Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 232139 AAA
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
238 PM PDT Tue May 23 2017

.SYNOPSIS...An upper level disturbance dropping south will move
inland through Washington and northeast Oregon tonight. The onshore
flow will strengthen this evening and tonight. The parent upper
trough will remain over the Pacific Northwest Wednesday for
additional cooling, more clouds, and spotty light precipitation. The
upper trough hangs back over the forecast area Thursday. High
pressure returns Friday for drier, sunnier and warmer weather. Warm
and dry conditions continue through the holiday weekend.
&&

.SHORT TERM...Tonight through Friday...Early afternoon water vapor
imagery showed the strong upper ridge that was over the Pac NW Sun
and Mon well east into the Northern Rockies and Great Basin. A low
pres area was also noted along the Southern British Columbia Coast
with the leading edge of the dry cold front near the N Washington
coast. As of 20Z the thermally-induced surface trough was located
from near KLMT through the Columbia Basin. Models shift this feature
to the Idaho border by late evening. Early-afternoon visible imagery
indicated stratus solidly along the coast and pushing well into the S
Washington and far N Oregon coastal valleys. The KTTD-KDLS gradient
has gone from nearly neutral at 15Z to 3.5 mb as of 20Z. West wind
has pushed through the Columbia Gorge into K4S2. Some of the more
wind-prone sites in the Willapa Hills and North Oregon Coast Range
have also noted stronger West to Northwest wind.

Models are in general agreement showing the low-level onshore flow
continuing to deepen along the coast and make more penetration into
the coastal valleys through the evening. The Hi-RES ARW suggests
stratocumulus and/or cumulus will develop and back-build from the
Cascade foothills late this afternoon and through the evening. Based
on latest trends have scaled back the drizzle potential for this
evening. Generally confined any drizzle threat west of the Oregon
Coast Range and SW Washington coastal valleys and Willapa Hills this
evening.

The strengthening onshore flow, in conjunction with increasing west
flow aloft, will result in breezy to windy conditions through this
evening. The strongest wind will be through the coastal gaps, the
Columbia River Gorge and over the mountains. Breezy to windy
conditions are also likely in the interior valleys as well,
especially those aligned west-to-east.

The primary short-wave moves south and east tonight, but, in its
wake, an upper trough carves out over the Pacific Northwest
Wednesday, which lingers into Thursday. The NAM would suggest a
marine layer to nearly 5000 feet MSL across the northern two-thirds
of the forecast area through early Wed afternoon. NAM forecast
soundings for KPDX and KSLE have the top of the marine layer close to
6000 feet MSL 19Z Wed. Models also show a short wave disturbance
rotating through the broad upper trough Wednesday, which should keep
clouds fairly solid over much of the area and possibly result in an
over-turning of the marine layer for more drizzle or even light
showers.

The forecast area will be on the back side the upper trough Thursday.
NAM indicates a fairly solid marine layer across the forecast area
Thursday morning, other than the southeast quarter. GFS and ECMWF
show the Cascades will get backdoored with the possibility of some
showers or even a few thunderstorms in the afternoon and evening
associated with the circulation on the back side of the upper trough.
The GFS and NAM are not overly excited about convective prospects
Thursday afternoon. NAM sounding for a point in the N Oregon Cascades
valid 23Z Thu shows little to no CAPE. Will leave the mention in for
now as not to yo-yo. The increased sunshine should push afternoon
temps inland back into the 70s. Friday looks to be quite pleasant
with minimal morning stratus or fog. Model 850 mb temps rise to +13C
in the far NW to +17C in the Lane County Cascades, which should
translate to lower to mid 80s in the interior valleys. Weishaar

.LONG TERM...Friday night through Tuesday. 500 mb high pressure
remains the dominant feature through the weekend. Sat looks to be as
warm as Fri, based on forecast 850 mb temps. Sunday is expected to be
a few degree warmer, pushing into the upper 80s for interior valleys,
Columbia River Gorge and Upper Hood River Valley. The ECMWF has the
500 mb ridge axis squarely over the forecast area Sat through Sun
morning, then slowly shifts the ridge axis east Sun afternoon. The
GFS shifts the ridge axis east a little sooner and also tries to
develop some minimal QPF near the Cascades, hinting at possible
convection near the Crest. The ECMWF maintains more of a SW mid-level
flow Mon, although it does hint at a short-wave off the N California
coast resulting in a slightly more unstable S-SW mid-level into the
Central Cascades. High pres aloft remains sprawled over the Western
U.S. Tue for continued warm and dry conditions. The ECMWF and GFS
maintain some light QPF over the Cascades Tue, but will not mention
thunder at this time due to higher uncertainty based on the GFS
ensembles and GFS and GEFS comparisons. Weishaar
&&

.AVIATION...Coastal TAF cigs are hovering between IFR at 008 and
MVFR at 015. May still see some periods of drizzle
this evening but am now beginning to question if there will be
much of a Vsby impact. Stratus has not penetrated inland to
speak of with Cig free skies largely continuing through the
evening and overnight hours. Will continue on bringing a stratus
layer inland after 09z for the norther inland terminals and
closer to sunrise for KSLE and south. Recent hi-res models are
starting to lean against the idea of impactful stratus, however,
so would not be surprised if the 00z or 06z TAF package removes
the mention entirely. Main impact inland will continue to be a
surge of winds through the Coast Range gaps. Expect several hours
of wind gusts 20-25 kt this evening as the marine layer surges
inland.

KPDX AND APPROACHES...VFR conditions most likely through this
evening under a rather strong marine wind surge. Expect northwest
gusts around 25 kt to prevail. Once winds began to ease that will
allow a stratus Cig to develop perhaps around 015 AGL. Am losing
confidence this overnight deck will develop as near-term hi-res
model runs are keeping rather dry conditions in place for a few
hours. If it does develop, then it appears to remain through at
least 24/18z. /JBonk

&&

.MARINE...High pressure over the NE Pacific and a thermal low
inland combine to continue SCA wind gusts through Wednesday and
likely a significant portion of Thursday. A weak cold front
reaches our northern waters later 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.

Have expanded the SCA for Seas through Wednesday morning across
all waters. a 4 foot background swell may produce a dominant 12
second period at times, but feel the 7 to 9 ft very steep/short
period/longer fetch wind waves will be prevailing much more
often. Will start to see a 9 to 10 second fresh swell mix in
later tonight and into Wednesday but had little confidence
regarding exact timing. Seas will ease by Thursday as winds ease
with little threat from winds or notable seas affecting the
waters through the weekend.

&&

.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
     Wednesday for Coastal Waters from Cape Shoalwater WA 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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