly E of the
terminal. This layer is expected to dissipate this evening as the
air mass becomes more stable. Surface winds will remain N-NW 4-10 kt
through 06z, then become NE 3-6 kt late tonight.

A weak trough will spread mid level clouds westward over the area
around 07Z with CIGS gradually lowering to around BKN050 around 10Z.
Some lower stratus is expected to form around 14Z with MVFR cigs.
Some light rain or showers should develop. Kam

&&

.MARINE...A weak surface trough will form over the N interior this
evening with slightly higher pressure along the coast. Models agree
that weak westerly flow will develop in the Strait of Juan de Fuca
this evening with speeds reaching borderline SCA levels in the
central and east strait. Weak W-SW onshore flow will develop across
the rest of W WA tonight. The surface trough will deepen into a weak
low over Vancouver Island by Saturday evening with W to SW onshore
flow continuing over W WA. The onshore flow should be a little
stronger Saturday evening with high end SCA winds likely in the
central and east strait. Higher pressure offshore will continue
through early next week allowing varying degrees of onshore flow to
continue. Kam

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small craft advisory central and east strait 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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