Scientific Forecaster Discussion

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

Synopsis...a frontal system will bring rain through today with
showers tapering off 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...rain will continue across western Washington today as
a Pacific frontal system moves inland. The warm front is lifting
through the region early this morning...and the cold front will
follow this afternoon. Onshore flow will increase behind the front
this evening with breezy to windy conditions. A Puget Sound sound
convergence zone will likely form over the central sound. Showers
will decrease after midnight tonight...although the pscz may linger
into Friday morning.

An upper level ridge will bring drier weather to the region Friday
afternoon. We should see some sun breaks with highs back near 60
degrees. The ridge will then shift east on Saturday as our next
front arrives. So expect increasing clouds throughout the day...with
rain spreading into the interior by late afternoon or early evening.
The trailing upper level trough will maintain scattered showers
through Saturday night. Weak ridging may bring another dry day on
Sunday...then another cold front will push inland Sun night. 33

Long term...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.

An 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...southwesterly flow aloft over western Washington with
warm front moving through the area this morning. Trailing cold front
reaching the coast this afternoon moving through the interior late
this afternoon. Upper level trough moving inland tonight with the
flow aloft becoming northwesterly behind the trough Friday morning.

Ceilings generally in the 1000 to 2000 foot range with local
ceilings below 1000 feet and visibilities 2 to 4 sm in light rain
and fog. Small improvement in the ceilings later this morning into
the afternoon hours. Cold front moving through the area during the
afternoon. Conditions improving behind the front with a 2500 foot
scattered to broken layer with another broken layer 4000-5000 feet.
Puget Sound sound convergence zone forming early this evening with
ceilings below 1000 feet and visibilities 3-5sm in showers near
kpae.

Ksea...ceilings 1000-2000 feet with visibilities 3-5 sm at times
this morning. Small improvement this afternoon. Frontal passage
around 23z with ceilings improving Post frontal. Southerly winds
10-15 knots with gusts to 25 knots near and just after the frontal
passage. Felton

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Marine...a cold front will move across the region today with Small
Craft Advisory winds over most of the waters. Gale force winds
developing in the central and eastern portion of the Strait early
this evening behind the front with the gale force winds lasting into
the early morning hours. Small craft winds over the remainder of the
waters easing Friday morning. Next front arriving Saturday for
another round of Small Craft Advisory winds. Large swell train for
this time of year behind the front with swell building to around 15
feet on Sunday. A weaker cold front will arrive on Monday. Felton

&&

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 remainder of the waters.



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