Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 230949

Area Forecast Discussion
National Weather Service Portland OR
249 AM PDT Mon Apr 23 2018

.SYNOPSIS...High pressure over the region will support mostly clear
skies through at least Wed and warm temperatures. Tuesday and
Wednesday will be the warmest days of the week with interior
temperatures likely peaking in the low 80s. Thursday will be slightly
cooler as an upper level low approaches. Then much cooler
temperatures expected Friday and through the weekend as an upper
level trough moves over the area.


.SHORT TERM...Today through Thursday. The general weather pattern will
change very little the next few days with high pressure aloft and a
surface thermal trough. This will result in dry weather with minimal
clouds (high and thin), and warming temperatures. Water vapor imagery
shows the upper ridge axis just west of the coast early this morning.
At the surface, high pressure is across Washington and northern
Oregon with an inverted trough across south and central Oregon.

The Troutdale to Dalles surface pressure gradient is currently weakly
negative (-1 mb), and the east winds will be fairly light this
morning. This gradient will increase this afternoon and evening to
around -5 mb as the thermal trough strengthens, with east gusts up to
20 mph possible near the Columbia River Gorge entrance. Clear skies
and a warming atmosphere will allow temperatures to warm quickly
today, with coastal temperatures possibly reaching 70, and the
inland valleys in the mid 70s.

Temperatures will warm even more Tuesday and Wednesday as the
thermal trough sets up over the Willamette Valley. These days will be
the warmest of the week, and of the year so far with interior valleys
expected to warm into the upper 70s or low 80s. Offshore winds will
weaken Tuesday afternoon, and a low off of the south Oregon coast
will result in south winds developing along the Central Oregon coast.
This will result in less warming for the Central Oregon coast and
south Willamette valley. The Central Oregon coast will likely peak in
the upper 60s or low 70s Tuesday and Wednesday afternoon with the
inland areas of Lane County peaking in the upper 70s. The overnight
temperatures will be warm too with minimum temperatures in the upper
40s or low 50s, and a subsidence inversion will allow mid slope
nighttime temperatures to be warmer than the valley temperatures.

A large upper level low approaches Wednesday night and Thursday
weakening the upper ridge. The thermal trough will be more over the
Cascades, and the offshore winds may not make it to the coast.South
winds will dominate the coast becoming slightly onshore in the
afternoon.  This will result in much cooler afternoon temperatures
for the coast, and slight decrease in temperatures inland. Diffluence
aloft may result in afternoon showers over the Cascades. ~TJ

.LONG TERM...No Changes. Previous discussion follows...Thursday night
through Sunday...The general idea of the cutoff low off the north CA
coast slowly approaching the coast late in the week suggests some
moisture spreading north in the  southerly flow aloft for increasing
chances of showers beginning late Thu or Thu night. The pattern with
a se flow aloft can also suggest the possibility for thunderstorms,
particularly over the Cascades, but for now will leave out the
mention as GFS soundings not indicating sufficient instability. Temps
will be cooling back down closer to normal as onshore flow resumes.
While models begin to show more significant differences by the
weekend, the net result is still plaint of clouds, chances for
showers, and seasonable temps.


.AVIATION...VFR conditions next 24 hours. Offshore flow will
increase today and continue tonight before easing.

KPDX AND APPROACHES...VFR. Easterly gusts peak at 20-30 kt from
23/19 to 24/12. /mh


.MARINE...Surface thermal trough will extend to the south
Washington coast today, then drift offshore tonight. This will
result in winds turning increasingly offshore. Main concern will
be wind gusts of 20 to 25 kt possible downwind of the coastal
river gaps including the mouth of the Columbia River. A long
period westerly swell will arrive around this time and it will be
interesting to see how the winds and swell will interact. Would
expect waves to stand up and become steep. As of now, wave
guidance indicates seas will not reach 10 feet or become square
with 7 second periods or greater, but worth paying close
attention to.

A southerly wind reversal may develop towards the
middle to latter half of this week with seas likely remaining
under 10 ft. Small craft advisory level winds, could return
towards the weekend, as a front moves into the north Oregon and
south Washington waters. /JBonk




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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. is the U.S. government's official web portal to all federal, state and local government web resources and services.