340
FXUS66 KPQR 310554
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
954 PM PST Thu Jan 30 2025

.UPDATED AVIATION DISCUSSION...

&&

.SYNOPSIS...Cloudy skies have overtaken the forecast area begin
as a cut off low moves across the region through the weekend.
This low pressure system will bring widespread rain and mountain
snow starting tonight. This low will press eastward through the
weekend and be followed by and Alaskan low. These systems will
maintain a cold and wet pattern through the middle of next week.
Expect rain for the lowlands and snow above 2500 ft through at
least Saturday night. Colder air will enter into the region
towards the start of next week and bring more snow to the
mountains as well as a rain/snow mix or possibly all snow for
the lowlands.

&&

.SHORT TERM...Tonight through Saturday night...The pattern
change is here. The pattern change is here. Cool, moist, onshore
flow has returned to the Pacific NW as the Omega Blocking
pattern continues to degrade. Looking at current IR and Water
Vapor Satellite imagery, we can see very dry air over the Great
Basin being pushed southeastward by a large cloud shield over
the region. Zooming further out, the cutoff, Alaskan low which
will be the primary driver of our weather through the Saturday
night is currently centered around 40N/147W is expected to push
eastward and will bring colder temperatures and precipitation.
As this system approaches the Cascades, snow levels will start
around 4000-5000 ft and slowly lower towards 3000-3500 by Friday
night. Also, heavier precipitation is expected to start late
Friday morning with a slow decrease in overall intensity through
Saturday morning. Heavy snow possible above 3000 feet, with
total snow accumulations between 6 and 12 inches, and up to 15
inches above 5000 ft. In addition to the snow in the Cascades
there is high confidence that 2 to 4 inches of accumulation will
be possible for the North Oregon Coast Range and the Willapa
Hills in Washington starting Friday evening. As a result, have
upgraded the Cascades to a Winter Storm Warning and issued a
Winter Storm Watch for the North Oregon Coast Range and the
Willapa Hills. Currently there are no signs of a high impact
snowstorm for the Cascades or Coast Range. While the Cascades
and Coast range will see snow, the lowland will deal with
persistent rain. Storm total precipitation for the Willamette
Valley look to range from 0.75 to 1.50 inches with 1.30 to 2.25
inches along the coast.

This pattern change will also bring cold temperatures to the
region and while the coldest temperatures are not expected to
arrive until the late Saturday night/Sunday morning. Expect
daytime highs to remain in the low 40s to low 50s with overnight
lows in the mid 20s to mid 30s. However, lows on Saturday night
will be sub-freezing for the forecast area with locations above
2000 ft falling into the low 20s. More info on what can be
expected through the middle of next week can be found in the
Long Term discussion below. /42

.LONG TERM...Sunday through Wednesday night...The long term forecast
is highlighted by a cool and showery weather pattern with low snow
levels, likely down to the valley floor or very near the valley
floor at times (500 ft or less, except 500-1000 ft in the southern
Willamette Valley and for the central Oregon coast). Despite the
lower snow levels, there are currently no signs of a significant
snowfall event for elevations below 500 ft, mainly due to the
onshore flow regime in place with southerly surface winds, marginal
temperatures, and the light/showery nature of the precipitation. The
grand ensemble mean for 850 mb temps ranges between -7 and -9
degrees Celsius, which is generally cold enough to support wet snow
down to the valley floor during a low-level onshore flow regime, or
at least snow mixed with rain. That being said, there is still
uncertainty regarding exact temps both aloft and at the surface,
albeit model spread is now a bit less compared to yesterday. The
LREF 10th-90th percentile for 850 mb temps ranges between -10 and -6
degrees Celsius on Sunday, -11 to -7 degrees on Monday, and -11 to
-5 on Tuesday.  In other words, around 10% of the ensemble space
suggests no snow at all down to the valley floor, however the vast
majority of ensemble guidance is cold enough to support at least
some wet snow down the valley floor. That being said, expect minimal
impacts and accumulation of snow for elevations below 500 ft
(generally only a dusting to one inch each night, with accumulation
generally limited to elevated surfaces and grassy surfaces). It is
also worth mentioning that any snow that does accumulate on any
given day below 500 ft will occur during the overnight/morning
hours, with melting of snow occurring in the afternoon/evening
hours. This will further limit overall travel impacts for elevations
below 500 ft. For elevations above 500-1000 ft, snow will be able to
accumulate much more readily, resulting in more significant travel
impacts. The West Hills in Portland are more likely to see 1-2
inches of snow (worst case scenario would be 3-4 inches; however the
chance of this occurring is only at 10%). In addition, anyone
traveling over the Cascade passes or the coastal mountains will need
to be prepared for winter driving conditions Sunday through Monday.
Mountain snow showers become lighter and less frequent Tuesday into
Wednesday, thus limiting travel impacts.

