324 FXUS66 KPQR 300548 AAA AFDPQR Area Forecast Discussion...UPDATED National Weather Service Portland OR 946 PM PST Mon Dec 29 2025 ...Updated aviation discussion... .SYNOPSIS...High pressure over the region through at least Wednesday will lead to frost and fog overnight with cooler temperatures. A pattern shift is in store starting as early as Thursday morning but higher confidence lies on Thursday evening or Friday morning. Precipitation persists through the weekend. && .SHORT TERM...Now through Wednesday Night...Broad ridging remains anchored over the region which is keeping conditions 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. Winds today have been a bit squirly with general offshore flow being experience throughout the area. Through the Willamette Valley winds are more northerly. As the ridge intensifies we will see easterly winds following suit. Surface analysis shows the ridge reaching it's peak on Tuesday morning which coincides with the strongest temperature inversion as well. Because of this, we will see a pressure gradient between Troutdale and the Dalles of around -6 to -7 mb. These gradients coincide with gusts in excess of 30-35 mph within the Columbia River Gorge and gusts around 20 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. With the HREF, the probability around Eugene is around 20-30% for overnight lows tonight into Tuesday morning while the NBM is closer to 60%. Likely this is due to the incorporation of cloud cover into the forecast. If skies maintain any level of cloud cover, very cold temperatures will be difficult to achieve. 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% 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. With the high pressure in place, a strong inversion builds which will persist through Wednesday. While we commonly associate air stagnation with the summer and summertime subsidence inversions, we do experience stagnant air in the winter. 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. However, if you are susceptible to any air quality concerns, it's best to view the observations at airnow.gov. -27 .LONG TERM...Thursday through Monday...While one would assume that the passing days would lead to higher confidence in the long-term forecast, unfortunately that is not the case. 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. 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. There are some ensembles that suggest the closed low will stay further south which would bring increased rainfall to California. But if it does track northward then we would see increased rainfall. 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 late Thursday, 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. As the weekend approaches, uncertainty remains high as the pattern remains very disorganized at this time. -27/42 && .AVIATION...High pressure over the region will maintain predominately VFR conditions through the TAF period. Winds remain light for most terminals tonight. An exception is in the far eastern Portland Metro, where tight pressure gradients are resulting in breezy easterly winds through the Columbia River Gorge and thus gusts up to 30 kt at KTTD. These breezy winds continue through the day on Tuesday. Temperatures falling to the upper 20s to low 30s across the Willamette Valley will favor widespread frost development overnight, especially over elevated metal surfaces. Because frost is more favored tonight, there is low confidence for fog formation. Guidance suggests a 15-25% chance for LIFR VIS/CIGs due to fog across the Valley at any given hour between 12-18z Tue, with the highest chances in the south Valley including KEUG. The lowest chances for fog (and frost) would be in the Portland Metro Area, where stronger winds through the Gorge would keep conditions mixed and maintain warmer temperatures. KPDX AND APPROACHES...VFR conditions through the TAF period. Easterly winds around 5 kt or less through 18z Tue, increasing to 8- 11 kt in the afternoon. Stronger easterly winds aloft around 40-45 kt at 1.5-2 kft may result in speed shear tonight through 18z Tue. -10 && .MARINE...High pressure remains in control of the region through the middle of the week. Winds expected to 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 the west. This will shift winds to be southerly on Friday with a 40-50% chance of wind gusts reaching 20 to 25 kts. These stronger winds will also push seas into the low teens for a time on Saturday. Details beyond this time are unclear but the unsettled pattern looks to continue. -19 && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...None. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland