Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 272139
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
239 PM PDT Sat Aug 27 2016

.SYNOPSIS...Much cooler temperatures today and through the
upcoming week as onshore flow persists across the area. A broad upper
trough shifts over the region and will remain with little change, as
a series of impulses rotate through the trough for a few rounds of
enhanced precipitation chances through the week.

&&

.SHORT TERM...(Tonight through Tuesday)...Water vapor satellite
imagery reveals a pair of disturbances in the vicinity of the region
- one moving south and east across southern British Columbia, and the
other sliding south from the Gulf of Alaska toward the western side
of Vancouver Island. The former, and its associated boundary have
spread some cirrus across the northern third of the forecast area.
Meanwhile, onshore flow has maintained plenty of marine stratus for
the coastal areas, as well as along the Columbia
River to about Kelso, though this does continue to slowly retreat
(especially along the central portion of the Oregon Coast). However,
with onshore flow continuing overnight, expect the marine layer to
continue to deepen and fairly widespread marine stratus into the
Willamette Valley overnight. Have maintained the chances for some
light drizzle or showers along the coast on Sunday. As the longwave
trough strengthens, the second of the aforementioned disturbances
remains off the western coast of Vancouver Island with an associated
front stalling offshore. Therefore, have decreased cloud cover across
the interior on Monday to reflect only partly cloudy skies with a
good bit of sunshine, though temperatures likely trend just a degree
or two cooler than Sunday. More significant cooling occurs Tuesday
and into the midweek period as the slowly moving upper trough
continues to slide across the region. Expect more cloud cover as the
first significant threat of showers moves across the area by Tuesday
afternoon for much of the area by the afternoon. Forecast
models continue to suggest the small chance of some thunderstorms to
clip the extreme SE portion of the forecast area - specifically, the
Lane County Cascades. While the flow aloft will be more south to
southwest, cannot completely discount the potential and despite the
best instability remaining east of the Cascade Crest, in
collaboration with neighboring offices, have introduced a slight
chance of thunder for this location.  Cullen

&&

.LONG TERM...(Tuesday night through Saturday)...Cooler and cloudier
with periods of precipitation will be the general rule for the second
half of next week. A break in precipitation is possible on Wednesday,
but maintained a few regions of slight chance PoPs due to timing
differences in the various model solutions. Chance PoPs persist for
the later portion of the week as various impulses rotate across the
region around the base of the mean trough. However, timing and
strength differences remain among the forecast models with a fair
amount of spread among ensemble members. As a result, less confidence
in the precise periods of higher chances of precipitation Friday or
Saturday. Regardless, temperatures will continue to remain several
degrees below normal for the extended period in the cooler air mass
that remains over the Pacific Northwest.  Cullen

&&

.AVIATION...Stratus has been slow to push back to the beaches so
far today. This has increased confidence that both KONP and KAST
will stay in the stratus today. Considering the stratus will
already be inland at sunset tonight, it should have no problem
expanding into the Willamette Valley. Ceilings could arrive at PDX
as early as 06Z and will expand southward near sunrise. By 14Z
tomorrow morning, most of the Willamette Valley will be filled
with stratus.

KPDX AND APPROACHES...VFR conditions expected for the remainder
of Saturday. MVFR cigs will develop between 8Z and 11Z tonight.
/Bentley

&&

.MARINE...Relatively light winds and small seas continue through
the weekend and into early next week as a breakdown in the NE
Pacific surface high weakens the pressure gradient across waters.
Seas in the 6-8 foot range will subside tonight as the 30 kt NW
winds off of Vancouver Island weaken. Only chance for marine
headlines in the next few days will be a few periods of gusty
south winds ahead of weak fronts Monday and Tuesday. Beyond
Tuesday model solutions diverge with a multitude of solutions.
Kept mostly south winds through the long term period as I do not
expect the NE Pacific surface high to re- establish until next
weekend at the earliest. /Bentley

&&

.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. The area is commonly
referred to as the forecast area.



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