Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 032330
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
330 PM PST Thu Dec 3 2020

.SYNOPSIS...High pressure expected to maintain mild weather through
Friday. Columbia Gorge gap flow will bring another round of breezy
to windy conditions to the western Gorge and portions of the
Portland/Vancouver Metro area through Saturday. A weakening front
will move across the area on Saturday bringing precipitation through
late Saturday night. Calm conditions expected Sunday before becoming
more active early next week.

&&

.SHORT TERM...Tonight through Sunday...A very mature upper low
center is coming inland near Haida Gwaii. It`s dragging a weakening
and separating front across the area this afternoon. Radar has
certainly shown a fair amount of echos but little, if any, is
reaching the ground as the upper portion of the front races eastward
ahead of the surface feature. Don`t expect any additional precip to
arrive after late this afternoon and anything between now and then
may end up as glorified sprinkles. The upper ridge will generally
shift westward again to keep conditions dry through Saturday.

Higher surface pressure east of the Cascades will continue to bring
breezy to windy outflow through the western Gorge and across
portions of the Portland/Vancouver Metro through Saturday. Pressure
gradients will increase again later tonight through Saturday with
gusts 45-55 mph possible in the most favored areas. At this time, do
not expect winds will reach advisory levels for the east Metro aside
from very occasional gusts around 40 mph, especially on Saturday.

Interior areas which are not seeing the Gorge outflow winds, will
continue to see inversions and light winds result in stagnant air
conditions. This includes most interior elevations below 1500 feet
plus the Central Gorge/Upper Hood River Valley, and portions of the
Metro. These conditions may ease later Saturday as south winds
increase and better mixing from low level instability occurs ahead
of an approaching upper trough. It`s far from a certain thing,
however, and there is a distinct possibility little improvement will
occur. Do have confidence the upper Hood River Valley and the
Central Gorge will keep enough offshore influence to limit mixing
and surface winds.

This upper trough is showing signs of splitting as a trailing
shortwave slides south along the western periphery of the trough. It
may have a hard time making the turn around the base of the trough
and then result in a cut-off low forming with the low center
somewhere near or over our area. Will go with chance to likely PoPs
for now, but have below average confidence in the overall scenario.

A couple things remain questionable for the weekend as a result.
First is if sufficient mixing will occur to clear out stagnant air
surrounding the valley. If not, then stagnant conditions may persist
into early next week. Second, will there be precip beginning
Saturday night over the Upper Hood River Valley as warmer southerly
flow increases aloft. Snow levels are currently projected to rise
above 5000 feet. Meanwhile, the low level easterly flow will keep
cooler Columbia Basin air banked up against the Cascade east slopes.
Saturday night will likely see temperatures in those two areas fall
below freezing. Most models bring very light (a few hundredths or
less) precip to that area, while some bring a tenth of an inch or
more. For now, will favor the solution that most models are showing
with the lighter amounts and thus include a slight chance of
freezing rain for later Saturday evening through Sunday morning.

A high energy west swell will continue sneaker wave conditions
through Friday afternoon. The seas will not generate large enough
surf to alarm beach-goers of the potential threat, and sudden
unexpected rises of sea water onto the beaches could catch those
recreating on the beaches off guard. /JBonk

.LONG TERM...Sunday Night through Thursday...Once the upper
trough or cutoff low depart, ridging extends back across the area
beginning Sunday with the region remaining on the northwestern
fringes. By Monday night, a shortwave will skirt across southern BC
and northwestern Washington potentially bringing some light precip
across our northern areas. Don`t expect to see much wind with this
system, but it may be enough instability to lift the inversions and
provide some stagnant air relief, if necessary. Thursday brings a
more defined trough across the region but models differ on the
details on how much precip will occur Have largely stayed with the
National Blend of Models through the extended until further clarity
becomes apparent. /JBonk

&&

.AVIATION...At 2130Z satellite imagery showed a band of mid-level
clouds moving into the Cascades. VFR to prevail over the inland
areas through at least 08Z Fri. Satellite imagery also revealed a
weakening frontal boundary just offshore. The will reach the coastal
TAF sites between 00Z and 03Z Fri and should result in a few hours
of MVFR conditions. Elsewhere, weakening surface gradients tonight
and gradually clearing skies will promote fog formation, especially
in the central and south Willamette Valley and also along the
Interstate 5 corridor in Cowlitz County. Will likely see areas of
IFR or lower conditions in these areas through Fri morning. All
areas to be VFR mid to late Fri morning through the afternoon as the
offshore surface gradient increases.

For detailed regional Pac NW weather information, go online to:
http://weather.gov/zse

KPDX AND APPROACHES...Mid and high level cloud band moving east
of the terminal as of 21Z. Increasing clouds late this evening,
but cigs to remain well within VFR thresholds. Wind at the terminal
will likely go calm or variable less than 5 kt late this afternoon
and there is the possibility a light northwest wind develops
overnight. The KTTD-KDLS gradient is expected to strengthen Fri
afternoon, which will result in 25-30 kt gusts between KPDX and
KTTD and 45-50 kt gusts at the west end of the Gorge. VFR to prevail
through Fri afternoon. Weishaar

&&

.MARINE...Wind speeds decrease tonight and become light and variable
through Friday. The offshore gradient strengthens again late Fri and
Fri night due to stronger higher pressure inland and lowering pressure
over the waters due to an approaching frontal system. This will likely
produce 20 kt gusts tonight into early Sat, especially for areas parallel
to coastal terrain gaps. South wind increases Sat with 25-30 kt gusts
possible Sat afternoon and evening. High pressure builds behind
the front Sun. Longer range models maintain some degree of high
pressure over the waters through mid-week, although a couple of
weak systems are expected to move across the area. These should
result in sub-advisory wind criteria. A stronger system looks to
impact the waters late in the week.

At 21Z buoy observations continue to confirm the presence of a
long period west swell across the waters. Dominant periods have
been 17-20 seconds. This swell will slowly subside to around 10
feet tonight then rebuild to around 13 feet Friday morning as
another swell train arrives. Seas are expected to subside below
10 feet Friday night, but wave heights exceed 10 feet again Sat
night. Longer range models are coming into better agreement
indicating a larger west swell will arrive early next week with
seas building into the upper teens. Weishaar/~TJ

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Air Stagnation Advisory until 7 PM PST Tuesday for Central
     Columbia River Gorge-Upper Hood River Valley.

     Air Stagnation Advisory until 10 AM PST Saturday for Cascade
     Foothills in Lane County-Central Coast Range of Western
     Oregon-Central Willamette Valley-Coast Range of Northwest
     Oregon-Greater Portland Metro Area-Lower Columbia-Northern
     Oregon Cascade Foothills-South Willamette Valley.

WA...Air Stagnation Advisory until 7 PM PST Tuesday for Central
     Columbia River Gorge.

     Air Stagnation Advisory until 10 AM PST Saturday for Greater
     Vancouver Area-I-5 Corridor in Cowlitz County-South
     Washington Cascade Foothills-Willapa Hills.

PZ...Small Craft Advisory until 4 PM PST Friday for coastal waters
     from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory until 8 PM PST Friday for Columbia River
     Bar.

&&

$$

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