Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 242140
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
241 PM PDT Tue Apr 24 2018

.SYNOPSIS...Strong high pressure over the region through midweek
will give way to an upper level low pressure system late in the
week. After a couple of sunny and warm days, showery weather returns
with the upper low, as well as more seaonal tempeatures lasting
through the weekend.

&&

.SHORT TERM...Tonight through Friday...Low level offshore gradients
weakn some tonight as weak surface low pres shifts north into the
thermal induced trough postion along the coast. The area of low pres
remains primarily along the coastal region tonight through Wed
night, perhaps allowing some marine clouds or fog to sneak north up
to the central Oregon coast as winds turn weakly onshore at times.
Otherwise the rest of the region should remain mostly clear with
warm temps.

On Thu, as the upper low off the north CA coast inches closer, the
surface trough finally make a more decisive move inland into the
Cascades. This turns the low level flow onshore across most of the
forecast area, with a sw push that will spread cooler air first into
the south Willamette Valley, then spread north over the interior
through the rest of the day and into Thu evening. GFS remains
decidely quicker at spreading chances for showers inland Thu night,
but ultimately, with the sse flow aloft spreading moisture in aloft
and the inland movement of the marine push providing a low level
boundary, will see a chance for some showers spreading in Thu night.
Model soundings are not particularly favorable looking for deeper
convection Thu night, but with modest diffluence aloft in the sse
upper level flow, will continue to indicate a slight chance for
thunderstorms Thu evening maily over the Cascades.

By Fri models have the upper low approaching the coast near the CA-
OR stateline. A deep marine air mass is evident in model soundings
Fri, coupled with some moist layers in the mid and upper levels.
This brings a general increase in the chances for showers, along
with a cooler marine air mass throughout the forecast area.

.LONG TERM...Friday night through Tuesday...Models consistent in
showing the upper low progressing slowly across the Pacific NW
through the early part of the weekend, augmented by another
shortwave attaching to the low Sat night and Sun. This will keep a
decent chance of showers over the region into Sun, which combined
with the low level onshore flow keeps temps a little below seasonal
normals. Sun night into Mon models in pretty good agreement that the
upper trough shifts east as a ridge of high pres presses in from te
west. This will allow temps to moderate some during the early part
of next week, while the chances for showers diminish.

&&

.AVIATION...Mainly VFR conditions are expected through the next
24 hrs. Marine stratus cloud to to work up the southern Oregon
coast tonight, and there is some chance that IFR stratus will
develop at KONP for a period overnight or early Wed morning.
Breezy offshore winds will continue through the day, but should
gradually weaken tonight.

KPDX AND APPROACHES...VFR conditions through Wed. Breezy
easterly wind gusts of 20-25 kt will continue through the
afternoon, then ease off tonight. Pyle

&&

.MARINE...Thermal low pressure over the waters is bringing breezy
offshore winds through the coastal gaps today, but gusts
generally less than 20 kt. The thermal trough will build
northward and weaken tonight. This may result in some 20 to 25 kt
gusts over PZZ270 this evening. Based on the latest model
guidance, a small craft advisory for winds is being issued with
the afternoon fcst update. A weak southerly wind reversal may
push into the central waters tonight. Then a more substantial
southerly surge is expected Wednesday night and Thursday morning,
which will likely result in widespread fog and low clouds.
Showery low pressure returns for Friday into the weekend. A weak
front is expected to arrive over the weekend, which may bring a
period of small craft advisory winds.

Westerly swell has pushed seas into the 8 to 9 ft range today,
with dominant periods around 15 seconds. The seas should
gradually subside tonight, then remain in the 5 to 7 ft range
through the remainder of the week. Pyle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory for winds until 2 AM PDT Wednesday for
     Waters from Cape Shoalwater WA to Cascade Head OR from 10
     to 60 NM.
&&

$$

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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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