Heavy Snow in Oregon, December 4, 2022

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
334 AM PST Sun Dec 4 2022

Updated Aviation and Marine Sections

.SYNOPSIS...A broad low will move down the coast through the
evening then swing northward over the OR/CA border will be the
weather maker for the next few days. A mix of precipitation is
expected ranging from snow to rain with everything in between.
Confidence increasing in the short term. Drier through Wednesday
before a robust low advects southward towards the coast from the
Gulf of Alaska late in the week.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...A dynamic winter weather
pattern continues to provide forecasting challenges through
Monday morning. Synoptically, setting the stage is the broad low
that is dropping southward along the Oregon coast into northern
California. This low will be the main driver of sensible
weather over then next 36 hrs. Models have a fairly good handle
on the location and track of this system which is building
confidence in the incoming pattern. They all show the low
tracking back northward and stalling right along the OR/CA
border on Sunday, but the progress has been slow. Clouds along
the frontal boundary are attempting to push inland from the
coast, but momentum just is not there yet. Thus, much of the
area has been under partly cloudy skies today allowing
temperatures to warm into the 50s in some inland locations where
wind is not present. Overnight, this front should rebound and
move into the southern Willamette Valley by late tonight, slowly
shifting northward through Sunday morning. With moisture
wrapping around the low behind the front, expecting a band of
precipitation to move from south to north Sunday morning which
will bring a mix of weather types. The predominate type south of
Salem will be rain given the lower snow levels, with the
Cascades likely will see a few inches of snow. With the
continued offshore winds ushering cold air from east of the
Cascades, there is ample cold air pooling at the surface which
will promote patchy freezing rain in some of the valleys of the
eastern slopes of the Coast Range and Lane County Cascades, and
the southern Willamette Valley.

This is where the the uncertainty comes into play, and to miss
sounding like a broken record, will say that little has changed
in the forecast. The east winds really will be one of the
determining factors in the north. Forecast soundings are
showing a very dry atmosphere due to the addition of the dry
air, and quite chilly. However, as the deformation band moves
northward with an inverted shortwave trough, precipitation that
does fall would likely evaporate. But, as that occurs, it will
cool the lower levels of the column and lower the wetbulb
temperature; thus, snow becomes a bit more likely through the
day on Sunday vs becoming virga. For those in the West Hills,
near Vernonia, and Forest Grove, chances are slightly higher for
accumulating snow around 1 inch. Cold air damming against the
terrain will lower snow levels and thus make it a bit easier to
get snow in those locations and others nearby. The lowlands of
the Willamette Valley and southern Washington Counties will have
a mix of precipitation. There has been some accumulation changes
for the Portland-Metro areas in the 12Z guidance vs that of the
06Z outputs. Snow amounts, if they were to occur, look to be
minimal and likely would be mixed with rain. There is higher
confidence for snow through the Columbia River Gorge and upper
Hood River Valley where 1-2 inches could accumulate above 500
ft; though, freezing rain is also possible at the lowest
elevations of the Gorge due to the cooling of the offshore
winds. Accumulation wise it likely will be minimal with around a
20% chance of seeing a trace of ice. Ultimately, some will see
snow, others will not but with continued inconsistencies in
track, temperatures, wind speeds, and atmospheric saturation, it
is frankly too difficult to identify specific locations with
any level of certainty.

By Sunday afternoon, the main band of the front will beginning
to dissipate and post frontal showers will remain through Monday.
Behind the front, snow levels will rise and showers will become
spotty. Will see light rain and/or snow depending on your
location. Accumulation will be minimal and no impacts are
expected.
	  -Muessle

.LONG TERM.../TUESDAY THROUGH SATURDAY/...Quieter weather
likely early next week as the area will be under light flow
aloft between split upper level flow on Tuesday. A shortwave
upper ridge then traverses the region on Wednesday, followed by
a digging upper shortwave trough. This feature will likely usher
in another Pacific frontal system sometime Wednesday night or
Thursday. The active weather pattern will likely continue
through the end of the week.

