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Area forecast discussion 
National Weather Service Seattle Washington
910 am PDT Wednesday Aug 20 2014

Synopsis...weak low pressure aloft helped create stronger low level
onshore flow for marine clouds and cooler daytime temperatures
today. Another low pressure system aloft is expected to swing
onshore Thursday and Friday that will bring a threat of showers
mainly to the mountains. Higher pressure aloft is forecast to build
onshore over the weekend and into next week for warmer weather and
more sunshine.

&&

Short term...the incoming weak upper trough was moving east near
the Idaho border this morning. It did increase low level onshore
flow late yesterday and overnight bringing a marine layer into
western Washington. Morning visible satellite images show the marine
deck is not solid - lots of breaks in it. Bases range from 2500 to
4000 feet with tops reported from ksea around 7000. Only spotty
drizzle reported. West to east pressure gradient has eased in
western Washington but still strong across the Cascades into eastern Washington. So
the bottom line - around 10 degree cooler inland today with some
sunshine as the day progresses. No change to the current forecast
package.

Looking upstream...the long wave ridge is in the NE Pacific and
trough just west of the dateline. Water vapor images show a weak
shortwave moving over the top of the ridge approaching 140w in the
Gulf of Alaska. 12z forecasts in thus far continue the trend of digging
this shortwave into the pacnw Thursday and Friday for continued low level
onshore flow...below average daytime temperatures and a threat of showers
mainly in the mountains. Buehner

Long term...from the previous discussion...no changes were made to
the inherited forecast...which essentially reflected near climatology. The
medium range solutions appeared to be in better agreement and showed
the County Warning Area under the influence of an amplified upper ridge centered
offshore. NC

&&

Aviation...northwest flow aloft will persist today as a weak upper
level trough moves into Idaho. Low level flow will remain onshore
with persistent high pressure over the offshore waters and lower
pressure inland. The air mass will remain generally stable...with
the exception of a slight chance of showers over the north Cascades.

The areas of 1k to 2.5k feet level clouds will continue to lift and
dissipate through the late morning. The middle level clouds...4-5k feet
will become scattered to broken by late afternoon.

Ksea...MVFR ceilings will lift through the late morning to around
4.5k feet by the early afternoon. Expect VFR conditions through the
evening with ceilings lowering to MVFR levels early Thursday
morning. South winds 6 to 8 knots will become southeast this afternoon
then ease to 4 to 6 knots around 06z/11 PM.

&&

Marine...onshore flow will prevail for the next several days with
high pressure offshore and lower pressure inland. Expect the typical
impacts of winds near Small Craft Advisory levels in the central and
eastern Strait of Juan Delaware fuca most evenings. Right now we are
expecting about 20 kts through the Strait this evening.

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...Small Craft Advisory outer coastal waters...central and east
Strait of Juan Delaware fuca.

&&

$$

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