838
FXUS66 KPQR 121045
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
345 AM PDT Wed Mar 12 2025

.SYNOPSIS...A parade of weather systems will keep an active
winter-like weather in place through the weekend into early next
week. Expect a myriad of conditions going forward with periods
of valley rain, ample mountain snowfall (even down to the
Cascades foothills and coast range Thursday/Friday), breezy to
gusty winds, and isolated thunderstorms. Out of all of these,
the primary impacts continue to revolve around mountain
snowfall, especially Friday through Sunday night, so if you
plan to travel over the Cascades the next 5-6 days prepare
winter driving conditions. -Schuldt

.DISCUSSION...Early this morning a broad amplified upper-level
trough continues to stream bands of light precipitation across
southwestern Washington and western Oregon while an associated
cold-frontal boundary remains offshore. As the day goes on, this
latter frontal feature leisurely approaches the coastline
before finally arriving onshore late this afternoon and evening
in conjunction with a distinct mid-level shortwave/vorticity
maximum rotating around the large parent trough further enhance
precipitation. All of these variables likely combine to
facilitate the rapid degradation of conditions across the
Cascades passes this evening as snow-levels crash to as low as
1500-2000ft overnight into Thursday morning. From there, rather
convective post-frontal showers persist through the rest of
Thursday into Thursday night. Confidence remains high in
advisory-level snowfall (6-10 inches) across the Cascades during
this period so the current Winter Weather Advisory will be
maintained.

Circling back to those convective post-frontal showers, the
atmospheric profile for Thursday still appears rather unstable
for March with 200-500 J/kg of CAPE extending up towards the
tropopause and little to no CIN shown by various deterministic
model soundings (NAM, GFS, ECMWF, GEM, etc). So, expect around
a 15% to 35% probability for thunderstorms across almost the
entire CWA Thursday into Thursday evening with short-lived
cells producing brief heavy rain/small hail/graupel, a rumble of
thunder or two, and locally breezy outflow winds. Overall this
activity is really nothing to write home about compared to other
portions of the country but its some excitement for us
lightning-starved meteorologists west of the Cascades.

Headed into Friday we turn our attention to the next weather
system barring down on the Pacific northwest while cooler air
aloft remains established overhead at least initially. This
likely brings another round of rain to the the lowlands and
heavy snow to the Cascades Friday through Saturday. With snow
levels already around ~1500 ft to start, could see some wet snow
mixing down to the upper elevations of the Coast Range, Upper
Hood River Valley (closer to 700-1000ft here), and Cascade
Foothills, but not expecting hazard level accumulations at this
time - we`ll need to watch this period closely. However as one
gains elevation, particularly in the Cascades, heavy snowfall
appears likely with preliminary storm total snow amounts ranging
from 1 to 2 feet of snow through at least Saturday evening for
locations above 3500 ft. Given these values and how weve seen
little change in model guidance resolving this system the last
few days, a Winter Storm Watch/Warning will likely need to be
issued in the next forecast cycle or two.

It should also be highlighted this Friday/Saturday system is
almost immediately followed by secondary and deeper upper-level
trough Sunday/Monday. There is added complexity of a warm-frontal
boundary initially pushing into western Oregon Saturday night
and Sunday morning which would briefly push snow-levels near
pass-level (4000-5000ft Oregon Cascades) before falling again
by Sunday night. However, along the Cascades crests this
secondary system could easily dump another 12 inches of snow or
more. The latest NBM probabilities for 12 inches or more of snow
accumulation has held around 60-80% 5pm Saturday through 5pm
Monday  closer to 40-50% US-26 near Government Camp. All in
all, winter certainly isnt done with the Pacific Northwest and
if you plan on traveling over the Cascades through the weekend
prepare for winter-like driving conditions and take the necessary
precautions to ensure a safe trip.

Moving back to the interior lowlands, still not expecting too
much in the way of impacts with these series of storms Friday
through Sunday/Monday other than breezy conditions and
additional rainfall. The latest 10-day HEFS (Hydrologic Ensemble
Forecast System) probabilities for local rivers to reach minor
flood stage are generally less than 5-10%. There remains a few
outlying scenarios in which strong winds of 50-55 mph or greater
reach the inland valley locations with the Saturday
night/Sunday disturbance due to variability in the low pressure
track. More importantly these probabilities point to a stronger
signal for gusty winds at the coast with the NBM projecting a
40-60% chance to see max gusts of 55mph or more across the
beaches and headlands Saturday night. Overall a progression of
this low further offshore (current ensemble consensus) would
just lead to just breezy south wind gusts of 25-40 mph for the
Willamette Valley and gusts of 40-50mph for the coast, while a
track close to or right along the coastline (10-20% chance)
would result in stronger winds being induced not just at the
coast but inland as well. At least there is good agreement
between both deterministic and ensemble guidance in a return to
slightly calmer conditions around Tuesday of next week due to a
transient ridge of high pressure shifting into the Pacific
Northwest.  -Schuldt/42

&&

.AVIATION...Satellite imagery shows the front draped over the
region which is producing rain and a mixture of MVFR and VFR CIGs.
Higher clouds have dominated which has kept many inland sites VFR
but that will change over the coming hours. Winds remain gusty
over the waters and the coastal terminals. For approaches, right
at 2000 ft and above, withs are southwest gusting as high as 35
kt. Some areas of LLWS are possible but directional shear is not
has favorable for those conditions. Through the day will see
fairly persistent conditions with the only real shift being in
regards to the coastal winds. Will see a lull in the gusts to 25
kt until another disturbance moves through after 06Z Thur. This
system will bring another round of rain and wind. Note that snow
is expected over the Cascades through the next few days.

PDX AND APPROACHES...A mix of VFR and MVFR conditions through the
day with no real trend. Overall, rain will be on the lighter side
so VIS will be less impacted than other areas. Around 18Z Wed,
winds between 1500-3000 ft will be southwesterly up to 30 kt.
There is not quite enough shear to justify LLWS, but those
susceptible to shear, may experience some challenges due to
cross winds at the surface. -Muessle

&&

.MARINE...The frontal system that has been bringing gale force
winds over the waters is beginning to shift inland. This will
cause winds to slowly ease. The Gale Warning currently is out
until 0500, however based on area observations, an extension may
be necessary - especially around PZZ273. Gusts have been reported
at buoy 46050 up to 43 kt which was a bit higher than forecast.
Seas have been hovering right around 9-10 ft. As the winds ease,
seas will rise with the addition of a westerly fresh swell. Cold
air will fill in behind this front which will bring an increased
chance for thunderstorms over the waters on Thursday.

Another frontal system arrives on Friday which is shaping up to be
stronger than the system today. There is a high probability for
Gale Force Winds with this system in PZZ272-273. Active weather
continues through the weekend with an even stronger system
arriving on Saturday. Winds will be amplified with increasing
chances for Storm Force winds and steep seas. -Muessle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

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

PZ...Small Craft Advisory until 5 PM PDT Thursday for PZZ210.

     Gale Warning until 8 AM PDT this morning for PZZ251>253.

     Small Craft Advisory from 8 AM this morning to midnight PDT
     Thursday night for PZZ251>253.

     Gale Warning until 5 AM PDT early this morning for PZZ271>273.

     Small Craft Advisory until midnight PDT Thursday night for
     PZZ271>273.

$$

www.weather.gov/portland

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