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Area forecast discussion ... correction to synopsis 
National Weather Service Seattle Washington
300 am PDT Tuesday Sep 30 2014

Synopsis...moist onshore flow will bring some showers to western
Washington today...especially in the mountains. A weak system moving
through western Canada will brush the area on Wednesday for spotty
light showers in the north part. Thursday should be dry and partly
sunny but a front will sag south into the region on Friday. An upper
ridge will help limit rain to the coast and mountains this weekend.

&&

Short term...an upper trough moved into western Washington
overnight...following the weak front that moved through the area
Monday afternoon. The upper trough axis is east of the Cascades
early this morning...and the Puget Sound sound convergence zone that it
spawned has weakened and shifted south with the veering upper flow.
At 2 am there were scattered showers across the forecast area...
especially at the coast and along the west slopes of the mountains.
Temperatures were in the upper 40s to middle 50s.

Northwest flow aloft will continue today as the upper trough moves
across eastern Washington and into the northern rockies. The air
mass will remain fairly moist below 700 mb. As often happens...the
convergence zone will probably reform over the north Puget Sound sound
area early this morning...then move south and dissipate. Showers
will be light and spotty across western Washington...except in the
Cascades where up to 0.25 inch is possible. Sun breaks will be
limited.

A shortwave ridge in the northwest flow will move across the region
tonight...and showers will become even more spotty. Then the extreme
south end of an upper shortwave trough digging across western Canada
will just touch the forecast area. It will bring a chance of light
showers to mainly the north part of the area...especially the
Olympics and north Cascades. The rest of the forecast area will
remain mostly cloudy with a few more sun breaks.

A more significant upper ridge will move into the offshore waters on
Thursday. A warm front immediately upstream of the ridge axis will
bring rain to the coast of British Columbia...and how far south that
feature will extend is highly uncertain. At this time we will stay
with a dry and partly sunny forecast for our area...but some rain in
the north part of the area is not entirely out of the question.
Mcdonnal

Long term...the warm front discussed above and its trailing cold
front should weaken and sag south into western Washington on Friday.
Model solutions differ considerably for the weekend and early next
week. The European model keeps a stronger ridge over the region...
with the frontal system remaining mostly to our north. In the GFS
solution the upper ridge weakens and shifts east of the Cascades...
allowing the frontal band to move into western Washington unimpeded.
And the GFS keeps some precipitation over the region as another
shortwave with another shortwave arriving Monday. For now I will
resist the temptation to update the Saturday through Monday
period...but it does look a bit optimistic based on the latest model
runs. Mcdonnal

&&

Aviation...an upper trough over the area will move into Idaho this
afternoon. An upper ridge will move over the region in its wake.
Strong northerly flow aloft and continued low level onshore flow. The air
mass will remain weakly unstable for scattered showers today.

Areas of ceilings in the 1-3k feet range will lift later this morning.
There was also patchy fog reducing visibilities into the 1-3sm range. The
fog will also dissipate later this morning. Expect areas of ceilings
near 3k feet with heavier showers and within the Puget Sound sound
convergence zone /pscz/ this afternoon. The pscz did drift S to ksea
overnight but it quickly dissipated. The pscz will likely redevelop
this afternoon.

Ksea...ceilings will be mainly in the 1-3k feet range through this morning.
VFR conditions are anticipated by 0000 UTC Wednesday.

&&

Marine...
a 1027 mb high over the offshore waters with lower pressure inland will
result in onshore or westerly flow through Wednesday. The flow will become
flat Wednesday night and remain that way on Thursday due to an area of high
pressure over western Washington.

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...Small Craft Advisory in effect until midnight tonight for the
central and eastern Strait of Juan Delaware fuca.

&&

$$

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

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