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FXUS66 KPQR 041209

AFDPQR



Area Forecast Discussion

National Weather Service Portland OR

410 AM PST Thu Dec 4 2025



.SYNOPSIS...A series of fronts will bring a period of wet weather 

Thursday through at least the middle of next week across southwest 

Washington and northwest Oregon. Ensemble guidance suggests a 

potentially impactful atmospheric river event next Monday into 

midweek, though uncertainty is very high.



&&



.DISCUSSION...Today through Wednesday...December is, on average, the 

wettest month of the year here in northwest Oregon and southwest 

Washington. So it should not come as a big surprise that the stretch 

of mostly dry weather to start the month would not last too long. 

Expect varying amounts of rainfall each day through this next week, 

with potential for heavier rain early next week, as a series of 

weather systems produces multiple rounds of widespread rain across 

the Pacific Northwest.  



An upper level ridge of high pressure remains over the northeast 

Pacific today, but eventually shifts southeast, flattens, and allows 

for more zonal flow into the region. A warm front associated with 

this initial system will stall near the coast today, spreading light 

rain across much of the area. Due to the northwest orientaion of 

winds, the Willamette Valley will likely be rain shadowed and see 

lower rain accumulations through this evening. The flow turns a bit 

more westerly on Friday, and an associated plume of deeper moisture 

will attend a cold front approaching the coast. IVT values are 

expected to peak around 500-700 kg/m/s, bringing a second round of 

widespread, heavier rain through Friday afternoon. Breezy southwest 

winds will also be possible with the frontal passage on Friday, with 

gusts most likely peaking around 20-30 mph.  



Showers expected to linger through Saturday as onshore flow 

continues. Then, another frontal system moves through the area on 

Sunday. Ensembles indicate IVT values could peak around 300-500 

kg/m/s with this frontal system which would produce another round of 

widespread rainfall. Total rainfall amounts Thursday through Sunday 

are most likely to be around 1.0-2.0 inches for interior lowlands, 

2.0-4.0 inches at the coast, and 3.0-6.0 inches for the Coast Range 

and Cascades. But, there remains a fair amount of uncertainty in 

both the low and high end potential. Impacts are not expected with 

rain amounts through Sunday, unless rain rates exceed 0.2-0.3 inches 

for multiple hours over certain watersheds leading to rivers that 

are prone to flooding quickly, such as the Grays at Rosburg and 

Willapa River. This will be something to monitor during heaviest 

periods of rain. 

  

The main period of concern for the forecast period is next week as 

ensemble guidance continues to indicate the potential for an 

impactful atmospheric river event Monday through Wednesday. 

Significant uncertainty still remains in the length of elevated IVT 

values, location of the moisture along the WA and OR coast, and 

exact peak of IVT values (the 00z GEFS mean is around 650 kg/m/s 

with +1 standard deviation around 800 kg/m/s, and the 12z Euro EPS 

mean is around 750 kg/m/s with +1 standard deviation around 850 

kg/m/s). These factors have jumped around between forecast runs over 

the past 24 hours and will likely continue to fluctuate until we get 

closer to the event. There is the potential that this ends up being 

another moderate atmospheric river event, similar to the preceding 

events. If this is the case, impacts are likely to be minimal once 

again. However, if a strong to extreme atmospheric river event 

materializes, there is the potential for widespread river flooding 

and/or wind damage or landslides as soils will already be saturated 

and river levels higher from the rain from the 4 days prior. 



The main period for river flood concerns is Tuesday into Wednesday 

as it takes time after rain begins for rivers to rise. HEFS guidance 

indicates a 10-30% chance of rivers reaching Minor flooding for 

Coast Range and Coastal rivers and 5-20% chance for Willamette River 

tributaries, mainly focused on Tuesday into Wednesday, possibly 

beyond depending on the rain forecast and/or response times of the 

rivers. Of note, HEFS guidance also indicates a 8-15% chance of 

exceeding Major flood stage at rivers such as the Nasselle, Willapa, 

and Wilson near Tillamook.  