Temperatures are expected to trend slightly warmer through mid-week,
but will generally remain near or below normal. Weak surface low near
the Washington coast may bring a slight chance to chance of light
showers on Tuesday. Snow levels will generally range from 2000-3000
feet through Wednesday. Ensembles indicate chances for precipitation
begin to increase early Thursday morning. Deterministic models are in
good agreement showing the next system dropping out of the Gulf of
Alaska on Wednesday with the leading front approaching the Pacific
NW early Thursday. The NBM PoPs of 70-80 percent seems reasonable for
Thursday. Showers will likely follow with northwest flow aloft.
Another frontal system is possible late in the week and into the
weekend. Snow levels are likely to fall as the colder air aloft moves
in, but should remain in the 1000-2000 ft range.
  -Hartsock

&&

.AVIATION...12Z TAFs: Low pressure centered well off the north
California coast and moderate to strong offshore low-level flow
through the Columbia Gorge has resulted in VFR conditions just
before 10Z. However, the offshore flow has resulted in up-slope
conditions for the central Gorge, with very light snow reported at
K4S2. -RA was noted at KRBG. Cigs will lower from south to north
as moisture continues to spread north from southern Oregon. Expect
increasing MVFR south of a KSLE to KONP line after 15Z. Precipitation
type will be highly variable. Interior areas north of KS12 will
have a better than 50 percent chance of seeing some snow.
Precipitation in the Columbia Gorge will be mainly snow Sunday
morning through Sunday night, but there is the threat for -FZRA west
of Multnomah Falls where the cold air is not as deep. No matter
precipitation type, flight conditions are expected to be mainly IFR
to MVFR over much of the area after 18Z Sunday. Temperatures moderate
Sunday afternoon and snow levels rise, such that any accumulations
should melt rather quickly late Sunday.

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

KPDX AND APPROACHES...VFR at the terminal and vicinity at 10Z.
Offshore flow continues through the Gorge with gusts 30-35 kt at
KPDX and KTTD. The KTTD-KDLS gradient will gradually relax after
15Z, decreasing to under -5 mb by 20Z. VFR to prevail early in the
12Z TAF period, but conditions expected to fall to MVFR between
16Z and 18Z Sunday. Temperatures should support snow initially,
but the odds of an inch of snow accumulating on grassy surfaces is
25%. Snow levels will slowly rise Sunday afternoon and any
accumulations should melt rather quickly late in the day.
Nonetheless, expect a mix of MVFR and IFR conditions Sunday afternoon
into Sunday evening. Weishaar

&&

.MARINE...Shortly before 10Z water vapor satellite showed a low
pressure centered near 42N 131W, or about 325 nm west of Pt. St.
George. The 09Z buoy reports showed wind gusts up to 30 kt,
strongest over the far northern waters. Expect small craft
advisory level gusts to persist over the far north zones through
18Z Sunday. Current advisories look good. Wind speeds diminish
below 15 kt across the waters by late Sunday afternoon. Wind
speeds then lower to 10 kt or less Sunday night and will remain
below 10 kt through Wednesday. Models are in good agreement
showing a frontal system impacting the waters Wednesday night or
Thursday morning. The National Blend of Models (NBM) v4.0
indicates a 70 percent chance of gale force gusts for buoy 46029
with this system. Will cap gusts at 30 kt this far in advance.

At 09Z seas were hovering around 10 ft. The offshore flow will
help to dampen wave heights. Expect seas to subside to 5 to 7 ft
by late Sunday afternoon. This takes into account a 1 ft added
bias to compensate for the trend of actual wave heights slightly
higher than model guidance. Seas then remain at or just below 5 ft
through mid-week and then build up to 12 to 14 ft late in the
week. Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Weather Advisory until 6 PM PST this evening for Central
     Willamette Valley-Coast Range of Northwest Oregon-Greater
     Portland Metro Area-Lower Columbia-Northern Oregon Cascade
     Foothills-Northern Oregon Cascades.

     Winter Weather Advisory until noon PST today for Cascade
     Foothills in Lane County-Cascades in Lane County-Central
     Coast Range of Western Oregon-South Willamette Valley.

     Winter Weather Advisory until 10 PM PST this evening for
     Central Columbia River Gorge-Upper Hood River Valley-Western
     Columbia River Gorge.

WA...Winter Weather Advisory until 6 PM PST this evening for Greater
     Vancouver Area-I-5 Corridor in Cowlitz County-South
     Washington Cascade Foothills-Willapa Hills.

     Winter Weather Advisory until 10 PM PST this evening for
     Central Columbia River Gorge-South Washington Cascades-Western
     Columbia River Gorge.

PZ...Small Craft Advisory until 10 AM PST this morning for Coastal
     waters from Cape Shoalwater WA to Cape Falcon OR out 10 NM-
     Columbia River Bar-Waters from Cape Shoalwater WA to Cape
     Falcon OR from 10 to 60 NM.

     Small Craft Advisory until 4 AM PST early this 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.
&&

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