High Wind in Oregon, December 18, 2021

Area Forecast Discussion...Updated
National Weather Service Portland OR
440 AM PST Sat Dec 18 2021

Updated Watches/Warnings/Advisories

.SYNOPSIS...Today will start off mild and breezy for most of the
forecast area as southwesterly flow aloft ushers in a relatively warm
subtropical air mass ahead of an approaching cold front. After a few
showers early on, rain will increase from north to south as the cold
front advances onshore and into NW Oregon later this afternoon into
this evening. The front is expected to stall over Oregon, with much
lower snow levels north of the front than to the south. While snow is
expected in portions of the Coast Range and Hood River Valley,
precipitation is likely to remain mostly rain for Willamette Pass
despite a much higher elevation. Snow and/or ice remain a possibility
near Hood River through at least Monday as easterly flow causes the
low-level cold air to linger despite warming temps aloft. An upper
level trough will linger nearby for much of the upcoming week,
keeping the weather unsettled.

&&

.SHORT TERM...Today through Monday...The stage is set for a complex
and challenging forecast over the next 2 to 3 days, and much should
be revealed by what transpires over the next 24 hours.

A weak warm front moved across mainly the northern half of the
forecast area Friday evening, bringing some light rain to the region.
The primary impact of the warm front has been to raise the snow
levels to around 6000 feet as a relatively mild subtropical air mass
moves in via increasing SW flow aloft. Now that the warm front has
passed, most of the precipitation has shifted into NW Washington and
the Seattle area. Much of the day today will probably be dry, mild,
and breezy, except for our north coastal zones which will be the
first to see rain increase again as a cold front begins to move into
our forecast area later this morning into this afternoon. The front
will continue southeast, spreading rain into at least the northern
half of the Willamette Valley during the late afternoon and early
evening hours. This is essentially the "easy" part of the forecast,
then things get much trickier starting this evening.

Latest water vapor imagery shows an upper trough to the southeast of
the Aleutians this morning. The energy associated with this upper
trough will be shunted southward by a strongly meridional jet stream
between 140W and 150W, forcing the core of the upper trough far
enough south and eastward so that it begins to interact with the tail
end of the above-mentioned cold front. Most guidance develops surface
low pressure along the front in the vicinity of 40N/130W this
afternoon and evening, causing the front to stall somewhere over
west-central Oregon.

There will be a sharp temperature contrast with the front, with the
06z NAM showing 850 mb temps of -3 deg C at Portland and +4 deg C at
Eugene 06z Sunday. Meanwhile, the stalling nature of the front and
the above-mentioned low offshore will likely develop some anafrontal
precipitation, potentially keeping precip going on the cool side of
the front for several hours. This is a scenario where - if everything
went absolutely perfectly - steady, heavy precipitation could cause
snow levels to lower all the way to the valley floor north of the
front due to the latent heat absorption required to melt snowflakes
as they fall below the snow level. The 00z suite of models raised a
lot of eyebrows in hinting that the Portland metro could see
accumulating snow this evening and possibly for a healthy portion of
the night. As mentioned - this is possible - but unlikely, as it
would take nearly 2 inches of rain over a short period of time to
achieve the cooling required to bring the snow level down to the
valley floors in such a marginal air mass. Given the 00z model suite,
it is now not surprising that the NBM is showing nearly 1 inch of
snow in the PDX metro area. We have opted to hold the snow level at
1000-1500 feet north of the front for now, but cool air should
eventually cause snow to reach lower portions of the Hood River
Valley Sunday or Sunday night as easterly flow jams cool air into
that portion of the Columbia Gorge.

