Heavy Snow and Winter Weather in Oregon, February 14 to 16, 2024

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1051 AM PST Wed Feb 14 2024

Updated aviation and hazards

.SYNOPSIS...Extremely challenging forecast in the cards for much of
SW Washington and NW Oregon, where cold air pushing southward from
British Columbia will meet milder, moist air from the Pacific. Low
pressure will spread precip NE across the district today, mostly as
rain south and west of Salem. The low will likely weaken and stall
just south of the Columbia tonight into Thursday, with precipitation
persisting near the Portland metro and the Columbia Gorge as snow
levels lower. Heavy snow will occur for the Cascades and Columbia
River Gorge/Hood River Valley later today through Thursday, possibly
persisting into Friday morning.

The forecast for the greater Portland/Vancouver metro is particularly
challenging. Forecast models range from an non-impactful cold rain to
impactful snow beginning as early as today. The best chance for
lowland snow appears to be north of Wilsonville, tonight through
Thursday. After a blustery and drier day Friday, a frontal system
will approach from the Pacific for the weekend, but split flow will
keep precip light. By then, subfreezing air should be limited to the
Gorge and Cascades.

&&

.DISCUSSION...Today through next Tuesday...Well, we mentioned a
nonzero chance of lowland snow in the last few discussions, and that
appears to be coming to fruition in what will be an extremely
challenging forecast for the lowlands north of about Salem. The
addition of high resolution guidance has significantly increased the
probabilities of snow accumulation for these areas, including for the
greater Portland and Vancouver metro area. Several inches of snow are
likely in the Columbia River Gorge east of Multnomah Falls, with over
a foot likely for the Cascades and upper portions of the Hood River
Valley by the time snow diminishes late Thursday or early Friday.
Meanwhile, latest probabilistic guidance continues to suggest very
low probabilities of impactful snow south of about Wilsonville. This
leaves SW Washington, the Portland/Vancouver metro area, and lower
portions of the Columbia River, where the forecast is so challenging
that 1-2 deg C of error for any portion of the lower atmosphere could
make the difference between a cold rain and several inches of heavy,
wet snow. Models have trended colder and snowier in the past 12-24
hours.

To set the stage, strong inversions have locked low clouds and fog
into most of our inland valleys. Based on latest (2 AM PST) RAWS
observations, the top of the inversion layer appears to be around
1500-2000 ft. Keep this inversion depth in mind, as it will be very
important to the potential for lowland snow today through Thursday.
The inversions are unlikely to break today as high clouds spread into
the forecast area from the southwest, leading to poor mixing. Many
locations in the Willamette Valley remained in the lower to mid 40s
Tuesday due to the strong inversions and persistent low cloudiness.
Fog and low clouds lingered for much of valley, expanding overnight.
Therefore it appears temperatures will struggle to climb much more
than a couple degrees from this morning's temperatures in the mid to
upper 30s. Areas of dense fog have increased over the past hour, so
we have issued a Dense Fog Advisory early this morning for the
Portland/Vancouver metro.

Synoptically, we have low pressure near 40N/130W, advecting mild,
moist air northeast toward SW Oregon. The deterministic 06z GFS, NAM,
and 00z ECMWF all keep the low near 1000 mb or slightly weaken it
while it approaches the Pac NW coast. This low appears to be right in
that "Goldilocks" spot where it will remain weak enough to prevent
strong warm advection, while remaining strong enough to enhance
frontogenesis, allowing significant QPF to impact much of SW
Washington and NW Oregon. Looking at Q-vector convergence in low-res
GFS, EC, and NAM data, there will be excellent forcing for synoptic
lift over nearly the entire CWA. Therefore the NBM mean QPF of around
1 inch for the inland valleys, 1-2" for the coast, and 1.5-3" for the
Coast Range and Cascades seems reasonable for totals through Thursday
night. NBM 90th percentile QPF runs from around 1.5" for the inland
valleys, 1.5-3" for the coast, and 2-5" of rain for the higher
terrain. These high QPF totals show the likelihood of sustained
moderate to locally heavy precip rates, which tend to gradually drag
down snow levels due to latent heat processes associated with melting
and evaporation. Areas where precipitation remains heaviest the
longest stand the best chance for snow levels to reach the suburban
hills or even valley floors - and at this point, that area appears to
be the greater Portland/Vancouver metro area where low-level
frontogenesis will remain the strongest for the longest.

