High Wind in Oregon, January 5, 2024

Area Forecast Discussion UPDATED
National Weather Service Portland OR
818 PM PST Fri Jan 5 2024

EVENING UPDATE, for aviation weather

.SYNOPSIS...A potent cold-frontal system will bring valley rain
and ample mountain snow to the region on Saturday before a
quick transition to a cooler and showery post-frontal pattern
Sunday. Early to middle of next week we’ll see several weather
disturbances hit the Pacific Northwest in quick succession
starting with a warm-frontal boundary on Monday followed by a
pattern extremely favorable for snowfall Tuesday through
Thursday – probabilities for an additional 24 inches of snowfall
over the Cascades through this period are extremely
high(70-99%). Our attention then turns to the potential for the
coldest temperatures of the season and lowland snow late next
week into next weekend as an abnormally cold airmass begins
moving southward from Canada, however uncertainty continues to
remain high as ensemble models struggle to determine exactly
where the cold airmass will penetrate into the northern/western
CONUS.

&&

.TONIGHT THROUGH SUNDAY NIGHT...After a brief lull headed
through the evening hours precipitation increases drastically
Friday night into Saturday morning as a robust upper-level
trough and associated cold front sweeps through western
Washington and Oregon. Deterministic guidance continues to
depict strong upper- level forcing with this system, and as such
it isn’t too much of a surprise to see guidance has increased
QPF and thus snowfall totals since the previous forecast.
Snowfall amounts for all but the immediate crest-line are still
within the bounds of high-end advisory criteria through this
event so the Winter Weather Advisory for the Cascades and Oregon
Cascade foothills has been left in place. It’ll be interesting
to see how snow levels react in the Cascade valleys/Canyons when
the frontal passage moves overhead during the morning hours
Saturday morning while precipitation rates are maximized. The
HREF has a 70-100 percent chance for 1inch or higher snowfall
rates during the morning period(roughly 4-10am) over the higher
terrain(2000ft+) and high resolution model soundings still
suggest the potential for isothermal snowfall driving snow
levels lower than otherwise would occur, especially as
deterministic guidance like the GFS and NAM depict a weak
offshore component to the surface winds as the front arrives.
With this in mind, snow levels were adjusted slightly to account
for this scenario - it wouldn’t be a shock to see a rain/snow
mix drop close to 1000-1500ft for a few hours but confidence in
this occurring is low.

Late morning into the afternoon we’ll switch to an unsettled
post-frontal weather pattern with shower activity persisting
across the region, most frequent over terrain features like the
coast range, foothills, and Cascades. Exploring high resolution
model soundings and deterministic MUCAPE vales suggest the
potential for weak embedded T-storm activity, peaking during the
afternoon hours when the cold-pool aloft(-35c at 500mb) shifts
overhead and daytime heating is maximized. These chances(15-24%)
for electrified convection persist during the overnight hours
near the coast. We’ll continue to see widespread shower activity
Sunday as well however the axis of the trough and associated
cold pool shifting well to the east ends any additional chances
T-storms. Still, impacts through Sunday and Sunday evening will
stay focused on mountain snowfall. If you are planning to travel
over the Cascades this weekend be prepared for winter driving
conditions. -Schuldt

.LONG TERM...MONDAY THROUGH THURSDAY NIGHT...Our next weather
disturbance in the form of a warm-frontal boundary arrives
Monday morning as the upper level flow regime becomes westerly
to west-northwesterly. This system brings another round of
widespread valley rain and high mountain Cascade snow, however
as alluded to snow amounts in the Cascades look somewhat limited
on on Monday with only a few inches at pass level due to
rapidly rising snow levels. Expect these to shoot up from around
2000 feet early Monday morning to above 4000-4500 feet Monday
afternoon. While this system will impact travel conditions over
the higher Cascades passes, significant impacts are unlikely.

That being said, much heavier and more impactful snowfall amounts are
likely to occur Tuesday into Wednesday as a trailing moisture-rich
cold front sends snow levels back down to around 1500-2500 feet and
brings 1-3.5" of QPF to the Cascades according to most ensemble
guidance (from Tuesday morning through Wednesday afternoon). An
onshore flow regime will likely keep snow ratios around 10:1 near
pass level, which would translate to roughly 12-35 inches of
snow. The NBM mean supports this well, showing 10-30 inches of
snow in the Cascades/foothills above 2000-2500 feet from Tuesday
morning through Wednesday afternoon (highest around and above
pass level). The NBM 90th percentile shows amounts around 35-50
inches, which appears to be a reasonable worst case scenario
given the moisture rich flow WNW to NW flow and low snow levels.
Regardless of the exact snow amounts, confidence continues to
grow that this system will bring the heaviest snow amounts in
the Cascades thus far this season and significant impacts to
those in the Cascades. It's also worth mentioning that the NBM
mean still shows around 1-4 inches of snow for elevations above
1500-2000 feet in the Coast Range, potentially impacting travel
conditions along portions of US26 and ORE6.

