Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 261643 AAA

Area Forecast Discussion
National Weather Service Portland OR
943 AM PDT Mon Jun 26 2017

.SYNOPSIS...High pressure over the Pacific Northwest will weaken over
northwestern Oregon and southwestern Washington. A marine surge will
continue to push inland overnight, for much cooler temperatures
today. There is a slight chance for some high-based thunderstorms
over the portions of the forecast area on Monday. Dry northwesterly
flow sets up later this week across the region, bringing more
seasonable temperatures and a pattern of morning clouds and afternoon


.SHORT TERM...Today through Thursday...Today is a transition day for
the weather pattern over the Pac NW. The strong upper level ridge
that brought record breaking heat over the weekend has shifted east
into the Rockies. This has opened the door for a shortwave trough to
approach the Oregon coast. We are seeing some pretty significant
thunderstorm activity this morning over portions of the north Oregon
coast and Coast Range, as well as for western portions of the
Willamette Valley. These storms are associated with elevated
instability and moisture in the southerly flow aloft ahead of the
trough. The 12Z KSLE sounding had a precipitable water value of 1.41
inches, which is near the maximum value recorded for the date. There
is also some diffluence in the upper level flow, which is likely
playing a part. The thunderstorm threat will continue through the
morning, and potentially into the early afternoon. We then expect any
thunder to become confined to the high Cascades later this afternoon
as the shortwave moves onshore and turns the steering flow
southwesterly. We also lose the diffluent flow aloft this afternoon.

In addition to the thunderstorms, the region is experiencing a
significant cool down today. Southwest low level flow overnight
brought cooler marine into the interior. The marine stratus was
slower to arrive than expected and has not filled the entire lowlands
, but it has been gradually expanding over the past few hrs. The
airmass over the region was still fairly warm this morning, but the
increased clouds will help to cool high temps today. There will also
be strengthening onshore flow and cold advection this afternoon. The
fcst models are suggesting highs over the interior only reaching into
the mid 70s. We didn`t go quite that low given the less than expected
cloud cover, but our fcst highs in the mid to upper 70s are still
20-25 degrees cooler than yesterday for the interior lowlands.

Northwesterly flow aloft sets up over the region for the remainder of
the first half of the week. Expect increased marine cloud cover and
even cooler temperatures Tuesday and Wednesday. With the thicker
marine layer in place, clearing may be delayed until later in the
afternoon with perhaps a few areas of morning drizzle possible along
the north coast and/or in the Cascade foothills. Forecast models
remain in rather good agreement for Thursday, with the next ridge
building over the region. This will bring warming temperatures and
skies will likely trend towards mostly sunny earlier in the morning,
though onshore low-level flow will maintain at least some clouds.

.LONG TERM...Thursday night through Monday...Little change to the
overall pattern into Friday as the upper ridge migrates east across
the Pacific Northwest and east of the Cascades later Friday. This
should bring afternoon high temperatures a little warmer over the
interior, but not much beyond 80F as heights only build to 582 dam.
Models begin to diverge a bit into the weekend, but at least moderate
confidence in the upper trough bringing increasing clouds and
probably some light showers at some point over the weekend. High
temperatures will trend closer to seasonal normals.   Cullen


.AVIATION...Onshore low level flow with LIFR to low MVFR conditions
at the coast in marine stratus, which is just reaching KPDX now
and trying to reach the south Willamette Valley. With an upper
short wave along the coast moving through today, may see the
stratus expand some more today before and cover more of the
valley. Cigs will probably raise to VFR this afternoon inland
could stay higher MVFR at the coast. Will also see some isolated
showers and thunderstorms across the inland areas that may last
into the afternoon in the valleys and the Cascades then move east
of the area late in the day.

The onshore flow should help the marine stratus lower tonight so
cigs should lower to MVFR tonight.

KPDX AND APPROACHES...MVFR cigs this morning will slowly lift to
lower VFR cigs later today and this evening then back to MVFR
after midnight tonight. pt

.MARINE...A very weak trough over the waters will give way to
high pres through the day today and remain through most of next week.
After some south winds along the Oregon coast into the afternoon,
this will bring a return to northerly winds. The winds may come close
to Small Craft Advisory threshold during the afternoon and evening
on Tue and Wed, but it looks very borderline. The high pres weakens
some late next week.

Seas to continue well below 10 ft for the next several days.
seas are around 5 to 6 ft with a 7 to 9 second dominant period.
Seas will be somewhat choppy but does not look like it will
exceed advisory criteria. There is the potential for another uni-
wave set-up Tue and Wed which usually brings fairly steep choppy
seas. mh/pt



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