Therefore it should not be a surprise that an increasing number of
models and ensembles, especially the high-resolution ones, are
beginning to show accumulating snow infringing on the Portland metro
and the Clark County Lowlands. 00z HREF means are showing anything
from a trace of snow for the inner metro to 2 inches I-205 eastward.
Ratcheting these totals up to HREF 90th percentile values now shows
1-2 inches of snow in Portland/Vancouver and 3-6" of snow east of
I-205. These snow accumulations assume a 10:1 snow-to-liquid ratio,
but the marginal temps and thus the wet nature of the snow should
yield ratios more in the 3:1 to 5:1 range. Therefore it is probably
safe to assume these models are actually depicting half the snow they
are showing. Regardless, this is a significant development, agreed
upon now by nearly all 00z ECMWF and GEFS members. Therefore, we will
be issuing a Winter Weather Advisory for the potential of up to 3
inches of snow in the Portland metro area tonight into Thursday.
Temperatures will be extremely marginal, if not too warm, for snow to
accumulate on roads. However, heavy precipitation rates can change
road temps quickly for similar reasons as for the atmosphere. The
most likely areas for accumulating lowland snow will be north and
east of I-205, elevations above 500 feet (including Portland's West
Hills), and perhaps far western Washington County along the east
slopes of the Coast Range and the Gales Creek/Banks area. That said,
any elevation could see accumulating snow where heavier precipitation
remains persistent in the Portland metro, along the Columbia River,
or in SW Washington. High-end (95th percentile) snowfall totals
approach 8 inches for portions of the Portland metro. All in all, the
setup with this system is very similar to the Jan 2017 surprise
Portland snowstorm, where decaying low pressure failed to advect
enough mild air to counter the lowering snow levels due to heavy
precip rates. That event led to 8 inches of snow at PDX, with over a
foot of snow in some portions of the metro area. With all this in
mind,

All this talk about snow should not mask the fact that there is still
a 40-50% chance elevations below 500 feet will not see any
accumulating snow. So don't be surprised if wet snow mixes in from
time to time but the p-type never fully changes over. While
confidence in all the details of this system remain remarkably low
for something that begins to affect us in Periods 1-2, confidence is
high in THIS IMPORTANT DETAIL: If precipitation fully changes from
rain to snow in your location, assume precipitation will remain snow
for the remainder of this event. This could happen as early as this
afternoon, but is more likely tonight into Thursday. If rain fully
changes to snow, it means the necessary work has been done by latent
heat processes to cool the column enough to bring snow down to your
elevation. Therefore, with the lack of significant warm advection,
the column should remain cold enough for precipitation to continue as
snow until it tapers off.

Whatever precipitation type prevails, it should start to diminish
Thursday night into Friday. However latest guidance is leading to a
shorter break in precipitation Friday. Stronger low pressure well
offshore will wobble closer to the coast while another cold high
pressure settles into the Columbia Basin. Easterly MSLP gradients
ramp up to around -9 mb KTTD-KDLS by Friday afternoon based on latest
guidance, which will support blustery east winds through the Columbia
Gorge and across the PDX metro. For Friday afternoon, NBM guidance
shows a 50-70% chance of east winds gusting 45-50 mph for the east
PDX metro. These easterly gradients will lock the cold air into the
Gorge and Hood River Valley so that it is available to some degree
when the next frontal system arrives over the weekend. Fortunately,
most models suggest this system will be torn apart by split flow as
its front moves onshore, with the heaviest moisture again aiming
toward California over the weekend and into early next week.