Another tricky portion of this forecast is the snow expected for the
Cascades. Given the sharp thermal gradient and the stalling nature of
the front, it is very possible we may see wet snow in the West Hills
of Portland (around 1000' elevation) at the same time it is raining
at Willamette Pass (5100' elevation). With snow levels starting out
so high ahead of the cold front, most of the snow will need to come
on the back side of the cold front, where winds are lighter and not
favorable for orographic enhancement. With this in mind, we opted to
convert the Winter Storm Watches to Advisories (rather than Warnings)
for the S WA/N OR Cascades, leaving the Lane County Cascades out of
advisories altogether due to the high snow levels. Latest guidance
suggests the heaviest snow will be north of Mt Jefferson, with 6 to
12 inches a good bet for Mt Hood above 4000 ft and across much of the
S WA Cascades. A few inches of snow are likely as snow levels lower
in the N OR Coast Range, so we added an advisory above 1500 ft - low
enough to affect the higher Coast Range passes.

The latest trend in guidance has been to spread precip back northward
as low pressure offshore strengthens and pushes the frontal zone back
northward Sunday and Monday. The position of the low will likely help
lock cool air in place in and near the Hood River Valley, but
temperatures throughout the column are marginal. At this time, we
kept freezing rain out of the forecast, in favor of a snow/rain
event. However, these Hood River cold pool events often end with a
period of freezing rain or freezing drizzle as the cold pool becomes
too shallow to support snow. As mentioned above, much will depend on
what transpires the next 24 hours - where the front stalls and where
the low develops offshore. At this point, it seems favorable for Hood
River to see at least some snow by Monday evening. The NBM 4.1
probabilistic guidance supports this to some extent, with the 50th
percentile near 1 inch of snow for Hood River proper. It is important
to note the upside potential shown by the NBM 4.1 - it is showing
roughly a 25% chance of 4+ inches and about a 10% chance of 8+ inches
of snow for Hood River Sunday night into Monday.

Not to be lost in all this is the concern about heavy rain and
resultant hydrologic issues. The front looks to move fast enough to
keep rainfall below what would prompt flood concerns, though rainfall
rates with the front itself may be heavy enough to cause brief
localized issues in areas of poor drainage. As the front stalls and
deep moisture continues to be forced over the front, there may be an
area of the Oregon Coast Range which receives particularly heavy
rainfall totals. It is difficult to say exactly where this would be
given all the moving parts to this forecast, but right now the area
between the Siletz and Siuslaw Rivers appear to have the greatest
chance of seeing more than 2 inches of rain.  Weagle

&&

.LONG TERM...Monday night through Thursday...A deep trough positioned
west of the north California, south Oregon coast will bring showers
across the area Monday morning through Tuesday afternoon. The low
will stay far enough offshore that it will keep east to northeasterly
winds across the CWA through Tuesday. This will allow cold air to
push west through the Columbia River Gorge dropping the overnight
lows into the mid to upper 30s by Tuesday and Wednesday morning.
Snow levels will lower across the Cascades to around 3000 ft but
meager QPF values from showers won't produce much snow accumulations
during this time.

Models diverge after Tuesday reducing confidence in the Wednesday
through Thursday forecast. In one solution (GFS and EC); the surface
low offshore will push north and phase with another low dropping
south off the BC coastline, strengthen and send another surge of
moisture over the area Wednesday. 850 mb winds will persist out of
the south pushing a warm nose north. This will keep snow levels
around 4000 ft over much of the CWA into Wednesday afternoon. This is
the solution the forecast is leaning towards. The second, colder
solution from the GEM shows a front dropping south producing slightly
below zero temperatures at 850 mb by Wednesday afternoon. This
coupled with moisture in the low levels ahead of the front would
produce higher snow accumulations in the mountains. By Thursday
afternoon, solutions show 850 mb temps over Eugene to be anywhere
from -2.5 C (GEM) to +5 C (GFS) So, one of the big questions in the
long term will be when will the cold air move in. -BMuhlestein

&&

.AVIATION...12Z TAFs: A mixture of conditions this morning as a
warm front pushes inland. This warm front is not overly deep, but
will allow for some mixing and increasing winds. Likely going to
see more consistent VFR CIGs aside from when showers pass by
this morning. Also expecting some LLWS wind shear due to
southerly winds at the surface and more of a westerly wind aloft.
Areas that will be most impacted will be through the Willamette
Valley and the Columbia River Gorge. Expecting quite a bit of
rain with this front which, when combined with the gusty winds,
could cause a decrease in visibility at times.

