494
FXUS66 KPQR 101818
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1018 AM PST Mon Feb 10 2025

.UPDATE...The forecast remains largely on track this morning as
we begin our transition to a much colder airmass over the coming
days. A heavily tilted upper-level shortwave trough is expected
to race southward from Canada into the region this afternoon/evening,
and while it won`t produce too many impacts latest model
guidance, particularly the HREF, is favoring the initiation of
isolated snow showers over the Cascades, northern Oregon Coast
range, and Willapa hills. Several HREF members show this
activity, steered by NE winds aloft and driven by a brief period
of instability as daytime heating wanes, drifting into the I-5
corridor by Longview/Kelso this evening before ending overnight.
PoPs were adjusted to better reflect this scenario although
little to no accumulation is anticipated. Otherwise, prepare
for cold overnight temperatures the next few days and increasing
chances for wintry precipitation starting on Thursday. The
previous forecast remains below. -Schuldt


&&

.SYNOPSIS...Overall mostly dry conditions are expected the next
few days as temperatures take a noticeable downtrend going into
the middle of the week. So, through Wednesday the vast majority
of our concerns revolve around the cold overnight conditions
exacerbated by locally gusty east winds through the Columbia
Gorge; temperatures 10 to 15 degrees below normal. This will
eventually set the stage for a the arrival of large-scale winter
weather event late Wednesday/Thursday through the end of the
work week bringing a smorgasbord of snow, sleet, and freezing
rain to the region. Confidence in the broad forecast details
during this time period continues to grow but finer points still
need to be worked out in the coming days as models come into
better agreement.

&&

.SHORT TERM...Now through Wednesday...While a few ongoing
stray showers may continue between now and Wednesday (mostly in
the Cascades), conditions should generally remain fairly dry
during this period. Any showers that do occur will not produce
any impactful accumulations. In the Willamette Valley, some
patchy fog may develop at times before around 10am Mon, but this
is not expected to be particularly dense or widespread.
However, this could reduce visibility locally, and combining
with below freezing temperatures, could locally degrade travel
conditions headed into sunrise hours - something to stay aware
of for the Monday morning commute.

Easterly flow through the Columbia River Gorge begins to increase
from Monday evening onwards. This increasing offshore flow will bring
increasingly colder overnight temperatures through at least Thursday
or Friday. On Monday night, temperatures throughout the lowlands see
a 60-90% chance of low temperatures below 25 degrees, particularly in
areas outside of major cities such as rural routes. The Cold Weather
Advisory that has been issued for Monday night remains on track.
Tuesday night looks to be colder, with near universal ensemble
agreement in temperatures throughout the CWA below 25 degrees.
Overnight temperatures likely remain below freezing through at least
the workweek. Further recurring Cold Weather Advisories can be
expected through at least Wednesday night.

With offshore flow increasing, strong easterly gusts can be expected
through the Columbia River Gorge from Monday night onwards, with a
TTD-DLS pressure gradient of -6 to -8 mb. Wind gusts peak on
Wednesday and Thursday, with gusts up to 50 mph possible there,
potentially up to 60 mph at exposed areas. Winds at the moment look
to begin decreasing on Friday, but there is some measure of
uncertainty that winds could remain easterly through the weekend.
With low overnight temperatures and gusty winds, there will be
enhanced risk for hypothermia and frostbite for those without
adequate shelter and winter clothing in and around the influence of
the Columbia River Gorge. /JLiu

.LONG TERM...Wednesday night through Sunday...Confidence continues to
increase with regards to a winter precipitation event beginning near
the end of Wednesday night into Thursday morning. A warm frontal
system pushes in at that time; combined with surface easterly flow
keeping a cold airmass at the surface, conditions are right for not
only snow, but freezing rain and sleet at times throughout Thursday
and Friday. GEFS, CMC, and EPS continue to get show more agreement,
with the low system likely pushing into the central Oregon coast,
increasing confidence in a winter precipitation event due to
generally northeasterly offshore flow. That said, QPF values have
come down slightly, with around 0.15-0.30 inches in the Willamette
Valley, closer to 0.50-0.75 inches in the Coast Range and Cascades.
The system sweeps from south to north, with better confidence in
timing now; precipitation begins near the end of Wednesday night in
the southern Willamette Valley, reaching the Portland metro area by
early to mid afternoon Thursday.

