High Wind in Oregon, December 9 to 10, 2022

Area Forecast Discussion
National Weather Service Portland OR
311 PM PST Fri Dec 9 2022

.SYNOPSIS...A warm front is slowly moving through the forecast
area bringing widespread precipitation. Snow levels will rise
through the night up to 4000 ft from Salem south. Will see a
more gradual rise in the north Oregon, south Washington Cascades
and the Columbia River Gorge. A cold occluded front will follow
late Saturday into Sunday. High pressure returns next week which
will bring much colder temperatures to the region.

&&

.SHORT TERM.../THROUGH SUNDAY NIGHT/...Radar imagery shows the
frontal boundary making landfall this afternoon with the
initial stages stretched from the Lane County Cascades up
through Columbia County. This front was slightly delayed in
comparison to model outputs, but overall remains on track. The
warm front is associated with a broad low pressure system that
is dropping down from the Gulf of Alaska. Satellite imagery
shows the low center sitting near 45.9N/-135.6W this afternoon,
with a steady northeasterly trajectory. With the low
approaching, offshore flow is increasing through the Columbia
River Gorge with gusts as high as 28 kt around Troutdale.
Interestingly though, Hood River remains light and variable.
This easterly flow will bring in cooler air from east of the
Cascades...in theory. However, observations show that
temperatures are not nearly as cool as previously expected.
Daytime temperatures west of the Cascades were about 4 degrees
warmer than yesterday, and a similar trend occurred outside of
our forecast area. With less of a cold air pool filtering in,
this may impact precipitation type. Will dive into this a bit
further in this discussion.

Before we can talk about precipitation type, must acknowledge
that there is still some uncertainty. While models are very
consistent on the track, precipitation amounts remains
variable. The ECMWF is the most robust solution with an excess
of 2 inches of QPF possible along the coast, and 1.5 inches
along the Eagle Creek burned area near Multnomah Falls. Mt St
Helens would see nearly 1.75 to 2 inches of QPF over the length
of the front. In addition, the ECMWF has a slower progression
and appears to stall between Saturday 18Z through Sunday 00Z. In
contrast, the GFS (one of the faster solutions) only shows
around 0.75 inches of rain over Newport and 1.0-1.25 inches of
St Helens. With all that being said, have trended towards a
median solution of the NBM 4.0 with some enhancement from
orographic effects.

Now the big question on folk's minds as they head out for
weekend activities "where, when and how much snow will we
receive?". The main areas that will be targeted with snow will
be the Cascades. As the low shifts northward, the main
precipitation bands will set up along the south Washington to
north Oregon Cascades. Here the warm front will intersect with
the cold air aloft and bring ample snowfall. Looking at warning
level snow with accumulations exceeding 12 inches throughout
much of the area above 3000 ft. Along the volcanoes, especially
Helens, will see an excess of 20 inches of snow from this
afternoon through Saturday afternoon (around 24 hours). Pass
level will see several inches though which may make travel
difficult at times, especially as it falls at night. The
Columbia River Gorge and Upper Hood River Valley on the other
hand has been much harder to get a grasp on. With the cooler air
in place due to easterly winds, and snowpack from yesterday's
snowfall, there is an increased likelihood that mixed
precipitation is possible. As it stands, there is high
confidence that locations above 1000 ft will see snow, and it
could incur additional accumulations up to 5 inches locally.
More widespread will see up to 3 inches. The challenge though is
in regards to freezing rain potential.

As the warmer precipitation meets up with the cold air at the
surface, could see another round of snowfall in the lowlands.
One challenge is that air is slightly warmer than expected so
there may be more of a trend towards a rain/snow/freezing rain
mix vs all snow. Another struggle is the fact that snow levels
are going to quickly rise as the front comes in. The Gorge will
be the last to see those rises, but still expected. The third
difficulty is that of freezing rain. With snowpack already at
the surface, the ground is cold enough that any super-cooled
water could easily freeze creating more ice than snow. At this
point, confidence is low in regards to what precipitation type
will be observed there so important to keep aware of your
surroundings as conditions could change very quickly.

As this first front exits eastward, another shortwave trough
associated with an occluded front will move inland. Will likely
see some slightly cooler temperatures which will lower
snow levels, but precipitation is not favored. Could see some
showers in the post frontal environment with snow along the
Cascades, but otherwise, just a fairly typical winter season
scenario. 											-Muessle

.LONG TERM /MONDAY THROUGH FRIDAY/...Overall there is good consensus
among the models that a drier and cooler pattern sets up next week.
The upper low slide will move out of the region, sliding over the
Great Basin on Monday as a steep upper ridge moves toward the Pacific
NW. Onshore flow in the lower levels could wring out some light
precipitation or drizzle in the higher terrain and along the coast.
Now, there is a shortwave trough that will be moving across the
northern Pacific which will attempt to undercut the upper level ridge
and form an upper low on Tuesday which could impact the weather mid-
week. The WPC cluster analysis shows quite a bit of variance with
this feature which tends toward lower forecast confidence.
Deterministic models develop a surface frontal system offshore but
there are multiple solutions, some keeping the low well offshore, but
others bringing some precipitation to the area. Looking at the
ensembles, the majority show dry weather, but there are around 20% of
the members that do bring in precipitation. Offshore flow then takes
over with drier weather more likely later in the week.

