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Area forecast discussion 
National Weather Service Seattle Washington
910 am PDT Monday Aug 31 2015

Synopsis...a frontal system will bring rain to western Washington
mainly from Puget Sound sound northward today through Tuesday. A large
upper level low will settle over the region Wednesday through Friday
for cool showery weather. The upper level low will weaken and move
east early this weekend for a gradual drying and warming trend.


Short term...a broad upper level longwave trough extends from the
NE Pacific across western Canada and the pacnw this morning...with
fairly strong west flow aloft over West Washington for late August. A frontal
system is hung up over SW b.C. This morning with the cold front
extending west-southwest from about central Vancouver Island. Radar is showing
steadier light rain over the north coast and north interior and smaller
areas of light rain crossing the central coast and Puget Sound sound

Water vapor imagery shows a well developed shortwave trough out near
49n/140w and the tail end of the cold front....that is digging southeast
toward north California. As the shortwave moves southeast it will will drag the
Vancouver Island cold front with it...southeastward across West
Washington...weakening the front in the process. For West Washington this means the
periods of rain...mainly over the north part will continue today...but
should be lighter in intensity and probably more intermittent
tonight and Tuesday. Per the GFS the much weaker front should be
just east of the area Tuesday afternoon. S winds will remain breezy
through this evening then gradually weaken and become SW through

The broad upper level longwave trough over the region will gradually
deepen and gain amplitude over the region this week. The trigger for
this is a small upper level low over the Gulf of Alaska today. This
small low will move southeast along the b.C. Coast through
Tuesday...eventually becoming the center of this large longwave
trough over central b.C. On Wednesday. The GFS shows a shortwave
trough associated with the developing low center crossing West Washington
Tuesday night. This shortwave will probably bring an increase in
showers over West Washington. The GFS shows another weak shortwave crossing West
Washington Wednesday...followed by a larger more developed shortwave trough
Wednesday night. This means the weather will become showery Tuesday
night Onward and there will be a cooling trend as the large trough
deepens over the region. Winds will be breezy at times as each
shortwave trough crosses West Washington. Kam

The snow level will be near 9000 feet today but fall to 5000 to 6000
feet by Wednesday. Continuing moist westerly flow of roughly 25 knots
between 5000 and 10000 feet will bring a foot or more of snow to some
of the high terrain of the Cascades and Olympics over the next three
days. Terrain above 8000 feet on Montana Baker will probably get 2 to 3
feet of snow between today and Wednesday. Mcdonnal

Long term...previous discussion from the 320 am afd...models all
show another shortwave digging into the base of the upper
trough...which will keep it over southern British Columbia and the
Pacific northwest through at least Thursday. Model solutions
separate beyond Thursday though. The 00z Monday GFS shows the upper
trough moving east of the Cascades Friday...with dry northwest flow
over western Washington Saturday and Sunday. The Canadian and
European models are similar...keeping an upper low over the region
through Saturday and holding off dry weather until at least Sunday.
Our forecast...for now anyway... leans heavily toward the GFS
solution. Mcdonnal


Aviation...a frontal system will be over the area today and
Tuesday. Areas of rain will fall...especially over the coast and
mountains...but the rain should spread to most of The Lowlands by
afternoon. The mountains and higher terrain will remain obscured and
the visibility will drop at times in the precipitation and mist...especially
coast. This last day of August is more like a typical day in the
rainy season.

Ksea...cloudy with rain at times...ceilings will drop a bit more as
the rain starts up today. Southerly wind a bit breezy at times.


Marine...a frontal system will be hung up over the area
today and Tuesday. Southerly gradients today are strong enough for
breezy weather and small craft advisories. Winds will probably ease
overnight as the front stalls over the area and starts to weaken.
A trough will move over the area later Tuesday and Wednesday. Weak high
pressure will build late in the week.


Sew watches/warnings/advisories...
Pz...Small Craft Advisory coast...East Entrance to the Strait...
Admiralty Inlet...northern inland waters...Puget Sound sound.
Small Craft Advisory for rough Grays Harbor bar.




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