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Area forecast discussion 
National Weather Service Portland or
300 PM PDT Wednesday Sep 17 2014

Synopsis...a low pressure system currently off the California coast
will move produce occasional sprinkles and light rain showers across
the area through Thursday. Coastal locations are likely to see more
frequent bouts of wet weather during this period than interior
locations. While hazy skies will continue to linger over the
Willamette Valley...onshore flow will keep the worst of the smoke
from the 36 pit and deception complex fires over the Cascades and
points east. High pressure and offshore flow will build over the
region towards the weekend so expect smokier conditions to return to
the Willamette Valley and temperatures possibly approaching 90f
again.

&&

Short term...for tonight...water vapor satellite imagery reveals a
large upper level trough off the Oregon/California coast...placing
the region in deep south to southwesterly flow aloft. Infrared and
visible satellite imagery show plenty of middle to upper level clouds
streaming northeastward across the region. The strongest area of
showers this afternoon is actively developing along the northern
California coastline. While a few embedded thunderstorms have
developed in this region...generally a tenth of an of rain or less
has fallen so far. This region of showers and isolated thunderstorms
is being enhanced by a distinct pv anomaly and the left exit region
of a 90kt jet that will brush our southeastern zones this evening.
As a result...will maintain a slight chance of thunder for eastern
Lane County.

Short term model guidance and even the global models suggest the
shower activity over northern California will struggle to bring much
more than sprinkles to perhaps a couple hundredths of an inch across
our forecast area this evening. As a result...probability of precipitation were kept highest
near the coast where deep layer moisture is greater...and mainly in
the chance category for interior locations.

For Thursday...models continue to hint that low level warm air
advection just offshore and a subtle low to middle level boundary will
concentrate shower activity across far northwest Oregon and southwest
Washington late tonight into Thursday morning. When coupled with
increasing westerly flow at 850mb...measurable rainfall looks likely
to fall along the north Oregon and South Washington coast...Coast
Range and Cascades Thursday. Wetting rains still do not appear all
that promising...but there is certainly a chance especially near the
coast.

For Friday...building 500mb heights Thursday night and Friday should
result in conditions drying out considerably across most of the area.
The lone exception to this will be across extreme northwest Oregon
and southwest Washington where a dying front pushing into the region
from the northwest may bring a shower or two Friday.

For Saturday...thermal low pressure over northern California will
build northward up the coast...bringing increasing offshore flow and
a return to much warmer temperatures. Areas of smoke will once again
become a concern over the Willamette Valley during this period.
/Neuman



Long term...Sunday through Wednesday...Sunday warm temperatures
will continue due to continued easterly flow. Sunday afternoon winds
will shift to more southwesterly with a marine surge along the coast
bringing clouds back to the coast. Southwesterly winds and marine-
sourced air will cool temperatures for Monday and Tuesday ahead of
our next weather system...an upper-level trough approaching the West
Coast on Monday. The GFS...ECMWF...and dgex are in good agreement
that while the longwave trough stalls in the Gulf of Alaska...a
series of shortwaves sourcing from the parent low off the West Coast
will bring several chances for rain across our area Tuesday and
Wednesday. The models recently have tended to generate a lot of
moisture further in the forecast period...then back off as the event
approaches. For this reason...I am hesitant to commit to this being
a productive rainfall event. -McCoy



&&



Aviation...increasing middle levels clouds continue through this
evening under widespread VFR conditions. Showers will gradually
increase late this afternoon and evening...particularly impacting
coastal sites. MVFR and possible IFR ceilings around 2k feet should
develop along the coast around midnight. Smoke from the 36-pit
fire will continue to impact the East Columbia gorge...Upper Hood
River Valley and around Montana Hood...but it is not as bad as
yesterday. Smoke from fires over SW Oregon and northwest California will
drift north to NE through Thursday...affecting slantwise visibility
from Cascades eastward.

Kpdx and approaches...VFR continues with increasing middle level
clouds around 10k feet. Scattered ceilings likely forming in the
4000-5000 feet range later this afternoon/evening with increasing
chances for a shower or two. /27



&&



Marine...quiet conditions prevail over the waters through at
least late Friday...as weak low pressure will maintain S to southeast wind
less than 15 knots. Not much change in wave trains as northwest swell will
mostly dominate through early Thursday...before varying and
shifting from SW to west by the middle of the week. Seas running 6
to 8 feet...with highest further offshore...with periods around 13
seconds.

High pressure will rebuild over the waters Friday and Sat...with thermal
low pressure over the coast and coastal waters. This will bring
another round of east to NE winds to the waters this weekend with
winds possibly gusting to 25 knots on Sat and Sat night.



&&



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 as the forecast area.

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