Heavy Snow in Washington, February 13 to 14, 2023

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
904 PM PST Sun Feb 12 2023

Updated Short Term and Aviation sections

.SYNOPSIS...A fast moving cold front will sweep across the region
early Monday. Cooler temperatures will follow with snow levels
lowering to near sea level overnight Monday night. Rain and snow
showers and a thunderstorm threat will persist into early Tuesday
morning before diminishing. High pressure then builds offshore but a
weak system will brush the area Thursday with some low to moderate
(25-55%) rain and snow chances.

&&

.DISCUSSION... EVENING UPDATE:	Little change to the current forecast
based on initial 00Z model runs. At 04Z water vapor satellite imagery
revealed a cold frontal boundary entering southwest Washington. Light
rain has been noted at Hoquiam, WA to Astoria, OR. The 00Z NAM shows
some slight differences from its previous couple of runs. The
aforementioned frontal boundary pushes southeast overnight through
Monday. The NAM now shows the secondary short-wave for Monday night
through Tuesday morning taking a slightly more inland trajectory.
Despite onshore flow, the air mass looks to be cold enough to support
precipitation as snow for the interior lowlands, even to south
Washington and north Oregon coast.

The 01Z run of the National Blend of Models (NBM) continues its
gradual increasing probability of an inch of snow for the Willamette
Valley late Monday night through 18Z Tuesday. The 01Z output shows a
general 30 to 50 percent chance of an inch, but up to 65 percent east
of Interstate 5 toward the lower Cascade foothills. Even the
immediate coast from near Raymond, WA to about Newport, OR has a 25
to 35 percent chance of at least a dusting. These probabilities
diminish to near 10 percent for Florence, OR. A majority of the
high-resolution models show some measurable snowfall for KPDX by
early Tuesday morning. The bulk of these high-res models suggest
one-quarter to three-quarters of an inch of snowdepth at KPDX early
Tuesday morning. The HREF ensemble-mean 12-hr snowfall valid 12Z
Tuesday paints 1-2 inches for much of the Willamette Valley, with the
potential for slightly higher amounts for the south Washington coast
and coastal valleys. Weishaar

REMAINDER OF DISCUSSION FOLLOWS: THROUGH WEDNESDAY:
Rain will arrive to the coast this evening, followed by a fast
moving cold front crossing the region overnight. Winds will be
rather breezy from the west/northwest around the time of the cold
frontal passing and for a few hours thereafter. Snow levels will
start dropping but the main action will occur beginning Monday mid-
morning.

Then, a second shortwave will push colder and moister air from the
Gulf of Alaska further southeast and over the region. Additionally,
colder air aloft will arrive with most areas west of the Cascades
proper showing enough instability to warrant a slight chance to
chance (15- 30%) mention of thunderstorms. Also by this point, snow
levels will have generally fallen to around 2000 feet as the overall
flow becomes northwesterly and orographics comes into play. Will
start seeing the first round of heavy snow accumulations across the
Cascades and foothills with several inches expected through Monday
evening. The higher Coast Range and Willapa Hills elevations will
also start seeing snow at this time. Elevations 2000 feet and below
should stay as rain for the time being, but may get some snow mixing
in or small hail under any thunderstorms and heavier showers.
Upgraded the watches to Winter Storm Warnings and also issued Winter
Weather Advisories for the higher terrain of Washington.

Monday evening will bring a bit of a break for the I-5 corridor as
rain shadowing sets up for a several hours.Snow will continue
accumulating over the mountains, however. And by midnight, a compact
and closed upper low slides south along the B.C. coastline then
crosses the region Monday overnight and early Tuesday. This
trajectory will be able to pick up some additional Pacific Ocean
moisture and dump it out mainly over NW Oregon. By then,
temperatures will have cooled enough to bring snow levels to near
sea-level. The low-level onshore flow, nearly perpendicular to the
terrain, and general weak instability (from -6 to -8 deg C 500 mb
air moving overhead) will help enhance the rain and snow rates (and
diabatic cooling for lower snow levels) but will also be somewhat
offset by the warmer air forced inland from the warmer ocean
surface. As such, this remains a complicated forecast as usual for
precipitation types across the coast and the I-5 corridor. The
better forcing could end up over the central and south Willamette
Valley, but leaving the stronger showers for points north. For now,
it seems the central coast will remain just a little too warm with
snow accumulation threat remaining below 1 inch.

