Area Forecast Discussion
Issued by NWS Portland, OR

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422
FXUS66 KPQR 050544 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
944 PM PST Tue Feb 4 2025

.SYNOPSIS...Stronger push of moisture tonight into Wednesday,
combined with colder overnight temperatures, may create slushy
lowland accumulations and defined snow above 500 feet.
Afterwards overnight temperatures will remain colder through the
end of the week, around 25-30 degrees in the valleys and along
the coast. Further precipitation becomes lighter through the
rest of the week so impactful accumulation is not expected.

&&

.SHORT TERM...Now through Wednesday night...Widespread
snowfall, becoming impactful in some areas, expected Wednesday
morning as a defined front pushes through the region on Tuesday
evening. Showers are beginning to move inland with most of the
precipitation falling along the coast and Coast Range. Snow is
already beginning to fall in Tillamook and in Newport, OR. The
coast will see a 60-90% chance of snow accumulations over 1 inch
Tuesday night and Wednesday morning, with a 40-70% chance of 2
inches or more. Temperatures are slightly warmer than the
previous night, so we may not necessarily see an exact rinse and
repeat. However, snow is still likely, and will accumulate on
roadways. The Coast Range sees much higher snow accumulation due
to orographic lift, with 85%+ chance of 4 inches or greater.
Given the cold air pool and the mass of cold air aloft mixing
down, could see anywhere from 4-10 inches of new snow which has
sparked a Winter storm Warning for the Coast Range and Willapa
Hills through Tue to 7pm Wed.

For lowland and inland locations (Willamette Valley, SW Washington
lowlands), it becomes much more complicated. The onshore
southwesterly flow could produce lower accumulations due to
precipitation shadowing, or higher amounts due to warmer and
moister air moving over colder air at the surface.
Precipitation will begin as rain tonight, but temperatures will
continue to drop behind the front from earlier today. A weak
easterly flow is bringing in lower temperatures a the surface
which should become cold enough for snow to mix in from
beginning around 12-3 AM Wednesday. For these lowland locations
under 500 feet (excluding the coast), expecting around an inch
of slushy snow. Most of the Willamette Valley (including
Portland) sees around a 40-60% chance of accumulations over 1
inch between 10pm Tuesday and 10pm Wednesday. Slightly better
accumulations at the West Hills and other areas of higher
terrain, up to around 2 inches possible there. Slushy
accumulations are expected to impact Wednesday morning`s AM
commute, and care should be taken while traveling, particularly
in hilly or elevated terrain.

A Winter Weather Advisory has been issued from midnight to noon
Wednesday for lowland areas as a result. The southern
Willamette Valley sees the highest uncertainty, as breezy
southerly flow beginning Wednesday morning could bring warmer
air into those areas. Depending on how strong this southerly
flow will be, we could end up with much less or even no
accumulations in the southern Willamette Valley if it warms
quickly enough. Otherwise, accumulations will be similar to the
rest of the Willamette Valley and Columbia lowlands. Overall for
the lowlands, there is low confidence in the exact locations of
snowfall, and exact accumulations. There is high confidence
that folks will see some kind of snow - whether it is slushy or
sticks - with locally higher amounts under heavier showers.
Confidence is also high that temperatures will warm enough
during the day that snow will melt.

Precipitation reaches the Cascades last, but orographic lift and
colder temperatures allow for much stronger snowfall above 2500
feet or so. Current conditions show somewhere between 6-12
inches at the Cascades, including all mountain passes. As such,
a Winter Weather Advisory has been issued for the south
Washington and north Oregon Cascades, from midnight tonight
through Wednesday night. Of note, currently watching potential
for higher snowfall as model runs are slightly increasing QPF
each run. Currently looking at 20-40% chance of the Cascades
seeing snowfall about 12 inches, which would reflect warning-
level snow. Will carefully be observing following model runs as
we get closer to snow beginning. -Muessle/Liu



