NWS Portland, OR wrote its last area forecast discussion before the first event at Jan 31, 9:48 PM PST, 2 hours 12 minutes earlier.
Area forecast discussion, Jan 31, 9:48 PM PST, 2 hours 12 minutes before
Area Forecast Discussion
National Weather Service Portland OR
948 PM PST Fri Jan 31 2025
.UPDATED AVIATION DISCUSSION...
&&
.SYNOPSIS...Rain and high-elevation mountain snow will continue
this afternoon as a cold front moves through the region. Snow
levels falls tonight behind the front, bringing more snow
showers to lower elevations. Rain and snow showers continue
tomorrow, with significant accumulations in the mountains which
will lead to hazardous travel conditions. Beginning tomorrow
night, the lowlands will likely see a rain/snow mix, with little
to no accumulation expected. This trend continues into next
week, with much colder temperatures.
&&
.SHORT TERM...Now through Saturday night...Radar and satellite
imagery as of 3 PM PST Friday depicts widespread rain and high-
elevation Cascade snow falling across northwest Oregon and
southwest Washington as a surface cold front pushes through the
region. This cold front is associated with a deep ~518-520 mb
trough centered over northwest British Columbia.
As the aforementioned front moves through, expect breezy
southerly to southwesterly winds with gusts to 25-30 mph for the
coast and interior lowland valleys, and 35-45 mph for higher
terrain. Snow levels are currently 5000-6000 ft but are forecast
to quickly drop behind the cold front. We'll see rain showers
transition to snow showers at lower elevations tonight as snow
levels fall to 1000-2000 feet across northwest Oregon and
500-1000 feet across southwest Washington. The I-5 corridor
north of Vancouver, WA through Cowlitz County may see a
rain/snow mix tonight; however, no significant travel impacts
are expected in the lowlands as we'll see little to no snow
accumulation.
Winter Storm Warnings remain in effect across the Cascades,
north Oregon Coast Range, and Willapa Hills through early Sunday
morning. In addition, a Winter Weather Advisory has been issued
for the Cascade foothills north of Lane County and the Columbia
River Gorge above 500 ft, also through Sunday morning. Please
refer to the posted headlines on our website to view expected
snow amounts within these warnings and advisories. If you plan
to travel through the north Oregon Coast Range, Willapa Hills,
or Cascades, prepare for winter weather conditions and refer to
ODOT/WSDOT for the latest road conditions and restrictions.
Post-frontal rain showers and mountain snow showers will
continue through the day tomorrow as the upper level trough over
British Columbia gradually treks southward with its axis
centered just offshore of Washington and Oregon. This cold
trough will be setting the stage for the upcoming wintry
weather. Saturday night will be the first of several nights
where interior lowlands (below 500 feet) will experience at
least some snow flakes in the air, especially the further north
you go. Low temperatures Saturday night into Sunday morning are
forecast in the upper 20s to low 30s. Since we'll be flirting
around 32 degrees across the Willamette Valley and southwest WA
lowlands, a rain/snow mix is the most likely scenario. However,
slight increase or decrease in temperatures can lead to an all-
snow or all-rain scenario. In addition, the showery nature of
the precipitation will lead to varying rain and snow amounts
across the area. Those that experience heavier showers could
end up with more snow than others. In general, still expecting
0.5" to zero accumulation in the lowlands Saturday night
(highest amounts further north toward the Portland-Vancouver
Metro and southwest WA). If there is snow accumulation in the
lowlands, it would likely be over grassy areas or elevated
surfaces and not lead to significant disruptions to travel.
-Alviz
.LONG TERM...Sunday through Thursday...The long term forecast
remains highlighted by a much colder and showery weather pattern
with low snow levels, likely down to the valley floor. Despite
the lower snow levels, there are currently no signs of a
significant snowfall event for the lowlands below 500 feet
elevation. This is due to the onshore flow regime in place with
southerly to southwesterly surface winds bringing relatively
milder temperatures. This is not like January 2024 where strong,
dry, offshore/easterly winds brought freezing Arctic air into
the area. Onshore flow always makes it harder for lowland snow
to form, however, the upper level trough dipping down into the
area is bringing air that's just cold enough to support lowland
snow. 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 a rain/snow mix.
That being said, expect minimal impacts and accumulation of
snow for elevations below 500 feet (generally only a dusting to
one inch each night, with accumulation generally limited to
grassy areas and elevated surfaces). It is also worth
mentioning that any snow that does accumulate on any given day
below 500 feet will occur during the overnight/morning hours,
with melting of snow occurring in the afternoon/evening hours as
we warm up. However, with temperatures falling to near or below
freezing each night, re-freezing of wet surfaces will may lead
to slippery surfaces.
For elevations above 500-1000 ft, snow will be able to
accumulate much more readily, resulting in more significant
travel impacts. For example, 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 still 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. High temps mid-week in the lowlands
are forecast to struggle to get above 40 degrees, while lows
begin to fall into the mid to upper 20s.
Some have been wondering if there will also be a threat of
freezing rain and/or sleet. In general, no (less than 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). Welcome to winter! -Alviz/TK
&&
.AVIATION...The region has fully transitioned to a showery post-
frontal regime late this evening with most terminals experiencing
a mix of MVFR to VFR conditions. Going forward, waves of shower
activity will persist through the rest of the TAF period with
guidance indicating a somewhat noticeable increase in activity for
inland sites after 14-16z Saturday into the afternoon. CIGs will
continue to bounce between VFR and MVFR as well - expect increased
potential for the latter category under and around these
aforementioned showers. Winds generally remain around 5 to 15
kts with gusts closer to 20-25 knots possible along the coast.
PDX AND APPROACHES...Showery conditions will continue through the
period although this activity should stay fairly infrequent before
15-16z after which point we'll see showers generally increase.
Guidance indicates VFR conditions will prevail most hours through
the TAF period, but underneath showers we'll likely see brief
periods of MVFR CIGs/VIS. Low confidence in the timing of any
deterioration however. -Schuldt
&&
.MARINE...Small Craft Gusts to around 30 kt expected to
continue across the waters at least into the early part of the
overnight. Gusts at buoy 50 continue to oscillate between low
end Gales to high end Small craft, generally as high as 35 kt.
Seas are running in the 10-14 ft range per available
observations, dominated by wind-driven, short period waves out
of the south. While winds are trending downward, a few low end
Gale gusts are possible. Winds will quickly shift around to the
west in the post frontal environment this afternoon, with short
period seas trending down relatively quickly as a result. Could
therefore see a bit of a lull in sea states this afternoon as
short period seas subside and mid period westerly swell remains
rather modest in the 4-6 ft range before building to around 8-10
ft Friday night in to Saturday. Broad area of low pressure near
Vancouver Island will maintain blustery conditions through the
weekend as cold air advection results in gusty west-northwest
winds and the building swell pushes seas back near 10 ft.
-Batz/CB
&&
.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning until 4 AM PST Sunday for ORZ106.
Winter Weather Advisory from 4 AM Saturday to 4 AM PST Sunday
for ORZ119-123-124.
Winter Storm Warning until 4 AM PST Sunday for ORZ126>128.
WA...Winter Storm Warning until 4 AM PST Sunday for WAZ203.
Winter Weather Advisory from 4 AM Saturday to 4 AM PST Sunday
for WAZ208.
Winter Storm Warning until 4 AM PST Sunday for WAZ211.
PZ...Small Craft Advisory until 4 PM PST Saturday for PZZ210-251-252-
271-272.
Small Craft Advisory until 10 AM PST Saturday for PZZ253-273.
&&
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