Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 231041
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
340 AM PDT Tue May 23 2017

.SYNOPSIS...Higher pressure of recent days will retreat and weaken as
a cool system approaches from the northwest later this morning and
afternoon and moves inland through Washington and northeast Oregon
tonight, increasing the onshore flow later today and tonight and
spreading clouds along the coast today and inland tonight. A little
drizzle or sprinkles are possible along the coast today and tonight,
and locally inland tonight. The parent upper trough will remain over
the Pacific Northwest Wednesday for more cooling, clouds, and spotty
light precipitation, lingering into Thursday with mainly some Cascade
showers. High pressure returns Friday for drier, sunnier and warmer
weather which will persist through the holiday weekend.
&&

.SHORT TERM...Today through Thursday...Today will be rather warm
again but probably not as warm as it was on Monday. The upper ridge
that had been over the Pacific Northwest will retreat and weaken as
the next system along the B.C. coast approaches today and moves
inland across Washington and northeast Oregon tonight. This system
spread low clouds along the coast today, with onshore flow increasing
inland later today and tonight. Clouds will spread inland through the
evening as well. Some drizzle is possible near the coast and locally
inland especially north overnight tonight.

It will be breezy to windy later today and this evening with the
associated marine push due to the increasing westerly flow aloft,
strongest through the coastal gaps and the Columbia River Gorge and
over the mountains, but some wind is expected in the valleys as well.
Highs inland will be difficult to forecast today as we have a warm
start this morning but onshore winds should increase by mid
afternoon. It should be a bit cooler than Monday but think we have a
chance at mid 80s before the onshore flow really takes hold later
today.

The initial main system moves eastward tonight, but an upper trough
carves out over the Pacific Northwest Wednesday in its wake which
lingers into Thursday. The models indicate a short wave swings
through the trough Wednesday which should keep clouds fairly solid
over much of the area and possibly produce some more drizzle or light
showers. Wednesday looks to be the coolest day of the week, with
inland temps probably no more than the mid 60s.

We are on the back side the upper trough Thursday. We should see some
sunshine break out in the afternoon after areas of morning clouds.
The models indicate the Cascades will get backdoored with some
showers or even a few thunderstorms in the afternoon and evening from
the circulation on the back side of the upper trough. The increased
sunshine should push afternoon temps inland back into the 70s.
Tolleson

.LONG TERM...Thursday night through Monday. High pressure builds
Thursday night and Friday and holds over the area through Monday.
Temperatures will warm into the mid to upper 70s on Friday then to
the low 80s for the holiday weekend. A low moving across southern
California will maintain a weak surface pressure gradient across NW
Oregon and SW Washington and prevent a strong thermal trough from
developing. This will result in light winds that are diurnal and
terrain driven allowing some marine influence in the afternoons and
nights, which will prevent the temperatures from getting excessively
hot. The models hint at some convection possible early next week but
the models are not all in agreement and the moisture source is a bit
in doubt so will hold off adding that for now.      TJ/Tolleson
&&

.AVIATION...Stratus associated with the approaching front will be
slower to make it to the Coast than previously thought. Will be
updating the TAFS to reflect this with the upcoming TAF package.
Could see some low clouds develop on the Central Oregon Coast,
bringing cigs and vis down to IFR this morning at ONP. Lesser chance
farther north, but can`t rule out IFR cigs at AST as well. If IFR
conditions do develop, expect them to continue as the marine clouds
ahead of the front push onto the Coast around 21Z today. IFR
conditions likely to continue along the Coast through tonight with
occasional drizzle reducing vis at coastal terminals as well. Inland
expect VFR conditions to continue for the next several hours. Expect
clouds to spread inland after 00Z this evening. Model guidance
predicts cigs with these clouds will be around 2500 ft, bringing MVFR
conditions to all inland terminals this evening. Expect MVFR cigs to
persist overnight tonight.

KPDX AND APPROACHES...VFR conditions through this morning and this
afternoon. Clouds move into the terminal around 03Z this evening,
with guidance suggesting cigs lowering to around 2500 ft. Expect MVFR
cigs to persist through the overnight hours. -McCoy
&&

.MARINE...High pressure over the NE Pacific and a thermal low inland
combine to continue SCA wind gusts through Wednesday. A weak cold
front clips our northern waters this afternoon, resulting in
increased NW winds, but still at small craft advisory speeds. Winds
will start to diminish late Wednesday into Thursday, but it may not
be until Thursday night before gusts drop below 21 kt across all of
the waters. Longer range guidance suggests lighter wind speeds
thereafter through this weekend.

Issued a SCA for hazardous seas this morning as buoy 50 has been
showing square seas for the past couple hours. Likely more of a mix
of steep 7 sec waves and a background swell train around 10 sec, but
will likely see this continue to be the case until late this morning
when dominant wave periods increase to 10 to 12 seconds. Will likely
see another chance for steep, square seas this evening through
Thursday as we see winds ramp back up and seas build up near 8 to 10
ft on Wednesday afternoon/evening. Seas gradually fall starting
Thursday and continue to fall through the weekend. -McCoy
&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory for winds until 5 PM PDT Wednesday for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     60 nm.

     Small Craft Advisory for hazardous seas until 11 AM PDT this
     morning for Coastal Waters from Cascade Head OR to
     Florence OR out 60 nm.

&&

$$

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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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