.LONG TERM...Thursday through Monday...Model runs continue to
show moisture on Thursday and beyond mostly shifting further
south, reducing potential for impactful snow. Only around a
10-20% chance in the Willamette Valley and Columbia lowlands for
accumulating snow between Wed 10pm to Thu 10pm. Each night
afterwards shows some minor potential for snow to mix in, but
precipitation remains too weak for any real impactful
accumulations anywhere but the mountains. Temperatures continue
to remain quite cold, in the upper 20s to around 30 in the
lowlands from Wednesday night through early next week. Thursday
night looks like the coldest night during this period, with NBM
showing a 40-50% chance of low temperatures Friday morning below
25 degrees in the Willamette Valley. Good model consensus that
cold temperatures remain with us through next week as well, with
current guidance indicating lows remain in the upper 20s early
next week. Still relatively low confidence in moisture next
week; snow remains a possibility for all areas next week, with
very high uncertainty in precipitation. -JLiu


&&

.AVIATION...A broad area of low pressure offshore maintains cold
and showery weather with southwest flow and generally VFR
conditions. Showers will increase through 09Z Wed starting out
predominately as rain, then slowly transitioning into periods of
-SN/-SNRA. Showers will result in increasing probabilities of
 MVFR/IFR conditions, along with a 15%-25% probability for
 intermittent LIFR conditions. Visibilities may also decrease
 with more intense snow showers through 18Z-20Z Wed. Widespread
 accumulations of 1-2 inches expected, with up to 3 inches in
 elevated terminals like KMMV, and K4S2. Due to the showery
 nature of the precipitation there could be a wide discrepancy
 of snow accumulations across the lowlands including chances for
 precipitation to stay as just rain.

Around 18Z-20Z Wednesday, expect predominately VFR and MVFR
conditions through the remainder of the TAF period. Snow
showers will also transition back to rain showers around the
same time. Southerly winds generally below 10 kt with light east
winds through the Gorge. Coastal locations could see southerly
gusts up to 25 kt starting around 20Z Wednesday.

PDX AND APPROACHES...Mainly VFR with occasional drops to MVFR in
passing showers. Showers increase in coverage after 06Z-08Z Wed,
with chances for MVFR conditions increasing to 60%-70% probability
along with a 15%-25% probability for intermittent IFR/LIFR
conditions. While probability for accumulating snow is low,
there is around a 30-40% chance of up to 1 inch of snow over the
next 24 hours. Snow that does fall will be slushy, and will
struggle to accumulate. Southeast winds expected around 4-7 kt.
/42

&&

.MARINE...A broad area of surface low pressure lingers offshore
of the Pacific Northwest today. Light to moderate southerly
breezes continue through tonight across the coastal waters,
while the sea state remains dominated by a mid period northwest
swell which has decayed to around 4 to 5 ft. Southwest winds are
expected to increase on Wednesday as a relatively weak but
compact low moves closer to the coast. Winds could gust as high
as 25-30 kt across the central Oregon coastal waters. This will
also likely bring an uptick in short period seas which could
briefly push combined seas into the 8 to 10 ft range. Therefore,
have issued a Small Craft Advisory through Wednesday evening.

Benign conditions follow on Thursday with seas subsiding again to
around 4 to 5 ft through the end of the week. The flow is expected
to turn more northerly by Friday as low pressure moves toward
central California. /DH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until noon PST Wednesday for
     ORZ101>105.
     Winter Storm Warning until 7 PM PST Wednesday for ORZ106-107.
     Winter Weather Advisory until noon PST Wednesday for
     ORZ108>112-114>118.
     Winter Weather Advisory until 4 AM PST Thursday for ORZ113-119-
     123>128.
     Winter Weather Advisory until 1 AM PST Thursday for ORZ120>122.
WA...Winter Weather Advisory until noon PST Wednesday for WAZ201-
     202.
     Winter Storm Warning until 7 PM PST Wednesday for WAZ203.
     Winter Weather Advisory until noon PST Wednesday for
     WAZ204>207.
     Winter Weather Advisory until 4 AM PST Thursday for WAZ208-211.
     Winter Weather Advisory until 1 AM PST Thursday for WAZ209-210.
PZ...Small Craft Advisory from 4 AM to 10 PM PST Wednesday for
     PZZ252-253-272-273.
&&

$$

www.weather.gov/portland

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