Additionally, there are signals for breezy winds with the frontal 

passage, although the ensemble spread remains very wide leading to 

low certainty. If winds do end up even around 30-40 mph, saturated 

soils could cause some localized impacts due to downed trees. Keep 

an eye on this forecast, especially if you live in flood prone 

regions.  DH/03



&&



.AVIATION...Moist, northwesterly flow aloft will push a front

across the area today. Light rain has already begun at the coast,

with mid-level clouds maintaining VFR conditions through at least

12z. Inland, LIFR to low-end MVFR stratus is expected to persist

through at least 14-16z. As rain spreads inland, CIGs are likely 

to lift to more widespread MVFR. However, there is about a 40-50%

chance that IFR conditions persist bit longer through 18-20z,

especially across the central and southern Willamette Valley.

Conditions at the coast are expected to trend downward toward MVFR

by 14z Thu near KAST and by 18z near KONP. Conditions are expected

to further deteriorate to IFR behind the frontal passage. Light 

winds will increase out of the south to southeast at around 5-10 

kt.



KPDX AND APPROACHES...IFR to low-end MVFR stratus expected to

persist through much of the day. Light rain expected to begin by 

14-15z Thu as a front pushes across the area. South to southeast

winds increasing to around 4-6 kt today. /02 



&&



.MARINE...Active weather returns as winds turn southerly this

morning ahead of the first in a series of fronts. Winds are

expected to increase to around 15-20 kt later this morning, with

gusts up to 25 kt across the coastal waters north of Cape Falcon

through early afternoon. Winds turn westerly behind the frontal

passage, lingering around 10-15 kt through tonight. West to

southwest winds then gradually increase on Friday to around 20-30

kt as the next front moves through. Have issued Small Craft

Advisories to account for these increased winds, today for the

northern zones, and on Friday for all coastal marine zones. Seas

around 5 to 6 ft today are expected to increase later Friday as a

northwest swell pushes into the waters through Saturday. Wave 

heights likely build into the lower teens by Saturday morning,

while there is around a 30% chance that seas exceed 15 ft.  



The parade of fronts continue through the weekend and into early

next week. There is around a 30-40% of wind gusts reaching 35 kt

on Sunday, while chances for Gales is higher (50-60%) later

Monday. Seas are likely to subside back under 10 ft by Sunday,

before building again Monday. /DH



&&



.BEACH HAZARDS...There is a Coastal Flood Advisory for minor 

tidal overflow flooding during high tide from 9 AM to 3 PM 

Thursday and Friday for the South Washington Coast and the Clatsop

County Coast in Oregon. Minor flooding up to 1 foot above ground 

level is possible during high tide in low lying areas near bays, 

sloughs, and the low reaches of the coastal rivers. Coastal 

residents in the warned area should remain alert for rising water 

and take appropriate action to protect life and property. Remain 

out of the water to avoid hazardous swimming conditions. -10



&&



.PQR WATCHES/WARNINGS/ADVISORIES...



OR...Coastal Flood Advisory from 9 AM this morning to 3 PM PST this 

     afternoon for ORZ101.



     Coastal Flood Advisory from 9 AM to 3 PM PST Friday for ORZ101.



WA...Coastal Flood Advisory from 9 AM this morning to 3 PM PST this 

     afternoon for WAZ201.



     Coastal Flood Advisory from 9 AM to 3 PM PST Friday for WAZ201.



PZ...Small Craft Advisory from 2 PM this afternoon to 6 PM PST this 

     evening for PZZ210.



     Small Craft Advisory from 8 AM this morning to 2 PM PST this 

     afternoon for PZZ251-271.



     Small Craft Advisory from 4 AM Friday to 4 AM PST Saturday for 

     PZZ251>253-271>273.



&&



$$



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