For now, the deterministic NBM seems to reflect the most likely
outcome well, with snow levels dipping to 100-300 feet Sunday
morning over southwest WA, the greater Portland/Vancouver metro,
Columbia River Gorge and Upper Hood River Valley. Deterministic snow
levels are nearly the same Monday morning and Tuesday morning,
albeit down to the lowest elevations of the valley floor. Although
snow accumulations look minimal below 500 ft, elevations above 500
ft in the West Hills are more likely to see 1-2 inches of snow
(worst case scenario would be 3-4 inches for the West Hills; however
the chance of this occurring is only at 10%).

Some have been wondering if there will also be a threat of freezing
rain and/or sleet. In general, no (<5% chance of these precipitation
types occurring). The overall pattern is not conducive for these
precipitation types, and forecast model soundings back this up by
suggesting precipitation will fall as snow, rain, or a rain/snow mix
(depending on elevation, as described above). -TK

&&

.AVIATION...Current observations show a mix of MVFR to VFR
CIGs/VIS this evening as a potent frontal system begins to spread
our first rainfall in quiet some time across the region. Expect
sub-VFR conditions to becoming fairly widespread later tonight
into Friday morning as rain intensifies with a high probability
(>70-80%) for IFR CIG/VIS at the coastal terminals and potentially
at KHIO as well. Winds likely increase out of the south at most
terminals on Friday, especially along the coast and central
Willamette Valley, with gusts of 20-30 knots possible through much
of the day. It`s also worth noting light easterly flow out of the
Columbia Gorge in combination with increasing S-SW winds just
above the surface will create some locally marginal LLWS concerns
roughly 12-22z Friday at KTTD. The more stratiform rainfall then
breaks to showers Friday late afternoon/evening with light
showers and primarily MVFR CIGS persisting into the end of the TAF
period and beyond.

.PDX AND APPROACHES...Conditions have improved back to VFR late
this evening (as of 05-06z) however confidence is high as rainfall
increase the rest of the night CIGs will eventually lower back to
MVFR around 10-12z Friday - MVFR CIGs then prevail the remainder
of the period. Light southerly winds will strengthen as well,
peaking after 20z with infrequent gusts up to ~25 knots
anticipated. -Schuldt
&&

.MARINE...A strong low pressure system centered around 40N and
147W is approaching the PacNW coast, sending a cold front toward
WA and OR. South winds will continue to increase through the
afternoon and into the overnight hours. Strongest winds are
expected overnight through early Friday morning, especially near
the central Oregon coast where this is high confidence for
winds gusting up to 40 kt. Will maintain the Gale Warning
through Friday morning, though a few models show gale force wind
gusts possibly persisting into Friday afternoon as the low
pressure center moves over the waters.

Seas beginning to slowly increase to around 5-6 ft as southerly
winds begin to pick up ahead of the cold front. Predominantly
wind driven seas continue overnight as southerly sustained winds
increase to 25-30 kt. Steep and choppy seas expected to around
10 to 14 ft through Friday.

Low pressure near Vancouver Island will maintain westerly onshore
flow Friday night through the weekend, along with a slight chance
for Thunderstorms Saturday morning and afternoon. A
northwesterly swell moves into the coastal waters Friday night,
maintaining elevated seas through Saturday. Seas expected to
gradually fall below 10 feet by late Sunday. -Batz/DH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Watch from Friday evening through Saturday
     afternoon for ORZ106.
     Winter Storm Warning from 4 AM Friday to 4 PM PST Saturday for
     ORZ126>128.
WA...Winter Storm Watch from Friday evening through Saturday
     afternoon for WAZ203.
     Winter Storm Warning from 4 AM Friday to 4 PM PST Saturday for
     WAZ211.
PZ...Small Craft Advisory until 10 PM PST this evening for PZZ210.
     Gale Warning until 10 AM PST Friday for PZZ210.
     Gale Warning until 10 AM PST Friday for PZZ251>253-271>273.
&&

$$

www.weather.gov/portland

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