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Area forecast discussion 
National Weather Service Portland or
906 am PDT Mon Aug 29 2016

Synopsis...an upper trough off British Columbia will dig south
and offshore through Tuesday. It then swings onshore during the
latter part of the week. This will maintain onshore flow over the
local area for at least night and morning low clouds and cooler
temperatures. Expect showers especially Tuesday night and beyond.
&&

Update...marine stratus was less extensive than forecast for the
second consecutive morning. Suspect a combination of weak onshore
flow and upper clouds may be to blame. Stratus will build back
towards the beaches this afternoon and may scatter out along the
north Oregon and South Washington coasts, but expect central Oregon
coast to remain cloudy through the day. /Bentley

Short term...(today through Wednesday night)...a closed upper low
has taken form off the British Columbia coast as has been forecasted
for the last several days. We are finally close enough the the main
event from this low such that mesoscale details are starting to come
in line among the various models. Unfortunately the marine stratus
precision still is not there aside from expecting clouds to bank up
along the coast and have varying degrees of inland impact over the
next 36 hours. Stratus has pushed back onshore tonight after
yesterday's retreat but has apparently not made it much further north
than Tillamook per surface observations. Unfortunately, passing mid
and high clouds are obscuring areas further north. Wherever the low
clouds are, do expect some patchy drizzle to come from them.

Unfortunately, the main mesoscale item of interest for Tuesday still
have notable variability with regard to position and strength. A
previous cutoff low has filled and has begun to drift northeastward
in response to the digging larger upper low. Most models agree to
bring it across the pacnw as an open wave but the varying positions
are making tuesday's forecast problematic. Decided to maintain the
most conservative approach coming from the GFS with the open wave
crossing the Oregon/California coast then sliding north along the
Cascades. This track brings showers and possible thunderstorms to
the central Cascades during the early afternoon. Model soundings are
marginal on their own, but an interesting wrinkle comes into play as
the open wave does carry a notable upper front along with it. A
tropopause fold down to 460 mb could act as a secondary trigger to
reinforce any upward motion created by afternoon heating. That said,
the bulk of models are pushing the threat of deeper convection well
east of the crest Tuesday afternoon and evening. This event is just
outside the available convective allowing models forecast data
window from the previous 00z runs, so it will be rather interesting
to see what today's cam runs bring.

Later Tuesday night and Wednesday, the primary frontal band has yet
again been delayed by the recent 00z models. May see some rain
develop along the north coast late Tuesday evening, but it's most
likely going to arrive later overnight and possibly closer to
daybreak Wednesday. The front will have lost most of its upper jet
support and will slowly March east Wednesday then finally crossing
the Cascade crest as late as midnight Thursday. Additionally, this
slower timing has also pushed the best threat for wetting rains well
north of our County Warning Area. Still would not be surprised if a couple areas pick
up a couple tenths of rain, but will not be advertising a widespread
soaking either. /Jbonk

Long term...no changes. Previous discussion follows... Thursday
through Saturday...continued cooler and cloudier with periods of
light rain will be the general rule for the second half of next week
as various impulses rotate across the region around the base of the
mean trough to the north. However, timing and strength differences
remain among the forecast models with a fair amount of spread among
ensemble members. As a result, less confidence in the precise
periods of higher chances of rain. Regardless, afternoon high
temperatures will remain in the low 70s inland during the extended
period, several degrees below normal for this time of year. With
cool and moist conditions, it will feel like an early taste of fall.
However, overnight low temperatures will be more seasonable, in the
50s. Bowen/Cullen
&&

Aviation...IFR to MVFR conditions continue for the coast and
coastal gaps, with some inland intrusion into the extreme southern
Willamette Valley and along the Columbia River about halfway to
kkls. Kast likely improves to lower VFR - perhaps even scattering
- after 20z; MVFR returns tonight. Konp likely will not improve
beyond high MVFR today. A stronger onshore marine push tonight
will likely yield widespread MVFR cigs inland Tuesday morning.

Kpdx and approaches...VFR prevailing through 11z Tue followed by
MVFR ceilings between 2000 and 3000 ft arriving after 12z. Cullen

&&

Marine...benign marine weather this week. Models have come to a
better agreement that winds will remain mostly south through the
week and even into next weekend. A period of stronger gusty winds
is possible on Tuesday and later Wednesday ahead of weak fronts.
Otherwise no small craft for winds concerns across our waters.

Seas will remain 5 feet or less through most of the week due to
light winds and a lack of any significant storm systems in the
North Pacific. Seas are a mix of long period (14 second) south swell
and northwest swell around 8 seconds. The longer period swell
will continue through mid week. Cullen

&&

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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