But, have issued snow advisories for all remaining lower elevation
zones for multiple reasons. First, the prime threat for
accumulations will occur after Midnight Monday night and persist
through about 7 AM, thus having a significant impact on Tuesday
morning's commute. Additionally, many areas below 1000 feet will
probably see 1/2 inch of snow accumulation or less while the other
areas may receive upwards of a couple inches on the high end. But,
could also see a scenario where the closed low sets up a deformation
band for a few hours and brings 2-3 inches of snow to a relatively
narrow strip of land, leaving the vast majority of places unscathed.
Or, trailing showers around daybreak could bring an inch+ of snow to
a relatively speckled patter of ground and nearly nothing elsewhere.
The expected reality, though, is a combination of these individual
events and felt it is better to be prepared given the time of day
the low elevation snow potential exists. What is known, is the
potential for accumulating snow certainly increases with elevation
and the terrain above 1000 feet should receive another few to
several inches of snow on top of what had already over the prior 18
hours.

Beginning around daybreak, the upper low center will quickly move
south and square over the forecast area. This will bring a fairly
rapid end to the shower activity. Very little in the way of showers
will remain much after 10 AM Tuesday. Skies will start clearing as
drier/more stable air invades the region and then allows sunlight to
reach the ground. This will help quickly melt any low elevation snow
as long as it remains exposed to the sun. Temperatures will bottom
out right around 32 degrees for the lowlands Tuesday morning but are
expected to recover back to the lower 40s Tuesday afternoon.

Tuesday night will bring a much clearer night sky which will allow
for strong radiational cooling and temperatures easily falling into
the 20s. While Tuesday morning will on the colder, breezy, and snowy
side, Wednesday morning will be notably colder for most areas as the
little bit of heat built up by Tuesday afternoon will easily escape
out to space. Wednesday will also remain relatively clear also
helping the temperatures rebound to the lower 40s. /JBonk

WEDNESDAY NIGHT THROUGH SATURDAY...
Upper-level ridging will bring dry weather across the region
Wednesday night into early Thursday morning. Deterministic and
ensemble guidance shows the ridge exiting by Thursday afternoon,
making way for the next system that will be coming towards the West
Coast. Currently, the majority of ensemble members are showing a
deep trough developing off- shore over the northeast Pacific
Thursday. Model guidance suggests that this trough will elongate far
enough south that it will split into a cut-off low off the
California coast along with a shortwave trough northeast of our
region. Essentially, we would be in between these two features and
trend towards less active weather. Moisture associated with the cut-
off low will be too far south to warrant any concerns for
significant precipitation, but am still waiting on the blended
guidance remove all QPF before pulling most PoPs from our forecast.
Similarly, ensembles are placing the shortwave trough axis over the
Canadian Rockies which would also keep precip away. Also, the GFS
ensemble has the shortwave trough axis extending the closest to us
compared to the Euro and Canadian ensembles, which would potentially
bring showers through Friday afternoon. Overall, chance for precip,
possibly with snow mixed in at times, remains 30% or lower for
Thursday and Friday.

As we head into next weekend, all clusters agree that we'll begin to
have northwesterly flow aloft and hint at potential ridging.
However, there is uncertainty with whether or not we will receive
precipitation. PoPs are currently 50/50 by Sunday afternoon, and
this could be due to the lingering influence of the exiting trough.
If any precip falls, expect it to be more stratiform than
convective. High temperatures are expected to be near seasonal
values, but lows will stay near or below freezing at least through
the early part of the long-term period. /Alviz

&&

.AVIATION...06Z TAFs: Quite the dynamic forecast so apologies for
the longer than normal discussion. The cold front has begun
moving inland with rain falling along the north coast. Ceilings
did lower farther than previously expected into IFR category
around KAST. However, behind the front, conditions should become
more steadily MVFR. Inland will be VFR until around 09Z when
conditions degrade to MVFR with the onset of the front. One tricky
component is the inland terminals. The region has been clear with
light winds for much of the day. It is possible that in the
central and southern Willamette Valley that some patchy fog could
form over the next few hours; confidence is quite low. Will note
that models have not performed well in predicting fog formation
over the last few days. Associated rain will turn to showers with
the arriving cooler unstable air mass after 12Z Monday. In the
post-frontal environment, conditions will become more convective
and there is a slight chance for thunderstorm activity after 19Z
Monday. The exact location and quantity of lightning or small hail
is too difficult to forecast, but stay tuned to area radar and
TAF updates.

Breezy west winds expected along the coast as well as over the
higher terrain after 06Z which will intensify through the day.
Towards the end of the period (after 20Z), expecting winds to
become gusty along the coast (up to 35 kt) and east/west aligned
terrain. At around 3000 ft, winds will be gusting as high 40 kt
from the northwest. With the cold air shifting in, snow level
will gradually lower down to 1500 ft by late Mon. With all the
clouds and showers, expect higher terrain to be frequently
obscured. At lower elevations, chances for snow is pushed off
until after 06Z Tuesday.

