Winter Storm and Winter Weather in Oregon, April 10 to 11, 2022

Area Forecast Discussion
National Weather Service Portland OR
255 PM PDT Sun Apr 10 2022

.SYNOPSIS...Unseasonably cool weather will continue throughout much
of the forecast period. Periods of rain and snow will produce winter
weather headlines across much of the area Sunday through early
Tuesday. Cool weather continues through the week.

&&

.SHORT TERM...Tonight through Tuesday...Low pressure has moved east
of the Cascades leaving the area under cool, showery northwest flow.
Periods of heavy rain and hail have been reported but no lightning
has been observed as of 2 PM. Webcams show snow falling across
Willamette and Blue Box pass with the last observed snow levels on
the Astoria profiler reading 2141 ft early this morning. Radar shows
showers across areas north of a line between Santiam Junction and
Lincoln City. Convective showers with a slight chance of thunder and
small hail will continue into this evening.

The bigger story comes tonight as a compact low moves directly over
the County Warning Area (CWA) bringing winter weather headlines
across much of the area. The low is currently near 46N/134W making
its way over a broad area of high pressure set up across the east,
central Pacific. This low is dragging a substantial amount moisture,
about 180% of normal, within its southwest quadrant out of the
subtropical Pacific. Total precipitable water from both the 12Z NAM
and GFS are matching well with those observed in polar orbiting
satellites. Models are coming into better agreement with each other
with regards to the track of the low with the 12Z deterministic
models, GFS, NAM, and CMC, as well as high resolution guidance
suggesting it will move onshore over Lincoln City. This will make
for dramatic changes over the CWA with regards to precipitation,
wind, and snow levels.

This is quite the unusual low especially for April. The 12Z GFS shows
a broad area of Q-vector convergence encapsulating the entire CWA
starting this afternoon and persisting through Monday. This lift
coupled with abundant moisture will keep rain across the area tonight
and into tomorrow morning. Embedded within this broad area of
convergence is strong areas of frontogenesis and slantwise
convection. These coupled with the convergence and high humidities
from the subtropical moisture will provide all the necessary
ingredients for heavy bands of precip. Hi-res guidance is showing 3
accumulations of 0.4-0.6 inches over much of the terrain and across
the lowlands north of McMinnville and Woodburn.

As the low tracks onshore there will be varying wind regimes and
differing snow levels as those winds push and pull different air
masses across the area. Strong winds within a bent-back occlusion
will bring winds gusting up to 40 mph across the central Willamette
Valley and along the coast. Wind gusts between 20 to 30 mph should be
expected elsewhere. Subsidence within occlusion will bring cold
temperatures aloft down to the surface and with evaporational cooling
from high precip rates snow levels will drop between 200-500 ft
across areas north of Salem. Looking at soundings, the layer above
freezing is roughly only 50mb deep and with precip rates of 0.25" per
hour as they are in the hi-res output that would cool the column by
2.5 degrees which is enough to dip low elevation temperatures in the
early morning to below freezing. That is the major concern. The
forecast temperatures are fairly warm right now so it may not seem
like snow could be an issue but if evaporational cooling and heavy
bands of snow materialize those temperatures will drop, the ground
will cover with snow before the Sun rises and accumulate more than
expected. This could warrant upgrades and new issuance of headlines
during the overnight hours in this scenario.

The previous Winter Storm Warnings across the Cascades and Cascade
foothills remain with an upgrade of the Lane County Cascades above
1000 ft. There will be some areas in the Coast Range that could hit
Warning criteria but confidence is not high enough at this time;
those above 1500-2000 ft should watch for potential upgrades. Winter
Weather Advisories have been issued across the Lower Columbia I-5
corridor in Cowlitz County, the greater Portland and Vancouver metro,
and the Columbia River Gorge. Elevations above 500 ft could see
accumulations between 3 to 6 inches with areas at river level likely
seeing around an inch. The central and southern Willamette Valley as
well as the south Washington and north Oregon coast could see mixed
precip. Less than an inch likely in these valley locations and little
to no accumulations along the coast. Because this is April and this
could, emphasis on could, turn into an unprecedented snow event those
driving during the early morning Monday commute should be cautious.

Showers and mountain and foothill snow will continue into Tuesday as
the low pushes east. Steep lapse rates within southwesterly flow
aloft will increase chances of thunderstorms across much of the area
tonight through Tuesday. -BMuhlestein

.LONG TERM...Wednesday through Sunday...A very active spring weather
pattern continues through the remainder of the week. Models and
their ensembles continue to show a cool and showery pattern
continuing through the end of this week and into the weekend. The
primary synoptic feature, is a forming closed low. Towards the end
of the week and potentially continuing into the upcoming weekend,
models are suggesting a shortwave ridge developing. This ridge could
bring a brief respite to the area, but given that blend outputs when
compared to operational models are showing significant differences,
with that in mind have deviated very little from the NBM model
output as uncertainty is high in the forecast through the extended
time frame. /42

