498 

FXUS66 KPQR 301229 AAA

AFDPQR



Area Forecast Discussion

National Weather Service Portland OR

429 AM PST Tue Dec 30 2025



.SYNOPSIS...High pressure over the region remains anchored over

the region through at least Wednesday. Cool and dry conditions 

will lead a favorable environment for frost and fog development

during the overnight hours through Wednesday. A pattern shift 

is in store starting as early as Thursday morning but higher 

confidence lies on Thursday evening or Friday morning. This 

pattern shift will bring relatively warm air along with 

precipitation back into the forecast through the upcoming 

weekend.



&&



.UPDATE...Have issued a dense fog advisory for dense freezing fog 

for Lower Columbia River and Cowlitz River Valleys, and Lane County 

Lowlands. Surface observations along with various webcams are 

showing patchy dense fog impacting certain parts of the I-5 

corridor this morning. Given that current temperatures are 

generally at or below freezing, any fog would likely be freezing 

fog. This fog should scour out around 10am this morning. If driving, 

slow down, use your headlights, and leave plenty of distance ahead 

of you. If driving, slow down and leave plenty of distance ahead of 

you. Pedestrians and cyclists should wear bright or reflective 

clothing to increase the chances of being seen by others in poor 

visibility conditions. Also, be aware that elevated, metal surfaces 

could be slick this morning. /42



.SHORT TERM...Now through Wednesday Night...Broad ridging 

remains anchored over the region which is keeping conditions

cool and dry with high clouds. Temperatures have been seasonable

with highs in the mid to upper 40s, and overnight lows in the 

30s. This trend will persist through Wednesday. High pressure

aloft is also resulting in a subsidence inversion developing

near the surface in addition to a thermally induced trough. This

is resulting in winds generally from the north to east. Winds

within the Willamette Valley and along the coast are favoring 

more northerly flow. However, robust easterly winds are 

continuing to be observed through the western part of the 

Columbia River Gorge, which is impacting the PDX/Vancouver Metro

area as well as Hillsboro. The easterly winds decrease the 

further you get away from the mouth of the Columbia River Gorge.

Model guidance will maintain a 5 to 7 mb gradient from KPDX to 

KDLS which will result in gusts up to 35 mph near the mouth of 

the Columbia River Gorge and gusts up to 25 mph through the 

Cascade passes. 



Overnight temperatures through Wednesday morning are forecast 

to fall below freezing throughout much of the forecast area. For

those with bulbs still blooming, these temperatures may be cold

enough to cause a "killing freeze". Areas most susceptible will

be within the central and southern Willamette Valley, rural 

portions of Clark, Columbia, Washington and Cowlitz Counties as 

well as the Upper Hood River Valley. There continues to be some 

disagreement in the probability for temperatures less than 30 

degrees F for Tuesday night into Wednesday morning. For example:

The HREF, has the probability for the Willamette Valley around

15-20% tonight into Wednesday morning while the NBM is 20-40%. 

It should be noted that areas like the PDX/Vancouver metro area 

in both scenarios are around 5-10%. There is high confidence 

though in the Upper Hood River Valley and rural portions of the 

I-5 corridor seeing temperatures below 30 degrees, and a 10-15% 

chance of lows of 25 degrees F or less in the Upper Hood River 

Valley. On Wednesday temperatures are not going to be nearly as 

cold as the ridge begins to break down as a series of frontal 

systems near the area. 



A feature we have talked about extensively remains the presence

of frost and fog. Frost continues to be the most likely forecast

component in the interior lowlands while fog is still possible

in other areas around Portland and Hillsboro. Again, it will 

come down to the temperature decreases and saturation. 



Lastly, a strong inversion will persist through Wednesday and

with stable air at the surface, we must consider that air 

stagnation could present an issue. In this case, conditions 

have some of the features of an environment that would cause 

stagnation: low mixing heights, light winds, and cold air 

pooling. Luckily, transport winds are elevated enough that 

mixing will be just enough to decrease concerns. Also, given the

relatively short period of time (less than 48 hours) will also

help to mitigate potential impacts. However, if you are 

susceptible to any air quality concerns, it's best to view the 

observations at airnow.gov. /42/27





.LONG TERM...Thursday through Monday...Uncertainty remains very

high as we look into the latter part of this week and through

the weekend. There are two different features we are tracking 

and each add a layer of complication to the forecast. Multiple 

model ensembles are showing a weak closed low forming off the 

coast of Southern California that will slowly track northward by

Thursday. At the same time, a frontal system stemming from a 

broad and very messy low pressure system within the Gulf of 

Alaska is dropping down at nearly the same time, although models

are continuing to have the Alaskan system follow the California

system. The combination of these two features is making the 

overall pattern incredibly messy and as result increasing the 

overall uncertainty of the long term forecast. The NBM is 

showing rain entering the forecast early Thursday morning with 

PoPs in the 15-30% range. However, the GFS ensemble brings rain 

into the forecast early Friday morning, while the ECMWF Ensembles

show rain by Thursday afternoon. It should also be noted that 

all ensemble solutions are showing 850 mb temperatures in the in

the 3 degree C to 8 degree C range which means that rain is to 

be expected for lower elevations with a rain/snow mixture for 

higher elevations. With these series of systems, we are not 

looking at impactful rainfall. This overall uncertainty continues

through the weekend and into the start of next week. /42/27



&&



.AVIATION...High pressure over the region will maintain

predominately VFR conditions through the TAF period. Ground

observations show isolated pockets of MVFR to LIFR conditions. 

Generally, northerly winds less than 10 kt across the forecast 

area for inland locations and areas along the coast south of KTMK 

through the TAF period. However, easterly winds through the 

Columbia River Gorge, will result in gusts up to 35 kt for KTTD 

and areas east of KPDX, in addition expect easterly winds for KAST

through at least the TAF period. Given the persistent, strong 

easterly flow, there is the potential for LLWS for KPDX and KHIO 

through the TAF period as well.



Temperatures falling to the upper 20s to low 30s across the 

Willamette Valley will favor widespread frost development, 

especially over elevated, metal surfaces. Because frost is more 

favored, there is low confidence for fog formation. Guidance 

suggests a 5% chance or less for LIFR conditions between 12Z-18Z 

Tuesday, with the highest chances in the south Valley including 

KEUG. 



KPDX AND APPROACHES...VFR conditions expected to persist with

generally north to east winds through the day. Strong easterly winds

from the Columbia River Gorge are not expected to impact the

terminal directly. However, given the persistent strong, easterly

flow, there is the potential for LLWS through the TAF period. /42



&&



.MARINE...High pressure remains in control of the region through

the middle of the week. Winds expected to be generally easterly 

through Tuesday night as a thermally induce trough remains in 

place at the surface. Winds 10 kts or less. Seas 5 to 7 feet at 11

to 14 seconds expected through Thursday. The pattern likely 

becomes more active heading into the weekend as high pressure 

shifts inland and disturbances approach from California and

Alaska. This will bring a southerly shift for winds on Friday 

with a 60-70% chance of wind gusts reaching up to 25 kts and 

around a 20-30% chance for gusts up to 35 kt. These stronger 

winds will also push seas into the low to mid teens by late

Friday/early Saturday. /42/19



&&



PQR WATCHES/WARNINGS/ADVISORIES...



OR...Dense Fog Advisory until 10 AM PST this morning for ORZ108-118.



WA...Dense Fog Advisory until 10 AM PST this morning for WAZ204.



PZ...None.



&&



$$



www.weather.gov/portland



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