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

KPDX AND APPROACHES...Cold front is advancing and should reach the
terminal by 09Z Monday. Expecting spotty rain becoming showers in
the post frontal environment. Will see a trend towards MVFR but
CIGs will likely sit right on the cusp of MVFR and VFR. No snow
for KPDX on Monday but heavier showers may bring small hail,
which could briefly accumulate. Chances for snow increase right at
the end of the forecast period, but probabilities sit right around
40% for more than a trace of snow.
-Muessle/Rockey

&&

.MARINE...Well, back to active weather, with a rough 36 hours in
store for the coastal waters. First up, a cold front will drop
across the region later tonight. Winds will turn westerly, with
gusts 25 to 33 kt for later tonight into Monday am. Seas will come
up a bit, but stay in the lower teens.

Then, another front will push across the region. As such, will see
pressure gradients tighten quite a bit, with solid west to
northwesterly gales for later Mon am through early Tue am. Gusts
35 to 45 kt expected, with strongest winds Mon afternoon into Mon
night. Now, with unstable air mass will feed the showers and
isolated thunderstorms, a locally brief stronger gust is not out
of the question. But, will maintain current advisories for tonight
into Wed, with Gale Warning for later Mon am through Tue am.

With all the winds, seas will build as well, with rather choppy
chaotic seas expected on Mon into Tue. Seas will gradually build,
running 15 to 20 ft by midday Mon, and then 20 to 23 ft for Mon
afternoon into Mon night. Seas will slowly subside Tue.

With these seas, and underlying swell still running with dominant
period of around 13 to 15 seconds, these waves and breakers will
pack somewhat of a punch for the surf zone. Will put up High Surf
Advisory for all of the coast for Mon into Tue. The seas will
gradually decreasing from north to south on Tue.

High pressure builds over the region from north to south later
Tue and Wed, with winds dropping back under 20 kt, and subsiding
seas, generally back to 10 to 13 ft for Wed.	    /Rockey

.BEACH HAZARDS...Increasing high energy westerly swells will
increase the chances for high surf conditions along the coast
Monday afternoon through Tuesday morning. A persistent westerly
wind will compound with the swell bringing waves to around 20 to
25 ft by 4PM Monday. A High Surf Advisory has been issued for the
area beaches. With these waves, while they may look exciting, can
be very dangerous. Destructive waves may wash over beaches,
jetties, and other structures unexpectedly. Stay tuned to the
forecast before heading onto area beaches.
-Muessle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...High Surf Advisory from noon Monday to noon PST Tuesday for
     North Oregon Coast.

     Winter Weather Advisory from 10 PM Monday to 10 AM PST Tuesday
     for Central Willamette Valley-Greater Portland Metro Area-
     Lower Columbia-North Oregon Coast-South Willamette Valley-
     Western Columbia River Gorge.

     High Surf Advisory from 4 PM Monday to 7 PM PST Tuesday for
     Central Oregon Coast.

     Winter Storm Warning from 10 AM Monday to 1 PM PST Tuesday for
     Cascade Foothills in Lane County-Central Coast Range of
     Western Oregon-Coast Range of Northwest Oregon-Northern
     Oregon Cascade Foothills.

     Winter Storm Warning from 4 AM Monday to 1 PM PST Tuesday for
     Cascades in Lane County-Northern Oregon Cascades.

WA...Winter Weather Advisory from 10 PM Monday to 10 AM PST Tuesday
     for Greater Vancouver Area-I-5 Corridor in Cowlitz County-
     South Washington Coast-Western Columbia River Gorge.

     Winter Weather Advisory from 10 AM Monday to 10 AM PST Tuesday
     for South Washington Cascade Foothills-South Washington
     Cascades-Willapa Hills.

     High Surf Advisory from noon Monday to 4 AM PST Tuesday for
     South Washington Coast.

PZ...Small Craft Advisory until 1 PM PST Monday for Columbia River
     Bar.

     Gale Warning from 1 PM to 10 PM PST Monday for Columbia River
     Bar.

     Small Craft Advisory until noon PST Monday for Coastal waters
     from Cape Shoalwater WA to Florence OR out 10 NM.

     Gale Warning from noon Monday to 10 AM PST Tuesday for Coastal
     waters from Cape Shoalwater WA to Florence OR out 10 NM.

     Small Craft Advisory until 7 AM PST Monday for Coastal waters
     from Cape Shoalwater WA to Florence OR from 10 to 60 NM.

     Gale Warning from 7 AM Monday to 10 AM PST Tuesday for Coastal
     waters from Cape Shoalwater WA to Florence OR from 10 to 60
     NM.
&&

www.weather.gov/portland

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