With the high degree of uncertainty in the forecast and the (albeit
low) potential of impactful snow as early as this afternoon/evening,
it is important for members of the public to keep a close eye on the
weather over the coming days. Make sure you have an emergency kit in
their car and be prepared for the possibility of travel impacts due
to snow. This is certainly one of those situations where you want to
hope for the best but prepare for the worst, as there is a 10-20%
chance of significant travel impacts for this evening's and/or
Thursday morning's commute.  Weagle

&&

.AVIATION...A pattern shift is underway as of 19z as a low
pressure system nears the coast and a warm front pushes
northeastward. Going to be a complex scenario though as easterly
winds will persist through the Columbia River Gorge impacting
K4S2, KTTD and KPDX, while southerly winds will begin in the
Willamette Valley south of KUAO. Precipitation is expected, heavy
at times after 06Z Thu but could be more of a mix of snow, rain,
and even some freezing rain in some locations. Blowing snow
through the Cascades, and heavy snowfall in the Columbia River
Gorge and around K4S2. For inland locations, if this precipitation
meets the cold air from the east, could see snow falling with
gusty winds around KPDX and KTTD. This would impact visibility due
to blowing falling snow, and accumulation on runways will be
delayed. If this pattern manifests, it will generally impact KPDX
and KTTD, but cannot rule our areas north of KUAO.

Note that the observations in Newport (KONP) are currently limited
to winds, temperature and pressure. Time of restoration is
unknown.

PDX AND APPROACHES...East winds will increase through the
afternoon with peak gusts after 06Z Thursday as the pressure
gradient increases. The challenge in the later half of the
forecast is going to be precipitation type. We will see rain and,
with the winds as strong as they are, will reduce VIS. However,
with the easterly winds bringing in cold air from the east rain
may transition to snow. Accumulation, if snow does begin to
accumulate, will reach around 1 inch or so. Confidence is low in
regards to all snow related impacts. -BKM/Muessle

&&

.MARINE...Winds and seas have settled around 10 to 15 kt and 5 ft
at 11 seconds. These conditions will begin to ramp up though over
the next several hours as a low pressure system drops down from
the northeast Pacific. This low will cause winds to ramp up over
the next several hours. Will see gusts up to 30 kt by Wednesday
afternoon. Winds will be more widespread southerly, but there will
be an easterly component through the coastal gaps. Seas will build
to around 12-14 ft at 14 seconds. As the low dissipates and
shifts inland winds and seas will ease. However, that will be
short-lived as another stronger front arrives on Friday. Seas
will once again build to near 15 ft and winds will increase as
well. Expecting winds to easily reach small craft advisory speeds,
and the outer waters (west of 30 NM) will likely reach gales.
Will continue to evaluate if they become more widespread. -Muessle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Weather Advisory until 4 PM PST Thursday for Coast Range
     of Northwest Oregon.

     Winter Weather Advisory from 8 PM this evening to 4 PM PST
     Thursday for Greater Portland Metro Area-Lower Columbia.

     Winter Weather Advisory until 10 PM PST Thursday for Northern
     Oregon Cascade Foothills.

     Winter Storm Warning until 10 PM PST Thursday for Cascades in
     Lane County-Central Columbia River Gorge-Northern Oregon
     Cascades-Upper Hood River Valley-Western Columbia River
     Gorge.

WA...Winter Weather Advisory until 4 PM PST Thursday for Willapa
     Hills.

     Winter Weather Advisory from 8 PM this evening to 4 PM PST
     Thursday for Greater Vancouver Area-I-5 Corridor in Cowlitz
     County.

     Winter Storm Warning until 10 PM PST Thursday for Central
     Columbia River Gorge-South Washington Cascade Foothills-
     South Washington Cascades-Western Columbia River Gorge.

PZ...Small Craft Advisory until 10 AM PST Thursday for Columbia River

     Bar.

     Small Craft Advisory until 4 PM PST this afternoon for Coastal
     waters from Cape Shoalwater WA to Florence OR out 60 NM.

     Hazardous Seas Warning from 4 PM this afternoon to 10 AM PST
     Thursday for Coastal waters from Cape Shoalwater WA to
     Florence OR out 60 NM.
&&

&&

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