Later this afternoon, the pressure gradient will increase as low
pressure drops down to the region, and a cold front advances on
the leading edge. This cold front will cool the environment quite
a bit and cause CIGs to lower to MVFR levels, if not IFR late
after 06Z Sunday. Most of the area will see rain, but with
temperatures dropping, there is a chance for a mix of rain/snow
through the Willamette Valley north of KUAO. More persistent
snow is expected along the Coast Range and the Cascades. Snow
levels are expected around 1000 ft. It is possible with the
cooling temperatures, that areas that saw mainly rain could
experience some patches of ice after 10Z Sunday...especially
along exposed metal surfaces. However, confidence is low on this
occurrence.

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

KPDX AND APPROACHES...VFR with chances of brief periods of MVFR
CIGs with passing showers. Winds are beginning to shift to the
south and increase which, when combined with westerly strong
winds aloft, is creating an environment suitable for some LLWS.
It looks like winds aloft around 2000 ft will be around 40 kt.
Winds at the surface will increase through the day as the warm
front passes. Chance for a return of offshore easterly winds
prior to the onset of the cold front due to an increased pressure
gradient. Strong easterly winds are not expected. Right around
the end of the forecast temperatures will begin to drop which may
bring a slight chance for a rain/snow mix. However, conditions
are trending a bit warmer and the vertical profile is not
favoring snow so will keep the forecast as just rain. -Muessle

&&

.MARINE...The warm front is moving over the waters this morning
which is causing winds to begin their gradual rise which will
persist through the day. Winds around buoy 46029 are already
gusting near 40 kt which will be the general trend through the
afternoon. Later in the day, a low pressure system will begin to
drop down from the Gulf of Alaska with a cold front on the
leading edge of the low. This addition of low pressure will cause
an increased pressure gradient along the near-shore waters
creating a slight coastal jet. The strongest winds stemming from
the south at 20 to 25 kt with widespread gusts to 35 kt are
expected through this afternoon. Generally the northern waters
will see the majority of the gusts at or near 40 kt. Behind the
warm front winds will ease quickly dropping to 10 to 15 kt with
gusts to 20 kt by Sunday morning. The cold front will then pass
over the waters on Sunday where small craft advisory level winds
are expected.

Seas will be a bit more consistent through the forecast as there
will be minimal change in the overall sea state. Expecting a
westerly swell around 8 to 10 ft through the next few days. Today
though, the southerly wind wave will be a big player which will
bring combined seas closer to 10 to 13 ft at 14 seconds. As the
winds ease through the day on Sunday, expecting wind waves to
follow suit but the primary swell will stay around 7 to 9 ft at
12 seconds with times of 10 or 11 ft seas in the southern
portions of PZZ275.   -Muessle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 6 PM this evening to 8 AM PST
     Sunday for Coast Range of Northwest Oregon.

     Winter Weather Advisory from 4 PM this afternoon to 10 AM PST
     Sunday for Northern Oregon Cascades.

     Winter Weather Advisory from 6 PM this evening to 10 AM PST
     Sunday for Upper Hood River Valley.

WA...Winter Weather Advisory from 2 PM this afternoon to 7 AM PST
     Sunday for South Washington Cascade Foothills-South
     Washington Cascades.

PZ...Gale Warning until 4 PM PST this afternoon for coastal waters
     from Cape Shoalwater WA to Cascade Head OR out 60 nm.

     Small Craft Advisory until 7 AM PST this morning for coastal
     waters from Cascade Head OR to Florence OR out 60 nm.

     Gale Warning until 7 PM PST this evening for coastal waters
     from Cascade Head OR to Florence OR out 60 nm.

     Small Craft Advisory from 1 PM this afternoon to 6 PM PST this
     evening for Columbia River Bar.

&&

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