Heavy Snow in Oregon, December 18 to 20, 2021

Area Forecast Discussion
National Weather Service Portland OR
245 PM PST Sat Dec 18 2021

.SYNOPSIS...Strong cold front will push inland this evening, but
stall over northwest Oregon overnight. This will maintain rain to the
south, with rain with low snow levels to the north of the front. Deep
moisture offshore will continue streaming inland Sunday and Monday,
with rain and mountain snow, through low snow levels may bring snow
to the the Columbia River Gorge and Hood Valley later Monday night
into Tuesday morning. Unsettled weather continues through rest of
next week, with snow levels lowering again later next week.
&&

.SHORT TERM...Tonight through Tuesday...Strong cold front now pushing
inland over western Washington and far northwest Oregon. As of 2 pm,
front extended from Puget Sound back across southwest Washington and
into far northwest Oregon down to around Tillamook. This front will
push farther inland and sink southward a bit more this afternoon into
the evening. But, with developing wave well off the south Oregon
coast, this front will slow its southward movement, becoming nearly
stationary overnight into Sunday. That is where the challenging
forecast issues, being where will it snow, and how low will the snow
level go, as well as the timing for the low snow.

Models have been bit mixy as to solutions, with a few ensemble runs
being way way too bullish on low elevation snow. But, will trend
forecasts more towards a mix, leaning with the NBM solutions. Now,
To the north of the front, snow levels will tumble, dropping as low
as 1000 to 1500 feet. As such, have various winter weather advisories
up for elevations above 1000 feet for northwest Oregon into southwest
Washington. At moment, looks like accumulations of 3 to 8 inches for
the Willapa Hills and far north Coast Range, as well as the Cascade
Foothills of south Washington. Heavier snowfall for the higher
Cascades, where 8 to 12 inches likely tonight into early Sunday. But,
with the front stalled, should be a band of somewhat heavier
precipitation over west central Oregon back to the northeast towards
the north Oregon Cascades. As such, precipitation may be heavy enough
to maintain a cooling effect to the air mass, allowing snow levels to
drop as low as 500 feet or bit more for areas just north of the
front. This low snow area seems to align from Chehalem Mountains
around Newberg northward to Vernonia, and eastward across the
Portland West Hills and higher terrain of eastern Clark County. With
that, seems good bet will see 2 to 5 inches of snow for areas above
700 feet for that area, with up to and inch possible across lowlands
of Washington County to far west/northwest Multnomah County (such as
Sauvie Island and Scappoose, as well as across parts of Clark County.
At this time, rain may mix with snow at times, but expecting little
if any snow accumulation for low elevation across rest of Portland
metro, including downtown Portland and the Airport. Timing is the
other question. As Will take some time to cool down, but think the
generally towards midnight, and mainly afterwards. Snow levels will
rise Sunday morning, but precipitation waning. So, will put up Winter
Weather Advisory for areas above 700 feet in the Portland/Vancouver
metro.

Now, along and to the south of the front, expect much higher snow
levels, with rain continuing. No low elevation snow for areas south
of Woodburn. Even across the Cascades, will see big differences in
the snow level. For example, snow levels running at 2000 feet for Mt
Hood area, but 4500 to 5500 feet towards Santiam Pass, and 6000 to
6500 feet at Willamette Pass. With accumulations of 8 to 12 inches
tonight into Sunday, with more expected Sunday into Monday morning,
will extend the current winter weather advisories for the South WA
Cascades and the North OR Cascades out to Monday morning. The stalled
front will lift north a bit Monday night into Tuesday, with not much
change in snow levels for the Cascades. As such, snow level will
generally above Santiam and Willamette Passes through Monday.

As earlier shift said:	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 2+ inches of rain.

Later Sunday night into Monday, will see the low pressure that sits
offshore slow draw bit closer. As it does, flow with the low will
gradually pivot the stalled front over northwest Oregon back to the
north. As it does, will see increasing low level offshore flow, with
drier cooler air at lower levels getting pushed back to the west
through the Columbia Gorge. As such, air mass supports low elevation
snow for those areas again. Snow levels will be down, running from
river level to 500 feet in the Gorge. NBM still supporting moderate
potential of accumulations in those areas, and think will have enough
to support winter weather advisories for the Gorge as well as Hood
River Valley later Sunday night into Monday. But, want to see a few
more model runs before issuing any winter weather for those areas.

Pattern not changing all that much for later Monday into Tuesday. Low
pressure offshore moves little, and in fact, looks to be replaced by
another low on Tuesday. But that low(s) appear to be getting kept bit
more offshore, and drifting to the south a tad. Front that was
lifting north across the region on Monday will be gone by Monday
night, lifting well to the north. Will see some moisture in the
southerly flow being pushed up against the south Oregon coast Monday
night, and lifting up across rest of western Oregon on Tuesday. Could
be dry period with not much precipitation for a while Tuesday morning
into the early afternoon. But, too early at moment to bite on that,
as will keep PoPs in 50 to 70 pct. But, any precipitation does seem
to stay on the light side.				     /Rockey

