536
FXUS66 KPQR 111035
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
335 AM PDT Tue Mar 11 2025

.SYNOPSIS...A progressive and active weather pattern is
anticipated going forward as a series of low pressures systems
take aim at the Pacific Northwest. Thisll bring a whole slew of
conditions to region over the next week with periods of valley
rain, moderate to heavy mountain snowfall (even down to the
higher foothills and coast range Thursday/Friday), breezy/gusty
winds, and isolated thunderstorms. Out of all of these, its
definitely worth highlighting the mountain snowfall impacts
particularly if you plan on traveling across the Cascade passes
Thursday through the weekend - prepare for winter driving
conditions! Confidence is also growing in gusty winds developing
along the coast this weekend as well. -Schuldt

.DISCUSSION..Broad zonal flow currently resides aloft with
fairly dry conditions across southwestern Washington and western
Oregon early this morning as we sit between weather
disturbances. This lull likely persists through at least the
morning hours today before an approaching upper-level trough and
accompanying surface low coupled with a warm-frontal boundary
increase light precipitation across the region late Tuesday into
Tuesday night ending any dry weather. From this point the
upper-level trough still offshore amplifies further aiming more
favorable moisture/forcing into northern California while the
aforementioned surface low caught within SW flow swings into
British Columbia leaving us in-between. Expect periods of rain
through Wednesday in the lowlands but nothing climatologically
abnormal.

Then, a strong cold-front arrives late Wednesday afternoon and
evening ushering in a rapid shift to post-frontal showers while
temperatures cool aloft. Both deterministic and ensemble
guidance are in excellent agreement showing 850 mb temps
dropping towards -4C to -5C by Thursday morning and punching
snow-levels to ~2000-2500ft. Its certainly possible in this
convective post-frontal regime wet snow or a rain/snow mix are
observed as low as ~1500ft within heavier shower bands,
however, impacts likely remain at higher elevations (2500ft+).
NBM probabilities are also still very high (80-95%) for the
Cascade passes to see Advisory level snowfall (>6in) Wednesday
night through Thursday evening therefore the Winter Weather
Advisory during this period will remain in effect. Probabilities
for warning-level snowfall (>12in) are only around 30-40% at
both Santiam and Willamette Pass, closer to 15-20% US-26 near
Government Camp. So while well need to watch how snowfall
totals trend the next couple of days it doesnt look like an
upgrade to a warning-level product during this first batch of
snowfall is warranted.

Its also worth highlighting the latest deterministic model
soundings from the NAM, GFS, ECMWF, and GEM show a consensus for
rather convective conditions (as far as March PNW setups go) on
Thursday with 200-500 j/kg of CAPE and little to no CIN. Thus
have maintained around a 15-30% probability for thunderstorms
across almost the entire CWA on Thursday. The sounding profiles
do show showing small, skinny CAPE and minimal shear supportive
of short-lived cells which may produce brief heavy rain/small
hail/graupel, a rumble of thunder or two, and locally breezy
outflow winds.

Now, with cooler air aloft established across the region all
eyes turn to quickly developing shortwave to our west expected
to race into the Pacific Northwest on Friday. With snow levels
around ~1500 ft initially we could see some wet snow mixing down
to the upper elevations of the Coast Range, Upper Hood River
Valley, and Cascade Foothills, but not expecting hazard level
accumulations at this time for these areas. But as elevation is
gained, especially in the Cascades, heavy snowfall is appears
increasingly likely through Saturday. While we are still 5 days
out from this event, preliminary storm total snow amounts are an
impressive 10 to 20 inches Friday through Saturday evening for
locations above 3500-4000 ft. If these snow accumulation values
are maintained then a Winter Storm Watch/Warning will likely
need to be issued in the next few days. And if that isn`t
enough snowfall for all you Cascade snow enjoyers, there is
moderate confidence in a secondary weather system in the form of
a deeper upper-level trough pushing overhead in the
Sunday/Monday time frame. The latest NBM depicts a 60-80% to
meet or exceed another 12 inches of snowfall 5pm Saturday
through 5pm Monday. So, please keep an eye on the forecast this
week and at minimum folks need to prepare for winter driving
conditions over the Cascade passes late week through the
weekend.

