Area Forecast Discussion
Issued by NWS Portland, OR

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000
FXUS66 KPQR 201643
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
942 AM PDT Mon Aug 20 2018

.SYNOPSIS...A weak upper level low pressure system over central
Oregon will move east today, eventually being replaced by another
disturbance diving south from British Columbia. Other than a slight
chance of thunderstorms in the Cascades this afternoon and evening,
these systems will be mostly dry for NW Oregon and SW Washington.
Offshore flow will develop across the region later today and Tuesday
for warming temps and an influx of smoke and haze into the region
from fires burning in BC and eastern WA. Cooler weather is expected
to return toward the end of the week as onshore flow returns.

&&

.SHORT TERM...Today through Wednesday...Forecast remains on track,
with two or three more hot days before a significant pattern shift
brings a return to cooler weather toward the end of the week.

An upper level low over central Oregon is producing some impressive
lightning activity to the northeast portions of the state. These
storms have access to the deepest moisture, being on the east side of
the upper low. Conversely, our forecast area will remain on the back
side of this upper low, with predominantly north to northeasterly
flow aloft. This system will move east into Idaho later today, only
to be replaced by another upper low or trough diving southward from
British Columbia. Despite moisture being somewhat lacking due to the
continental origins of this system, we maintained a slight chance of
thunder for our Cascades zones, as the secondary system looks to come
with some decent forcing aloft. The 00z GFS is placing our Cascades
in the left exit region of a 60-80 kt jet streak diving down from BC,
which should enhance lift somewhat this afternoon, giving an assist
to instability which will be modest due to paltry moisture and weak
surface heating due to increasing smoke. Due to the northeasterly
steering flow, cannot completely rule out a thunderstorm or two
developing over the S WA Cascades then drifting into the Clark County
lowlands and eastern portions of the PDX metro. However the odds of
this appeared too slim to include a mention of thunder in the
forecast for the lowlands. Thunder chances decrease considerably
after this evening, as the upper trough/low digs south toward the
OR/CA border and dry northeasterly flow increases.

Speaking of the smoke, it will be on the increase today as low to mid
level flow turns northeasterly, advecting plenty of smoke into our
area from the numerous wildfires over British Columbia and eastern
Washington. Upstream, air quality has deteriorated rapidly across
much of Washington, and there appears to be no reason why this won`t
be the case when the thicker smoke arrives into our forecast area
today and Tuesday. We have relayed Air Quality Advisories (PDXAQAPQR)
from our partnering air quality agencies, valid through noon Wed. You
can find the latest information by checking with your local air
quality agency, or by accessing the smoke blogs
(http://wasmoke.blogspot.com for Washington and
http://oregonsmoke.blogspot.com).

As offshore flow intensifies later today through Tuesday,
increasingly hot and dry weather is expected throughout NW Oregon and
SW Washington. Even the coast has a decent chance to reach the 80s by
Tuesday, especially Lincoln City northward. Offshore gradients do not
appear strong enough to completely deter a seabreeze from reaching
the coast, which should allow them to avoid the same magnitude of
heat felt more than a couple miles inland. The smoke will also affect
temperatures as it becomes thicker across the region. Most model
guidance, particularly MOS guidance, does not take the smoke into
consideration. Therefore our forecast is considerably lower than
guidance for high temperatures, especially Tuesday. Regardless,
offshore flow will still make it hot, but temperatures should stay
below 100 degrees Tuesday. We may need a Heat Advisory for a large
portion of our CWA Tuesday, but due to the uncertainty of the impact
the smoke will have on temps, will hold off on this for now.

The second upper low/trough is expected to be kicked eastward Tuesday
night and Wednesday as flow across the NE Pacific becomes
increasingly zonal. Shortwave ridging will likely keep it hot for
one more day Wednesday, though the coastal valleys should be much
cooler as offshore flow wanes. There are strong signs of a
southwesterly push of marine air on the 06z NAM, which may hold
Eugene and Corvallis in the 80s Wednesday while most other inland
areas reach the lower to mid 90s again. Even if PDX fails to reach
90 degrees Wednesday, it still appears very likely that PDX will
break the record for 90+ degree days in a calendar year (29 days set
in 2015) by the time this spell of hot weather is done...possibly
tying it as soon as this afternoon.  Weagle

.LONG TERM...Wednesday night through Sunday...Ridge starts breaking
down late on Wednesday with a shortwave trough approaching, and a
pretty strong marine push Wednesday night into Thursday morning.
With flow generally northwesterly, stratus will likely push into the
north Willamette Valley, but will probably not make it down to Salem
or Eugene. Temperatures on Thursday will be much cooler, with highs
generally in the upper 70s to low 80s inland.

A northwesterly upper-level flow pattern sets up for later this
week, meaning onshore flow and westerly winds aloft.  Good news
about this is that it should keep smoke out of our area going into
next weekend. Seasonable temperatures should continue as well, with
highs inland in the upper 70s to low 80s, and morning clouds on the
coast breaking up in the afternoons. Next piece of good news is the
potential for a deep upper-level trough that the models are showing
could bring some rain either late next weekend or early the
following week. For those who want rain, don`t get your hopes up too
high just yet. This is still a long ways out, and as the models have
shown in recent weeks, much can change this far out in the forecast.
-McCoy

&&

.AVIATION...IFR marine stratus currently found along the coast
should retreat to the coastline this afternoon before dissipating
entirely late tonight and Tuesday morning as offshore flow increases.
However, the developing easterly low level flow will bring smoke
from east of the Cascades across the entire forecast area. This
will likely result in a mix of MVFR and IFR visibilities at most
taf sites through 18z Tuesday.

KPDX AND APPROACHES...Increasing offshore flow will bring smoke
from east of the Cascades and result in a mix of MVFR and IFR
conditions through 18z Tuesday. /Neuman


&&


.MARINE...High pressure over the northeast Pacific and thermally
induced low pressure over northern California and the Great
Basin will continue through the upcoming work week. This will
result in few changes little day to day change in conditions.
Gusts will generally remain below 20 kt through the week with
seas in the 4 to 6 ft range. Seas will become mixed and choppy
starting tonight as multiple wave trains with periods around 7
seconds and 12 seconds move across the waters. A storm system
dropping southeastward out of the Gulf of Alaska may result in
winds picking up next weekend. /Neuman


&&


.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...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. This area is
commonly referred to as the CWA or forecast area.


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