Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 271759 AAB

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1058 AM PDT Tue Oct 27 2020

Updated Aviation Discussion

.SYNOPSIS..High pressure will remain over the Pacific Northwest
through the middle of this week bringing dry weather and slowly
warming temperatures. A weak cold front at the end of the week
brings the next chance for rain, but a quick return to dry weather
over the weekend appears likely.


.UPDATE TO SHORT TERM...Radar observations this morning show light
radar returns moving across the south WA coast and approaching the
far north OR coast, with light drizzle already being observed around
Grays Harbor via CWOP stations and webcams. Forecast NAM BUFR
soundings over the next 24 hours at Astoria show a saturated layer
that is deep enough to support light drizzle with the presence of
weak rising motion. Therefore, a slight chance of light drizzle has
been included in the forecast for today through Wednesday morning
along the south WA and far north OR coast. In addition, minor tweaks
were made to the sky and temperature forecast to better match
observations. -TK

.SHORT TERM...Today through Thursday...Satellite images early
this morning showed marine stratus slowly spreading into the south
Washington and far north Oregon coastal areas with a weak northwest
onshore flow. As clouds slowly spread further southeast this morning
expect the marine clouds to reach into the lower Columbia Valley and
further down the north Oregon coastline this morning. Over the next
couple of days models suggest low level moisture spreading further
south and inland, to include much of the Willamette Valley Wednesday
and Thursday morning. Subsidence from the upper ridge shifting inland

will generally keep the moisture confined to a shallow layer in the
low levels. Expect some retreat in the low clouds back to coastal
areas during the afternoon hours. Beyond the marine cloud
considerations, the only other notable change over the next few days
is a slow moderation in temperatures back closer to normal due
primarily to subsidence warming.

.LONG TERM...Thursday night through Monday...00Z operational models
and EC ensemble mean all continue to indicate a shortwave trough
moving across the region Friday, dragging a weak cold front south
into the region. The front returns a chance for rain to the region
Friday, at least across the northern portions of the forecast area.
Ensembles suggest this is just a one and done system though, as
there is good agreement in the upper ridge rebuilding over the
Pacific Northwest over the weekend for a return to dry weather with
a continuation of seasonable temperatures. Confidence in the upper
ridge continuing into Monday begins to wane some, so will allow for
a slight chance of showers in the north coastal region on Monday,
but leave the rest of the area dry.


.AVIATION...Satellite observations show a surface low near the
Olympic Peninsula, that is steadily pushing a mixture of
MVFR/IFR conditions across the area. KAST is currently
experiencing SCT/BKN skies around 2500 ft. These conditions will
slowly move south along the coast but, KONP will likely maintain
offshore flow which should keep the stratus off the coast
through 12Z Wednesday. A mixture of VFR/MVFR cigs from 2500 to
3500 ft fluctuating from SCT/BKN is already impacting the
northern Willamette Valley. Expect VFR/MVFR stratus to likely
reach KSLE around 19Z Tuesday and approach KEUG around 23Z
Tuesday. There is the possibility for some IFR conditions
manifesting within the Willamette Valley after around 12Z
Wednesday especially for locations south of KSLE.

For detailed regional Pac NW weather information, go online to:

KPDX AND APPROACHES...VFR expected to persist through most of
the day, with cigs between 3000 and 5000 feet. VFR/MVFR cigs
between 3000 and 2000 feet develop around 05Z Wednesday. /42


.MARINE...High pressure offshore will maintain northerly winds
across the waters over the next several days, with the strongest
winds expected over the coastal waters south of Cascade Head.
Seas will stay in the 3 to 6 ft range, with the highest seas
well offshore. Do not expect much change in this pattern through
early Thursday.

A front will approach the region later Thursday and shift across
the coastal waters Thursday night. This will bring the return of
light to moderate southerly winds to the waters on Friday,
sustained around 15 to 20 kt. Seas will see a bump upward, but
will most likely peak between 6 and 9 ft Friday night. Seas and
winds should east this weekend. -BPhillips/TK




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