Dense Fog in Oregon, December 16 to 17, 2023

Area Forecast Discussion...Updated
National Weather Service Portland OR
506 PM PST Sat Dec 16 2023

.UPDATE...As of 5 PM, fog was already becoming dense in the
Willamette Valley roughly Albany southward. Fog was also beginning
to expand into southern portions of the Salem metro area. This trend
has prompted us to issue a Dense Fog Advisory for the central/south
Willamette Valley through noon Sunday. Confidence is fairly high
that fog will remain dense as the 500 mb ridge axis moves SW-NE
across the forecast area, increasing subsidence and pinning the fog
to the valley floors. Fog will probably begin to lift later Sunday
morning as 500 mb heights begin to decrease and subsidence relaxes.
Weagle

&&

.SYNOPSIS...Strong ridge of high pressure aloft and low-level
offshore flow are leading to unseasonably warm conditions this
weekend across much of SW Washington and Oregon. That said, strong
inversions have developed, with cooler air trapped in many inland
valleys. These valleys stand to see occasionally dense fog through
Monday morning, mainly during the night and morning hours. Low
pressure will lurk offshore through much of next week, leading to
occasional chances for rain as wraparound moisture drifts northward
from California. Snow levels will remain unseasonably high through
much of next week.

&&

.SHORT TERM...Today through Tuesday...Not much to add to previous
discussion, which does a good job explaining the situation at hand.
Only minor changes were made to the forecast, mainly to incorporate
more persistence into the forecasted temps/RH through Monday.
Offshore gradients still stand to ramp up over the next 24 hours,
with the latest NAM suggesting the magnitude of MSLP gradients
peaking at around -8 or -9 mb. This should support wind gusts of up
to 40 mph for Troutdale, 65-70 mph for Corbett and potentially 80+
mph for Crown Point, as the previous AFD mentioned. The short/long
term discussions below are more or less unchanged from this morning's
discussion.

A persistent weather pattern will maintain dry and seasonably mild
weather this weekend. The upper level ridge extending across the
Great Basin into the Pacific Northwest will hold firm while low level
offshore flow continues. Below the subsidence inversion, particularly
across the southern and central Willamette Valley, fog and low
stratus likely to develop each night and persist well into the day.
An air stagnation advisory is in effect for these zones, as well as
the lower Columbia River Valley through Sunday night. Breezy offshore
winds and above normal temperatures expected above the inversion and
along the coast. Gusty east winds will persist near the mouth of the
Columbia River Gorge during this time as colder air remains trapped
in the Columbia Basin and pressure gradients between KDLS and KTTD
remain around -4 to -6 mb over the next 24 hours. Model guidance
suggests the gradient strengthens on Sunday with gusts possibly
pushing 80+ mph at Crown Point.

The weather pattern begins to change starting Sunday night into
Monday. A large cutoff low pressure currently spinning off the coast
of California lifts northeastward toward the Pacific NW. This will
bring increasing rain and high mountain snow (as snow levels are
expected to remain 6500-7000 ft). Could see precipitation begin as
early as Sunday night across southern parts of the CWA, while most
areas will see rain on Monday. Amounts will be variable as this
pattern remains fairly complex and dependent on where the heavier
showers develop. For example, at PDX the NBM 10th to 90th
percentiles suggest the range of 24-hour rainfall amounts could vary
from 0.10" to 0.75", while ensembles indicate less than a 5% chance
that rainfall would exceed one inch through Monday night in
Portland. There is a slight chance for thunder, mainly along the
central coast as an upper shortwave trough ejects northward later
Monday. In the central Columbia River Gorge and upper Hood River
Valley, there is also a slight chance (~15%) that precipitation
could fall as freezing rain initially on Monday. Current
temperatures and dewpoints do not support it, but there if enough
clearing upstream in the Columbia Basin that cooler temperatures may
end up materializing over the next 2-3 days so will keep the low
probability mention in the forecast for now.

As the low pressure system continues to lift northward, expect
showers to persist across the area through Tuesday morning, then
dissipating through Tuesday evening. /DH/Weagle

.LONG TERM...Wednesday through Saturday...There is general agreement
among the GEFS, EPS and CMCE ensembles that another shortwave trough
will dig southward over the eastern Pacific and stay far enough west
and south of the region in the middle of the week to dry us out for a
could days. Nonetheless, there is a 10-20% chance that this shortwave
trough does not dig far enough south for us to dry out completely
and/or the zonally oriented polar jet stream to our north drops just
far enough southward to result in at least a storm system or two
clipping the region. NBM PoPs and temperatures appear on track with
everything described above so few deviations were made. All in all,
pretty quiet weather for mid-Dec.  DH/Neuman

&&

.AVIATION...Little change in the overall pattern, as broad low
pressure remains off the north Calif coast. This will maintain
south to southwest flow aloft, with variable high clouds across
the region into tonight. Light offshore flow near the surface
will keep region dry. Widespread VFR conditions throughout the
region, except for persistent low stratus remaining in KEUG.
Beginning around 03-04z Sun, guidance suggests chances of LIFR
fog for areas south of Portland around 50-70%, with all other
inland areas around 10-30%. Areas of localized freezing fog
remain a concern for tonight into Sunday morning.

PDX AND APPROACHES...VFR with variable high clouds into tonight.
Light offshore flow will maintain east winds out of the Columbia
Gorge, enough to keep areas to east of PDX free of fog. Chances
for MVFR conditions throughout the TAF period remain under 20%.
-JH

&&

.MARINE...High pressure will remain over the inland PacNW today
through Sunday, while broad low pressure sits well to the
southwest of the region. This pattern will maintain east to
southeast winds 10 kt or less on the waters, but 15 to 20 kt
closer to shore in the gaps of the coastal terrain. Overall, seas
holding at 6 to 8 ft, with higher seas south of Newport on the
outer waters. Seas may up a bit more in that area later Sunday,
with 9 to 10 ft.

More active weather returns next week, with first system lifting
northward offshore. This will bring 20 to 30 kt winds to the
waters later Monday into Tuesday, with gusts to 35 kt on the outer
waters. Then, another break as high pressure builds over the
region for Wed to Fri. But, another strong front likely to arrive
later next Fri into Sat.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Air Stagnation Advisory until 4 AM PST Monday for Central
     Willamette Valley-Lower Columbia-South Willamette Valley.

     Dense Fog Advisory until Noon PST Sunday for Central and
     South Willamette Valley.

WA...Air Stagnation Advisory until 4 AM PST Monday for I-5 Corridor
     in Cowlitz County.

PZ...Gale Watch from Monday afternoon through Tuesday morning for
     Coastal waters from Cape Foulweather OR to Florence OR out
     10 NM.Coastal waters from Cape Foulweather OR to Florence
     OR from 10 to 60 NM.
&&

www.weather.gov/portland

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