&&

.AVIATION...00Z TAFs: Showers in a moist airmass and
northwesterly flow aloft is producing a mix of VFR and MVFR
ceilings across the forecast area, as well as plenty of mountain
obscuration. The main aviation concerns begin tonight into
tomorrow morning, when a strong closed surface low is expected to
push inland near Lincoln City. This low will bring lowering cigs,
increasing winds, rain, and snow to northwest OR and southwest
WA. East winds ahead of the low will help bring colder air into
the northern Willamette Valley and interior lowlands of southwest
WA, with snow or a rain/snow mix expected down to the valley
floor. Right now, it looks like the best chance for snow in the
lowlands is between 11-17Z Monday for locations to the north of
KSLE. While mainly rain is expected for KSLE and KEUG, mainly snow
is expected for KTTD, KPDX, KUAO, and KHIO Monday morning. In
fact, snow may become moderate to heavy at times in the early
morning, which will reduce visibilities down to 1/2 SM to 1 SM at
times. Expecting anywhere from a trace to an inch or two of snow
accumulation in the lowlands to the north of KSLE. Southerly winds
will also gust to around 30 kt over the Willamette Valley, with
even stronger gusts expected along the coast.

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

KPDX AND APPROACHES...VFR cigs and light rain showers will
continue through this afternoon, before evolving into a steady
rain later this evening with cigs lowering into the IFR range by
06Z. The forecast becomes tricky thereafter, but models are now
honing in on a period of snow at the KPDX terminal between 12-17Z
Monday morning, potentially heavy at times. Expect significantly
reduced visibilities during that time due to a combination of
breezy winds and falling snow. Expect anywhere from a trace to
one inch of accumulating snow at the terminal. -TK

&&

.MARINE...A closed surface low was located around 45N/135W early
Sunday afternoon, which was on track with the latest suite of model
guidance. Models have finally honed in on the expected track and
strength of this low, which is expected to push into the coastal
waters this evening before making landfall around Lincoln City
shortly after midnight. This low will bring increasing winds and
building seas. As this low moves into the coastal waters, wind
gusts to 40 kt will develop across the outer waters, with gusts
potentially as high as 45 kt near the center of the low. Wind
gusts to 40 kt are also expected over the inner waters due to a
combination of a coastal jet ahead of the incoming low and strong
northwesterly winds on the backside of the low. Although
northwest winds will ease slightly Monday night, it will still be
quite breezy with gusts to 35 kt or so. The exception appears to
be the northern inner waters, where gusts should top out closer to
30 kt Monday night.

The aforementioned low will also bring rapidly building seas.
While most guidance is suggesting combined seas topping out around
15 ft, would not be surprised if seas end up peaking closer to 20
ft due to building wind waves and fresh swell. This is especially
true for the central waters from Cascade Head to Florence, where
winds and wind waves will be maximized due to the track of the
low. Wind gusts should finally fall below 30 kt Tuesday morning,
but a northwesterly swell around 11 to 14 ft will keep seas
elevated. -TK

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 8 PM this evening to 2 PM PDT
     Monday for Central Coast Range of Western Oregon-Coast
     Range of Northwest Oregon-Upper Hood River Valley.

     Winter Weather Advisory from 3 AM to 10 AM PDT Monday for
     Central Columbia River Gorge-Greater Portland Metro Area-
     Lower Columbia-Western Columbia River Gorge.

     Winter Weather Advisory until 5 PM PDT this afternoon for
     Cascade Foothills in Lane County-Cascades in Lane County-
     Northern Oregon Cascade Foothills-Northern Oregon Cascades.

     Winter Storm Warning from 5 PM this afternoon to 5 PM PDT Monday

     for Cascade Foothills in Lane County-Northern Oregon
     Cascade Foothills.

     Winter Storm Warning from 5 PM this afternoon to 6 AM PDT
     Tuesday for Cascades in Lane County-Northern Oregon
     Cascades.

WA...Winter Weather Advisory from 8 PM this evening to 2 PM PDT
     Monday for Willapa Hills.

     Winter Weather Advisory from 3 AM to 10 AM PDT Monday for
     Central Columbia River Gorge-Greater Vancouver Area-I-5
     Corridor in Cowlitz County-Western Columbia River Gorge.

     Winter Weather Advisory until 5 PM PDT this afternoon for South
     Washington Cascade Foothills-South Washington Cascades.

     Winter Storm Warning from 5 PM this afternoon to 5 PM PDT Monday

     for South Washington Cascade Foothills.

     Winter Storm Warning from 5 PM this afternoon to 6 PM PDT Monday

     for South Washington Cascades.

PZ...Small Craft Advisory until 3 PM PDT this afternoon for Columbia
     River Bar.

     Small Craft Advisory until 10 PM PDT this evening for coastal
     waters from Cape Shoalwater WA to Florence OR out 60 nm.

     Gale Warning from 10 PM this evening to 5 PM PDT Monday for
     Coastal waters from Cape Shoalwater WA to Cascade Head OR
     out 10 nm.

     Gale Warning from 10 PM this evening to 11 PM PDT Monday for
     Coastal waters from Cascade Head to Florence OR out 10 nm-
     coastal waters from Cape Shoalwater WA to Florence OR from
     10 to 60 nm.

&&

&&

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