With regards to snow accumulations, the lowlands in general shouldn`t
expect to see too much; from about Salem northwards through SW
Washington, there is only a 10-30% chance of accumulations over 1
inch from 4am Thu to 4am Fri. South of Salem, freezing rain looks to
be more prevalent, with snow being less of an issue. NBM has higher
accumulations in the Columbia River Gorge and Upper Hood River
Valley, with around a 40-60% chance of snow accumulations over 3
inches there during that same period. Do note that these
probabilities are likely underdone as the NBM
does not handle these types of warm air overrunning scenarios
very well locally. The Cascades will see slightly higher
accumulations, closer to 3-6 inches during this period.

The biggest concern at the moment will be freezing rain throughout
the lowlands and in the Gorge Thursday and Friday. Precipitation
likely begins as mostly snow, but as the warm front overruns the cold
airmass, freezing rain accumulations look likely from Thursday
afternoon/evening onwards, allowing for sleet and freezing rain.
Exact timing on the transition is still uncertain at the moment. LREF
still has a 30-60% chance of accumulating freezing rain in the
Willamette Valley and adjacent foothills/valleys, with the best
chance in the central Willamette Valley between Eugene and Salem. The
90th percentile ice accumulation of the LREF has right about a
quarter inch of ice accumulations between 4am Thu to 4am Fri, which
seems a reasonable worst case scenario for now. In Portland, 90th
percentile has slightly lower accumulations, closer to 0.1-0.2.
Overall confidence in the timing and exact amounts/locations is
rather low, and will be carefully monitoring model runs in the next
few days. Potential for winter precipitation through Friday should be
taken into consideration if making travel plans in the middle of next
week. Most ensemble members are showing a break in precipitation near
the end of next week, with possibility of more moisture on Sunday or
Monday. In some scenarios the low level flow never truly turns
onshore which means there is a chance (20-35%) the Columbia River
Gorge and Hood River Valley may be dealing with freezing rain into
early next week.  -Schuldt/JLiu

&&

.AVIATION...Rather chaotic flight conditions across the airspace
this morning as high pressure slowly builds over the airspace
today. Expect any locations that have lowered flight conditions to
slowly improve to VFR through 20Z Monday. Afterwards, expect
predominately VFR. High pressure aloft will also result in a weak
thermally induced trough manifesting within the Willamette Valley
and will bring easterly winds within the Columbia River Gorge and
could bring gusts up to 30 kt for KTTD starting around 04Z-06Z
Tuesday.

PDX AND APPROACHES...Current MVFR conditions at the terminal
expected to improve to VFR over the next hour. Afterwards, VFR
conditions expected to dominate. Increasing easterly winds will
result in a LLWS threat as winds aloft will be northerly and much
weaker than the 2000 FL winds starting around 05Z Tuesday. /42

&&

.MARINE...Conditions are expected to be relatively benign until
at the middle of the week. High pressure will persist offshore
and maintain northerly winds through today. Seas will remain
around 4 to 6 ft. Once the high pressure weakens Tuesday, winds
will become offshore. Seas will begin to increase towards 8-10
ft for Thursday into Friday as a westerly swell moves in and
southerly winds pick up. A low pressure system approaching the
Oregon coast could support Small Craft winds with a 50-70%
chance Wednesday evening through Thursday. -Batz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Cold Weather Advisory from 7 PM this evening to noon PST
     Tuesday for ORZ101>118-120>125.
WA...Cold Weather Advisory from 7 PM this evening to noon PST
     Tuesday for WAZ201>210.
PZ...None.
&&

$$

www.weather.gov/portland

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