&&
.LONG TERM...Wednesday through Friday...With diverging model
solutions there isn't too much confidence moving into the long term
forecast. Over the last 24 hours the 12Z EC has become more inline
with the 12Z GEM solution; these solutions show a deep upper level
trough moving south along the B.C. coastline and shifts inland near
Vancouver Island early Thursday morning. While the 12Z GFS shows the
trough staying offshore and dropping south towards northern
California Thursday morning. The latest 12Z run of the GEM is
stronger, slower and drops the trough axis right across the Pacific
NW into Thursday morning. The associated surface front will move on
to the north Oregon coast Wednesday afternoon [according to the GEM]
and bring a cool and showery pattern across the area Thursday.
Temperatures aloft will drop to around -3 to -5 C at 850 mb Thursday
which will aid in dropping snow levels to around 2500 to 3000 ft by
Thursday. With post frontal showers, snow will have a good chance
for accumulating up in the Cascades.

A secondary, weak front will move through the northwesterly flow and
provide enough lift to bring another round of heavier showers Friday.
Snow levels will also progressively drop Friday as the arctic air
mass over Canada pushes south. Overnight lows across Willamette
Valley sites will likely dipping into the mid 30s. Snow levels will
drop to around 2000 ft and potentially 1500 ft by end of day Friday.
NBM shows a 10-15% chance that lowlands in the northern Willamette
Valley will see very light (around 0.1 inches) snow through the day
Friday. 			  /BMuhlestein

&&

.AVIATION...Widespread MVFR cigs continue along with increasing
precipitation. Low Level Wind Shear due to southerly winds at
the surface and more of a westerly wind aloft for most TAF
locations through 00Z Sunday. Areas that will be most impacted
are the northern Willamette Valley and the Columbia River Gorge.
Gusty winds and precipitation could cause a decrease in
visibility at times.

The cold frontal passage has already been observed at KAST, which
now has strong westerly winds. Expect a similar wind shift
through the overnight hours as the front moves inland. The cooler
environment behind the front could cause IFR conditions after 00Z
Sunday. Precipitation will mostly be rain, but with temperatures
dropping, there is a high probability for a rain/snow mix through
the Willamette Valley north of, but not including KUAO after 06Z
Sunday. KHIO might see some snow stick to the ground with light
accumulations. More persistent snow expected along the Coast Range
and the Cascades as snow levels are expected to be around 500-800 ft
through at least 18Z Sunday.

Expect a slow warm up across the area after 18Z Sunday as well as
winds becoming easterly, which is being caused by a surface low
and associated warm front pushing northward along the coast.
Also, could see some gusty east winds near the mouth of the
Columbia River Gorge.

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

KPDX AND APPROACHES...Predominately MVFR CIGs, with the chance
for IFR conditions after 06Z Sunday. Gusty southerly surface
winds combined with strong westerly winds aloft, are creating an
environment suitable for Low Level Wind Shear through 00Z
Sunday. Tonight, temperatures drop which will likely result in a
rain/snow mix after 06Z Sunday. Slow warm up across after 18Z
Sunday and winds becoming easterly. Could see some gusty east
winds near the mouth of the Columbia River Gorge. /42

&&

.MARINE...A low pressure will push inland into western Canada
through the overnight hours. As the low moves inland, it will
drag a cold front across the waters and will form a coastal jet
through Sunday morning. As of this afternoon, a westerly wind
shift has been observed at Bouy 46029 and a similar shift is
expected at Buoy 46050 this evening. Currently, westerly small
craft wind gusts are present in zones PZZ270 and PZZ250 along
with southerly gale force winds in zones PZZ275 and PZZ255. These
gale force conditions should subside slightly and become
westerly this evening as the front moves eastward.

Sunday, winds will become offshore and then southerly as a
surface low moves northward towards the Olympic Peninsula
through Monday. At this time winds are expected to be below small
craft advisory criteria Sunday and increase by Monday.

Seas, in general, are expected to remain in the 8 to 12 ft
range. However, expect choppy and chaotic conditions as multiple
swell trains and varying wind directions will impact all waters
through the start of the week. This means that there could be
periods of time when seas are in the mid teens in overall wave
height. /42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 6 PM this evening to noon PST
     Sunday for Coast Range of Northwest Oregon-Upper Hood
     River Valley.

     Winter Weather Advisory until noon PST Monday for Northern
     Oregon Cascade Foothills-Northern Oregon Cascades.

     Winter Weather Advisory from 9 PM this evening to 10 AM PST
     Sunday for areas above 700 feet in Greater Portland Metro.

WA...Winter Weather Advisory from 6 PM this evening to noon PST
     Sunday for Willapa Hills.

     Winter Weather Advisory until noon PST Monday for South
     Washington Cascades.

     Winter Weather Advisory from 9 PM this evening to 10 AM PST
     Sunday for areas above 700 feet in Greater Vancouver Area.

     Winter Weather Advisory from 7 PM this evening to noon PST
     Sunday for I-5 Corridor in Cowlitz County-South Washington
     Cascade Foothills.

PZ...Small Craft Advisory until 8 PM PST this evening for coastal
     waters from Cape Shoalwater WA to Cascade Head OR out 60
     nm.

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

     Small Craft Advisory until 6 PM PST this evening for Columbia
     River Bar.

&&

www.weather.gov/portland

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