Across the interior lowlands, still not expecting too much in
the way of impacts with the series of storms over the weekend
other than breezy conditions and additional rainfall. Current
10-day HEFS probabilities for local rivers to reach minor flood
stage are generally 5-10% or less. That said, were still
monitoring an outside chance (10-30% based on NBM and LREF) for
strong winds (>55mph) inland associated with the Saturday
night/Sunday disturbance. More importantly these probabilities
have sharply trended higher along the coast with the latest
suite of ensemble guidance projecting a 40-60% chance to see max
gusts of 55mph or more 11pm Saturday to 11pm Sunday. At the end
of the day this will all hinge on strength and placement of the
surface low along the coast the latter half of the weekend and
there has been decent run-to-run and model-to-model variability
in the location of this feature the last few days. Still a
broader signal certainly exists for windier than normal
conditions this weekend and confidence in the forecast out on
days 6-7 will only improve headed into the middle/end of the
week. -Schuldt/42

&&

.AVIATION...The leading edge of a cold frontal system is nearing
the coast producing a high stratus shield over the region. Clouds
have been variable with lower stratus setting up over the southern
Willamette Valley and broken skies aloft. Because of this, we are
seeing a mix of MVFR and more widespread VFR conditions. Some
backbuilding of stratus is possible which would cause CIGs to drop
in the northern Willamette Valley. Over the last few hours, areas
with clearer skies have begun to develop some patchy fog reducing
VIS to MVFR. Not expecting dense fog anywhere due to some
lingering stratus but cannot rule it out. Models continue to
struggle depicting this overall environment.

The front will make landfall around 00Z Wed at the earliest which
is when we will see more widespread MVFR CIGs and rain. Winds at
2000-3000 ft will be south at 45-55 kt. Due to the more
convective nature of this system, could easily see these winds mix
down to the surface along the coast causing windier conditions
there.

PDX AND APPROACHES...Predominately VFR through the period with
variable mid-level clouds. There continues to be a chance for
MVFR CIGS (30-50%) between 10-16Z today. However, because we have
yet to see this stratus in the surrounding area, have decided
against including it in the TAFs. Note though it may be within the
vicinity of the terminal. Rain chances increase after 00z
Wednesday. Around 10Z Wed southwest winds around 2000 ft will be
25-35 kt will meet with a southerly wind less than 10 kt. Will
likely experience some speed shear but have omitted from the TAF.
-Muessle
&&

.MARINE...Conditions will remain consistent until later this
morning as a cold front nears the coast line. Will see a quick
ramp up of winds at that time. South winds 15 to 20 kt with gusts
up to 25 kt expected. Seas will hover just below 10 ft then
eventually rise. Gale force winds will then develop late
tonight/early Wednesday morning. The challenge here is that the
speeds are sitting right around 35 kt. There is around a 30%
chance that winds could fall below 35 kt slightly. Because of the
impacts, have issued a Gale Warning starting around 8 PM Tuesday.
These winds will be slow to start and will actually see a period
of hazardous seas as winds increase. Given the short time frame of
these hazardous seas, have started the Gale Warning a bit earlier.
Winds will be strongest from Tillamook south in the outer waters.

Winds will quickly ease before yet another low pressure system
arrives on Friday. This system is significantly more robust. If
the low pressure system and significantly colder air take a more
southerly track and gets closer to the shore, wind speeds will be
higher. At this point there is around a 50% chance of storm force
winds, but confidence is low at this point due to the
discontinuity of the low track. There is still ample time for this
pattern to come into better agreement. -Muessle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Weather Advisory from 5 PM Wednesday to 11 PM PDT
     Thursday for ORZ126>128.

WA...Winter Weather Advisory from 5 PM Wednesday to 11 PM PDT
     Thursday for WAZ211.

PZ...Small Craft Advisory from 1 AM to 10 AM PDT Wednesday for PZZ210.

     Small Craft Advisory from 11 AM this morning to 8 PM PDT this
     evening for PZZ251>253-271>273.

     Gale Warning from 8 PM this evening to 5 AM PDT Wednesday for
     PZZ251>253-271>273.
&&

$$

www.weather.gov/portland

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