textproduct: Portland

SYNOPSIS

Strong high pressure remains firmly established over the Pacific Northwest and will continue to dominate the weather pattern through much of the week. This will maintain an extended period of dry weather, light winds, and temperatures well above climatological normals aloft. At the surface, however, persistent low-level inversions will support widespread fog and low stratus across the inland valleys, leading to cooler daytime temperatures in those areas. Elevated terrain will remain notably warmer, drier, and sunnier. The stagnant pattern will also promote areas of poor air quality and locally hazardous travel conditions in the mornings due to fog. Little change is expected through at least the weekend.

DISCUSSION...Now through Sunday night

The overall forecast remains on track this afternoon, with strong confidence in the continuation of a blocked, high- pressure-dominated pattern across northwest Oregon and southwest Washington. An anomalously strong upper-level ridge remains centered over the region, effectively suppressing storm systems and maintaining dry conditions area-wide.

At the surface, conditions continue to diverge sharply between the lowlands and higher terrain. Dense fog and low stratus persist across portions of the Willamette Valley and other sheltered inland valleys this afternoon, with some locations struggling to fully erode cloud cover. Where fog remains entrenched, temperatures have remained a tad cooler than surrounding areas and visibilities locally reduced. Confidence remains high that fog and low clouds will redevelop again tonight and persist into Wednesday morning, especially in the southern and central Willamette Valley and other fog-prone locations. While some partial afternoon improvement is possible on Wednesday, the inversion is expected to remain strong enough to support repeated overnight fog formation through at least Thursday.

Above the inversion, conditions are markedly different. A pronounced subsidence inversion near 2500 to 3000 ft is trapping moisture below and producing very dry air aloft, with relative humidity values remaining low across the Cascade foothills and higher terrain. These areas will continue to experience abundant sunshine, mild afternoons, and unusually dry conditions for midwinter. The persistence of low RH above roughly 2500 ft may have implications for fuels and fire behavior in elevated terrain, and this will need to be monitored closely as the stagnant pattern continues.

Temperatures will reflect this strong vertical contrast. Inland valleys will generally remain cooler, with highs often held in the upper 40s to low 50s due to cloud cover, while higher elevations and locations above the inversion, including the Cascade foothills and portions of the Coast Range, will see highs well into the 50s and locally the lower 60s. Overnight lows will remain mild at elevation, while colder air pooling in sheltered valleys will allow temperatures to dip into the 30s.

Frost potential continues to be a concern for cold-prone areas, particularly the Upper Hood River Valley and nearby sheltered locations. Light winds, clear skies, and strong radiational cooling beneath the inversion will allow temperatures to approach or fall below freezing during the overnight and early morning hours through most of the week, notably Thursday and into the weekend for the Upper Hood River Valley. Localized frost formation is likely, especially in areas that decouple early in the evening.

Winds will remain generally light and terrain-driven through midweek, though increasing easterly flow is expected in the western Columbia River Gorge, Cascade gaps, and portions of the east Portland and Vancouver metro area later this week as offshore pressure gradients strengthen. While winds are not expected to be impactful, they may locally reduce fog coverage near the Gorge.

The persistence of this stagnant pattern will continue to limit vertical mixing and dispersion, allowing pollutants to accumulate near the surface. As a result, air quality concerns remain elevated, particularly across the Willamette Valley, where an Air Stagnation Advisory remains in effect and could be extended or expanded if conditions persist.

Looking ahead, model guidance continues to indicate little change through the weekend. While weak disturbances may brush the periphery of the ridge, they are not expected to meaningfully disrupt the dry pattern. A potential pattern transition towards cooler temperatures remains hinted at beyond the current forecast period, but confidence remains low regarding timing and impacts. Until then, expect continued dry weather, valley fog, and mild conditions aloft across the region. ~12

AVIATION

High pressure over the region will be the weather driver for the next week. Over the next 24 hours, the major weather feature to watch is fog. Throughout the day, the Willamette Valley has struggled to break out of the fog completely with LIFR clouds lingering and patchy VCFG. Due to the level of saturation already occurring, there is a high probability of fog redevelopment overnight. Based on high-resolution models, many lower elevation locations will struggle to break out of the fog. There is around a 30% chance of fog lasting past 18Z Wednesday within the central Willamette Valley. Along the coast is a little bit of a different story. There will be a slight easterly drift, especially around KONP, which should keep conditions dry enough that fog is less of a threat. Cannot rule out lower stratus in some locations dropping CIGs to low-end MVFR or high-end IFR status. However, those winds will increase after 18Z Wednesday which will scour out any fog or low stratus.

This effect will be coupled inland through the east-west aligned gaps and the Columbia River Gorge. In these locations, the threat of fog is significantly less.

KPDX AND APPROACHES...Higher confidence in fog formation tonight around KPDX with a 60% chance of fog forming as early as 06Z Wednesday. One caveat will be whether there is enough of a dry air intrusion from the easterly winds from the Gorge. This could be another scenario where the west side of the terminal sees fog while easterly approaches do not. Any fog that does form will struggle to erode, and once it does, there is around a 30% chance of IFR CIGs lingering. Exact timing is quite difficult to tell at this point. -27

MARINE

High pressure will keep conditions fairly settled through the week. Seas are generally 7-9 ft at 13 seconds with isolated waves up to 11 ft in the far inner waters. These conditions will persist through Wednesday will a brief uptick in wave heights starting Wednesday morning. A thermal trough begins to develop causing winds to shift to the north. The increased wind with associated wind wave will amplify those seas. Will see another round of Small Craft Advisories on Wednesday for seas, then transition to wind based criteria. These conditions will persist through at least Thursday. One thing to note on Thursday, a easterly wind will gust through the coastal gaps causing periodic wind increases in the inner waters and around moorages.

Very calm conditions are expected Friday through Monday as strong high pressure stays locked in place over the region. Winds should stay under 10-15 kt this weekend with seas around 4-6 ft. -27/23

PQR WATCHES/WARNINGS/ADVISORIES

OR...Dense Fog Advisory from 10 PM this evening to noon PST Wednesday for ORZ104-108>111-114>118.

Air Stagnation Advisory until 4 AM PST Friday for ORZ109>111- 114>118.

WA...Dense Fog Advisory from 10 PM this evening to noon PST Wednesday for WAZ204>206.

Air Stagnation Advisory until 4 AM PST Friday for WAZ205-206.

PZ...Small Craft Advisory from 11 AM to 4 PM PST Wednesday for PZZ210.

Small Craft Advisory from 7 AM Wednesday to 4 AM PST Thursday for PZZ251>253-271>273.