textproduct: Portland

SYNOPSIS

High pressure is firmly established over the Pacific Northwest early this morning and will remain the dominant weather feature through the end of the work week. This pattern will support dry conditions, limited cloud cover outside of valley fog and low stratus, and unseasonably mild temperatures. Light winds and strong low-level inversions will promote areas of fog, frost and air stagnation in sheltered lowland valleys. In contrast, strengthening easterly flow through the Columbia River Gorge will lead to windy conditions in the eastern Portland/Vancouver Metro and adjacent terrain through Thursday.

SHORT TERM

This morning observations indicate increasing low-level moisture trapped beneath the inversion, leading to fog and low stratus beginning to develop at Corvallis and Eugene. Expect continued fog development this morning, becoming prevalent across portions of the Willamette Valley where high level clouds have cleared. These conditions are favored in areas removed from terrain-driven flow, where winds remain light and radiational cooling is efficient. Fog and stratus are expected to gradually erode later this morning, though some locations may see slow improvement due to the strength of the inversion.

A strong upper-level ridge centered over the region will maintain dry weather through Friday. Subsidence associated with this ridge will reinforce the inversion, limiting vertical mixing and allowing low clouds to redevelop each night. Above the inversion, conditions will remain mostly clear, particularly today and Thursday, as any mid-to-high level cloudiness remains confined well north of the forecast area.

The pressure gradient between the interior Columbia Basin and western Oregon will continue to tighten today, resulting in increasing easterly winds through the Columbia River Gorge and into the eastern Portland/Vancouver Metro area. Model guidance indicates the strongest gradients developing late tonight into early Thursday, with Troutdale to the Dalles values reaching -7 to -8 mb and locally stronger in some ensemble solutions. These gradients support a 40-60% percent probability of wind gusts approaching 45 mph in the eastern metro area, with high probabilities of gusts near or above 50 to 55 mph along exposed ridgeline such as Crown Point and Three Corner Rock. While these probabilities still suggest uncertainty, increasing pressure gradient and winds aloft, Tuesday evenings wind gusts around 40 mph at Troutdale, and the potential for northeasterly downsloping winds improves confidence for impactful winds. As a result, a Wind Advisory has been issued for winds 25 to 35 mph with gusts up to 40 to 45 mph from 10 PM this evening to 1 PM Thursday for the East Portland Metro. Strongest wind gusts will be located from Troutdale east, with the highest winds occurring after 3 AM Thursday. Winds are expected to ease Thursday evening into Friday as gradients relax.

Away from the influence of easterly winds, stagnant conditions will persist. Light surface flow combined with shallow mixing heights will continue to limit dispersion, increasing the potential for deteriorated air quality. An Air Stagnation Advisory remains in effect through Friday afternoon for the Southern and East Central Willamette Valley, Cascade foothills, North Clark County lowlands, and the Lower Columbia and Cowlitz River Valleys. The Portland/Vancouver Metro area and West- Central Willamette Valley remain excluded due to better mixing from easterly and northerly flow.

Clear nights and calm winds will also maintain a favorable environment for frost development in colder valleys, particularly Thursday and Friday morning. The frost greatest potential continues to be in the Upper Hood River Valley, with more localized and patchy frost possible across portions of the central and southern Willamette Valley.

Into the weekend, ensemble guidance continues to favor a transition toward cooler and wetter conditions as an upper-level trough approaches and the ridge shifts east. While confidence is high in a pattern change, uncertainty remains regarding the timing and coverage of precipitation. Current ensemble solutions range from limited precipitation near the coast Saturday to more widespread rainfall by Sunday. Snow levels are expected to lower into the 3000 to 4000 foot range as cooler air arrives on Monday, supporting renewed snowfall in the Cascades. ~12

AVIATION

VFR flying conditions continue across the region as high pressure builds over much of the West. For all terminals except KEUG, KSLE, and KHIO, expect VFR conditions through at least 06Z Thursday. As for the aforementioned terminals, deteriorated conditions will persist until 18-20Z Wednesday, then improve back to VFR. Specifically, KEUG will experience LIFR fog, KSLE may experience MVFR VIS, and KHIO may experience LIFR CIGs. There is less certainty for KSLE and KHIO due to low agreement between probabilistic and deterministic guidance. Later in the TAF period around 06-09Z Thursday, most terminals may see LIFR fog development, with the exception of terminals affected by the Columbia River Gorge outflow (KPDX, KTTD). Otherwise, expect easterly flow at terminals along the Columbia River and a variable to northerly in the central and southern Willamette Valley. Winds will generally be 5-10 kt, with KTTD being the exception with 20-25 kt winds and gusts up to 30-35 kt.

KPDX AND APPROACHES...VFR conditions through the TAF period. Easterly to southeasterly winds around 5-10 kt. Winds from the Gorge will keep the terminal clear of fog. With increasing winds in the Gorge near the end of the TAF period, could see gusts up to 20 kt around 12Z Thursday. ~12

MARINE

Variable winds with gusts up to 15 kt will be the trend through Friday. A long-period westerly swell has pushed into the waters today, increasing seas to around 9 to 11 feet at around 12 to 15 seconds. Seas will remain elevated above 10 feet through the end of the week. A Small Craft Advisory will remain in effect from now until 4 PM PST Friday for all the waters. Into the weekend, Small Craft winds likely with a 40-60% chance of Gale Force winds. ~12/03

BEACH HAZARDS

A long-period westerly swell has pushed into the waters today, causing a high threat for sneaker waves. A Beach Hazards Statement is in effect until 1 AM PST Thursday for the coasts of northern/central Oregon and southern Washington. Waves will be able to run significantly farther up on a beach than normal, including over rocks and jetties. Sneaker waves can suddenly knock people off of their feet and quickly pull them into the frigid ocean which may lead to serious injury or drowning. Caution should be used when in or near the water, especially those who will be razor clamming this week. ~12

PQR WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory until 4 PM PST Friday for ORZ108- 115>118-123>125. Wind Advisory from 10 PM this evening to 1 PM PST Thursday for ORZ112. WA...Air Stagnation Advisory until 4 PM PST Friday for WAZ204-205. PZ...Small Craft Advisory until 4 PM PST Friday for PZZ210-253-273. Small Craft Advisory until 4 PM PST Friday for PZZ251-252-271- 272.