Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Seattle Washington
910 am PDT Thursday Apr 17 2014

Synopsis...a frontal system will bring rain through today with
decreasing showers tonight. Drier weather is forecast on Friday with
weak high pressure overhead. Additional fronts will arrive later
Saturday and later Sunday...keeping the weather generally wet. An
upper trough will bring showery weather early next week.

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Short term...a front will move onshore today with rain changing to
showers in the evening behind the front. Strong onshore flow will
prevail behind the front for breezy conditions this evening most
areas. Showers will taper pretty fast tonight...becoming mostly
confined to the mountains and a Puget Sound sound convergence zone after
midnight. A weak upper ridge will build over the area on Friday for
a mostly dry day. Skies are expected to become partly sunny later in
the day as the low levels dry.

A second front will bring rain later Saturday. Models have been
pretty consistent with the idea of rain reaching the coast around
middle-morning and spreading inland in the afternoon. Sunday will
probably be a mainly dry day in between weather systems. A third
front will be approaching from the west but it looks like it will be
slow moving and not spread rain into the area until around Sunday
night or even Monday. Schneider

Long term...previous discussion...an upper level low will form over
the NE Pacific on Monday while a weak cold front crosses through western Washington
for more rain. The low will then gradually shift inland Tuesday through
Wednesday for a chance of showers. The air mass will remain cool and
unstable with snow levels near 3000 feet. However with the low
overhead we should have good mixing and will probably see afternoon
sunbreaks. 33

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Hydrology...river flooding is unlikely for the next 7 days.

The exception is the North Fork of the stillaguamish river east of
the sunrise 530 landslide near Oso where a Flood Warning remains in
effect until further notice. Flooding continues for this river as a
result of water pooled behind the debris dam that partially blocks
the river.

The North Fork of the stillaguamish near Arlington is forecast to
rise today with a crest just under 9 feet this evening. The river
level will begin falling overnight and continue falling on Friday as
high pressure moves over the area.

Additional rainfall for the slide area through tonight will be on
the order of about one inch.

Additional fronts will affect the slide area later Saturday and
Sunday.

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Aviation...an upper trough offshore will move across the area
tonight. Strong SW flow aloft will become northwest later tonight. A cold
front offshore will sweep across the region this afternoon. Low
level onshore flow become strong behind the front.

Expect a Puget Sound sound convergence zone /pscz/ to quickly develop
between kawo and kpae behind the front late today. The pscz should
become quasistationary near the King/Snohomish County border this
evening.

Ceilings are expected to be mainly in the 1-3k feet range during much of
the day today. Rain will reduce visibilities into the 2-4sm range at times.
Some improvement is anticipated this evening.

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Marine...a cold front about 130 nm offshore will move across the
area this afternoon. Expect westerly winds to rise to gale force
over parts of the Strait of Juan Delaware fuca behind the front. A 1019 mb
high along the Washington coast and lower pressure east of the Cascades
will result in light onshore flow on Friday. Expect a strong cold front
to impact the region on Saturday.

There is a possibility of high surf conditions on the coast late
Saturday night through Sunday. The wave watch model indicated that
seas could reach 15 to 20 feet by Sunday morning before subsiding
Sunday afternoon. The current forecast slightly undercuts guidance.
If model trends continue...will need to bump up wave heights on the
coastal waters.

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Sew watches/warnings/advisories...
Washington...Flood Warning remains in effect for the North Fork of the
stillaguamish river east of the sunrise 530 landslide.
Pz...Gale Warning central and eastern Strait.
.Small Craft Advisory all other waters.

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You can see an illustrated version of the forecast discussion at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html