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Area forecast discussion 
National Weather Service Portland or
216 PM PDT Wednesday Oct 1 2014

Synopsis...northwest flow aloft will be across the Pacific northwest
this afternoon and tonight in the wake of a recent low pressure
system. An upper ridge will approach the coast Thursday and move
onshore Friday for dry and warmer weather. Weak offshore flow is
expected to develop Thursday and continue into Friday. The ridge may
weaken enough over the weekend to allow some light precipitation over
the South Washington and far north Oregon coast and Coast Range.
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Short term...the cloud pattern from this morning has adjusted some
based on overturning from daytime heating and also from the
northwest flow over the area. The clouds are most solid in the
northern forecast zones and over the northern Cascade areas and a
good part of the Coast Range. The northwest flow will persist
tonight as the main upper ridge axis remains to our west. The models
indicate that there might be some brief decrease in the clouds this
evening with the loss of daytime heating...but overnight cooling
will lead to a thickening of the low clouds across our northern
zones...around Portland north...and especially around the higher
terrain. In areas that do clear tonight...especially south of
Portland...will likely see areas of fog and low stratus redevelop
late tonight and Thursday morning.

The models forecast a light offshore flow pattern developing Thursday
and Friday...partly due to a system passing well to our north into
Montana and its trailing higher pressure tonight and Thursday
morning...and due to good radiational conditions the next couple of
days that will lead to a cooler air mass east of the Cascades.
Gradients will remain very light tonight and early Thursday. The
models show more of a northerly flow developing down the valley with
a light east flow in The Gorge on Thursday. The gradient turns more
easterly Thursday night and Friday and the east wind through The
Gorge will increase then...with gusts to 35 miles per hour or so possible
around Corbett Thursday night through Friday. The offshore gradient
decreases late Friday.

The cool air mass and good radiational conditions will lead to cool
nighttime temperatures the next few nights. The models forecast dewpoints
staying in the lower 40s so this may limit how cool we do get...but
some upper 30s are possible tonight and middle to upper 30s Thursday
night.

A weak front tries to sag into our northern coastal waters this
weekend...but the models have not been very certain with this.
Otherwise the remainder of the area will see dry and mild conditions
except cool in the morning with local morning fog or low clouds
around. Tolleson

Long term...looks right now like northwest Washington and British
Columbia may get an atmospheric river event over the weekend. Most of
the moisture and rainfall should miss US...but the surface high
pressure system to our west to southwest will push some of the
moisture southward to the southwest Washington and northwest Oregon
coast. This brings a chance for showers along our north coast
starting late Saturday night through Tuesday.

Elsewhere upper level ridging over the West Coast will keep weather
fairly benign. Calm winds over the Willamette Valley will make
conditions favorable for fog to develop each night...and this fog
will lift to stratus during the day but will be slow to clear. -McCoy
&&

Aviation...mostly VFR conditions this afternoon except stratocu may
occasionally result in patchy MVFR ceilings. Think these clouds will hang
on into the early evening before clearing leads to radiation fog
overnight...especially for the interior valleys. Lingering surface
moisture...calm winds and clear skies is all it takes for IFR fog to
develop. The main uncertainty is whether there will be enough
clearing. The fog should be shallow and clear late in the morning or
early afternoon...especially as offshore winds increase. Kttd will
likely have easterly gusts up to 30 knots Thursday afternoon.

Kpdx and approaches...VFR conditions through this evening. Radiation
fog will likely develop late tonight with possibly IFR conditions
after 9z. One exception is if the clouds this afternoon do not clear
tonight...then expect ceilings to lower to MVFR around 1k feet or so
rather than shallow fog. Tj
&&

Marine...high pressure over the waters today with a thermal trough
over the southern Oregon waters will maintain north-NE winds over the
waters. The thermal trough will shift northward Thursday and the
winds will become more offshore...and slightly stronger south of
Cascade Head. Gusts may reach 25 knots south of Cascade Head Thursday
evening...mainly beyond 20 nm. There will be little change in the
pressure pattern Friday...except for slightly weaker gusts. A front
will decay over the northern waters this weekend and weaken the
surface pressure gradient. A Post-frontal swell will push seas back
up to 7 or 8 feet Saturday night into Sunday. High pressure will
rebuild north of the waters Monday and the northerly gradient will
strengthen. Tj
&&

Pqr watches/warnings/advisories...
or...none.
Washington...none.
Pz...none.
&&

$$

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This discussion is for northwest Oregon and southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.

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