Temperatures are likely to remain below normal through next week, as
the NBM shows a steady decline of both highs and lows. Expect the
coolest temperatures by the end of next week with Friday morning
temperatures possibly in the lower to mid 20s, except in the teens
near the central Columbia River Gorge. Day time temperatures are not
likely to reach 40 degrees, except along the coast where more
modified air will keep temps slightly warmer. One thing to watch with
likely cold air in place will be what happens to the upper low later
in the week. The NBM is indicating around a 10% chance of low
elevation snow potential, while the EC ensembles also show a few
members with lowland snow mid to late next week. 	-Hartsock

&&

.AVIATION...00Z TAFs: A warm front associated with a rapidly
deepening shortwave offshore will bring a mix of flight conditions
across the area through Saturday morning. Expect widespread MVFR
with light rain across the area with conditions deteriorating as
the cold front approaches overnight. Heavy rain ahead of the front
will cause visibility to drop into IFR/LIFR criteria for short
periods of time. LLWS and gusty conditions will be a concern as
the warm front lifts north through much of the TAF period.

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

KPDX AND APPROACHES...Expect predominantly MVFR ceilings through
the forecast period. Rain will become heavy at times reducing
visibility. Strong southerly winds aloft will produce strong speed
shear through the 2000 ft layer. 					-BMuhlestein

&&

.MARINE...Satellite imagery as of 1400 shows a rapidly deepening
shortwave centered near buoy 46036. A nearly 10 mb gradient across
the coastal waters is producing sustained winds of 30-35 kt with
gusts just now reaching into the Storm Force category slightly
later than expected. Buoy 46089 has been verifying with a peak
wind gust of 49 kt around 1300. Guidance is doing fairly well
with regards to the pressure tendency with most indicating the low
will hit its minimum of 980 mb around 2100 off of Vancouver
Island. The wind-sea will build into the upper teens, low 20s
within the southerly, warm sector. Gradients are expected to peak
this evening and remain elevated until roughly midnight. Strongest
winds are still expected to be south of Depoe Bay with seas rising
to near 25 ft just south of buoy 46050. Some of the guidance,
particularly the EC ensembles, persist in showing Hurricane Force
winds across PZZ273 for a brief period but there is not enough
confidence to elevate the Storm Warning. Mariners should avoid
this zone during the overnight hours. The strongest winds are not
associated with a coastal jet as the cold front lags in arrival. A
fresh northwesterly swell will disperse across the waters and
build overnight into the Saturday morning making for a confused
seas state.

The low will become vertically stacked, start to fill, and weaken
into Saturday morning. The low will drop south beyond 60 nm and
the cold front will move ashore Saturday morning. The The low
will move onshore over northern California and more settled
weather is expected by Saturday evening. Persistent northwesterly
winds expected into early next week with seas just below 10 ft.
														-BMuhlestein

&&

.BEACH HAZARDS...A brief period of coastal flooding is expected
during the afternoon high tide Saturday. The total tide at Toke
Point is expected to reach 11 ft around 130 PM, Astoria to reach
11 ft around 2 PM, and Garibaldi to reach 10.5 ft around 130 PM.
This will be marginal at best but coastal residents in the area
should be alert for rising water for a few hours around high tide.
														-BMuhlestein

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Coastal Flood Advisory from noon to 4 PM PST Saturday for
     ORZ001.
     Winter Storm Warning until noon PST Saturday for ORZ011.
     Winter Weather Advisory until noon PST Saturday for ORZ013.
     Winter Storm Warning until 10 AM PST Saturday for ORZ014.
     Winter Weather Advisory until 4 AM PST Saturday for ORZ015.
     Winter Weather Advisory until noon PST Saturday for ORZ016.
WA...Coastal Flood Advisory from noon to 4 PM PST Saturday for
     WAZ021.
     Winter Storm Warning until noon PST Saturday for WAZ019.
     Winter Weather Advisory until 4 AM PST Saturday for WAZ045.
     Winter Weather Advisory until noon PST Saturday for WAZ046.
     Winter Weather Advisory until 4 AM PST Saturday for WAZ040.
PZ...Gale Warning until 4 PM PST this afternoon for PZZ210-251.
     Storm Warning until 1 AM PST Saturday for PZZ210-251.
     Hazardous Seas Warning from 1 AM to 10 AM PST Saturday for
     PZZ210-251>253-271>273.
     Storm Warning until 1 AM PST Saturday for PZZ252-253-271>273.
&&

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