The weather forecast becomes much more uncertain from Wednesday
night onward. This is when our attention turns to the potential
for the coldest temperatures of the season so far beginning
late next week into next weekend (Jan 11-14) as an abnormally
cold arctic airmass begins moving southward from Canada into
portions of the northern CONUS. However, "potential" is the key
word here as the forecast remains highly uncertain as models
struggle to determine exactly where the coldest air will
infiltrate. In fact, some grand ensemble members are showing
near normal to above normal temperatures late next week with 850
mb temperatures rising above freezing. On the flip side another
subset of ensemble members suggest surface temperatures in the
teens or even single digits across the lowlands (and of course,
a threat for lowland snow too). However, don't be fooled by
singular extreme deterministic model runs as several recently
like yesterdays 00z EURO have been outliers within the larger
ensemble space. To illustrate the degree of uncertainty even
more, the NBM 10th to 90th percentile for low temperatures
Wednesday night ranges from the mid teens to mid 40s, single
digits to mid 40s Thursday night, and single digits to upper 40s
Friday night. You cannot get much more model spread than this –
almost 50 degrees between solutions! Still it’s worth noting
the ensemble means overall have trended slightly towards the
colder solution today. For now, it's best to monitor the
forecast over the next week to see if conditions trend towards
the warmer model solutions or colder model solutions. From
there, details regarding the potential for lowland snow will
hopefully become more clear although probabilities for at least
an inch of snow in Portland have been trending higher since
yesterday – stay tuned!
-Schuldt/TK

&&

.AVIATION...at 8 pm, it was still dry with VFR inland. But, rain is
increasing along the coast, with CIGS dropping to MVFR. This all due
to strong cold front well offshore that is fast approaching. Rain
with mix of MVFR and IFR along the coast and into the coastal mtns
through 10Z or 12Z Sat. Rain and lowering CIGS will spread across
the interior between 10pm and midnight.

Cold front will push onshore around 09Z to 10z, then race inland to
the Cascades by 15Z Sat. With that, will see flip of winds from
southerly to more west-northwesterly. CIGS will improve a bit, as
rain transitions to showers. Overall, mix of VFR and MVFR on
Saturday, with lowest conditions in/around showers. Will maintain
isolated thunderstorms along the coast later tonight through Sat,
with a smaller chance of such farther inland. With snow levels
dropping behind the front, will see snow above 2500 ft. Any heavier
showers will produce small hail for lower elevations.

PDX AND APPROACHES...VFR for few more hours this evening. But, CIGS
will gradually lower to MVFR around or just after midnight as rain
increases. Cold front pushes across OPs area around 12Z, with a
switch from south winds to westerly winds. CIGS will improve a
behind the front, with mix of VFR and occasional MVFR with showers
(some with hail) after 8 am Sat.                    /Rockey

&&

.MARINE...A very active weather pattern is expected through the
next week as a series of frontal systems move through the waters
as a low moves towards Vancouver Island this weekend. The first
front is expected to cross our waters tonight through Saturday.
This will be a fairly fast moving front, but will cause seas and
winds to increase. Will see widespread winds of 20-25 kt with
gusts to 30 kt. There is a 60% chance of seeing gusts up to 40 kt
through the inner waters from late Friday to Saturday early
morning. These winds could be observed from 10-20 NM as well, but
less probable and they will be less widespread. Seas will continue
to be hazardous with heights of 17-19 ft at 18 seconds. There may
be a few observations of seas touching 20 ft, but not expected to
be persistent. Thunderstorm chances range from 20-30% through
Saturday both on the leading edge and behind the front. Small hail
and gusty outflow winds would accompany any storms.

As the front exits the area, the winds will ease and seas will
slowly lower. Seas will reach around 10 ft by Sunday evening.
However, this break will be short lived as yet another, more
robust series of fronts will advance over the region through the
week. Tuesday looks to be a "high sea day" with seas around 20-25
ft in the outer waters. The reaction of the seas will greatly
depend on the incoming low pressure system. 			-Muessle

&&

.BEACH HAZARDS...A series of frontal systems will bring a robust
westerly swell that will impact the OR/WA waters the next week.
Seas build towards 18 to 20 ft at 16 to 18 seconds, resulting in
high surf conditions along the coast. Breakers as high as 20 ft
are expected, and with these high energy waves, sneaker waves
remain a threat. While the High surf conditions themselves will
last only a few hours, the sneaker wave threat is a bit more
prolonged. Conditions will ease late Saturday through Sunday,
though they will become even more active next week. Will add that
the region is entering the period of Perigean tides.
														-Muessle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...High Surf Advisory until 10 PM PST Saturday for Central Oregon
     Coast-North Oregon Coast.

     Winter Weather Advisory from 10 PM this evening to 4 AM PST
     Sunday for Cascade Foothills in Lane County-Cascades in
     Lane County-Northern Oregon Cascade Foothills-Northern
     Oregon Cascades.

WA...High Surf Advisory until 10 PM PST Saturday for South Washington
     Coast.

     Winter Weather Advisory from 10 PM this evening to 4 AM PST
     Sunday for South Washington Cascades.

PZ...Hazardous Seas Warning until 10 PM PST Saturday for Coastal
     waters from Cape Shoalwater WA to Florence OR out 10
     NM-Columbia River Bar-Coastal waters from Cape Shoalwater
 WA to Florence OR from 10 to 60 NM.&&

www.weather.gov/portland

Interact with us via social media:
www.facebook.com/NWSPortland
www.twitter.com/NWSPortland