textproduct: Portland

SYNOPSIS

High pressure builds over the Pacific Northwest and persists through Friday, returning dry and mostly sunny weather. Light winds and limited air movement will result in air stagnation for parts of the Willamette Valley and southwest WA lowlands. Each night through Friday morning, clear skies and calm winds will result in potential frost or fog development. An exception would be the Columbia River Gorge and eastern Portland/Vancouver Metro, where easterly winds will increase with gusts peaking Wednesday night into Thursday.

DISCUSSION...Today through Monday

An upper level ridge is building over the Pacific Northwest today which will persist through the end of the work week and bring dry weather through Friday. Satellite imagery as of early Tuesday afternoon depicts patchy areas of lingering fog and low stratus across the Willamette Valley and southwest Washington lowlands. Fog and low stratus will gradually dissipate throughout the afternoon as daytime heating progresses. Otherwise, expect mostly sunny skies elsewhere. There will be some lingering mid to high level clouds today as a system in the northeast Pacific rides the northwest periphery of the ridge and treks into western British Columbia, but sunnier days are expected on Wednesday and Thursday. Afternoon high temperatures are forecast to peak in the upper 50s to low 60s for most interior lowland valleys this week, but some locations around 1000-2000 feet elevation may even reach the mid to upper 60s due to a strengthening inversion from the upper ridge. These temperatures are around 10-15 degrees above normal for this time of year.

Winds throughout the duration of the upper level ridge will generally be light across most of the forecast area, except in and near the Columbia River Gorge and eastern Portland/Vancouver Metro Area. Strengthening east-west pressure gradients between Troutdale (KTTD) and The Dalles (KDLS) will lead to breezy easterly gap winds through the Gorge, particularly on Wednesday and Thursday. Deterministic and ensemble guidance suggests winds will peak sometime Wednesday night into Thursday morning. Guidance shows pressure gradients between KTTD and the KDLS around -5 to -6 mb on Wednesday and increasing overnight to around -7 to -8 mb by Thursday morning. A few ECMWF ensemble members even indicate gradients as strong as -9 mb Thursday morning. Based on high resolution guidance (REFS) and NBM 5.0, there is a 30-50% chance for peak wind gusts up to 45 mph across the eastern Portland/Vancouver Metro, with a 50-70% chance for gusts up to 50 mph for exposed ridgetops like Crown Point or Three Corner Rock. There are times where models tend of underestimate how tight the pressure gradient will be, so NBM 90th was blended into the forecast to increase these easterly winds. We'll continue to monitor these pressure gradients closely and see if a Wind Advisory is warranted. Either way, conditions will be breezy so make sure to secure any loose outdoor items if you're in a east wind-prone location. Winds appear to decrease on Friday as pressure gradients ease.

For locations away from the influence of easterly winds, there are two main concerns: fog/frost potential and air stagnation. First, clear skies and calm winds will maintain a favorable set- up for fog and frost formation across interior lowland valleys. The highest threat for frost will be Thursday and Friday mornings, where chances for temperatures falling below 36 degrees are around 40-60% for the central/southern Willamette Valley, 20-40% for the northern Willamette Valley and southwest WA lowlands (excluding the Portland Metro), and greater than 80% for the Upper Hood River Valley. Frost would most likely be seen over grasses and metal surfaces.

When it comes to air stagnation, light winds and limited air movement due to the inversion from the ridge will lead to a period of stagnant air conditions, which may lead to deteriorating air quality. The Air Stagnation Advisory has been remains in effect through 4 PM Friday across the Southern Willamette Valley, East Central Willamette Valley, North and Central Oregon Cascade Foothills, North Clark County lowlands, and the Lower Columbia River/Cowlitz River Valleys. Based on soundings and model guidance, mixing heights in these locations are forecast below 2000 feet with transport winds around 5 kt or less. The Portland/Vancouver Metro Area and West-Central Willamette Valley remains excluded from the advisory as these areas will remain better mixed from the increasing easterly winds this week (these winds turn more northerly as it gets into West-Central Valley).

A pattern change returns this weekend as the majority of ensemble members show an upper level trough approaching the Pacific Northwest and shifting the upper ridge eastward. There is high confidence that we'll turn cooler and wetter, however, there is still uncertainty with the exact timing of when precipitation will arrive and exactly how much we'll get. On Saturday, 55% of members show precipitation just clipping the far northwest Oregon and southwest Washington coast, while 35% of members show a soaker across much of the area, and 15% of members show conditions remaining completely dry. By Sunday, the majority of ensemble members return widespread precipitation to the area. Cooler temperatures will also bring snow levels down to 3000-4000 feet, which would return some much needed snow to the Cascades. -10

AVIATION

VFR flying conditions continue across the region as high pressure builds over much of the West. Bkn high-level clouds and largely calm winds will again allow for areas of valley fog to develop inland overnight. The highest confidence in LIFR vis within fog remains across the southern Willamette Valley (KEUG, KCVO), with lesser but still moderate confidence (40-60% chance of IFR vis) throughout the rest of the Willamette Valley (KSLE north to KHIO). In the Portland area, east winds will lessen the chances for fog, but there is a 10-15% chance for light north winds at KPDX overnight, which would favor more widespread fog. Any vis restrictions will improve by 19-20z Wed as fog mixes out and conditions return to VFR beneath largely clear skies. Along the coast, VFR conditions are expected to persist through tonight.

KPDX AND APPROACHES...VFR flying conditions are expected to continue through Wednesday morning beneath bkn high clouds at 20 kft. East winds around 5 kt at KPDX and near 20 kt with gusts near 30 kt at KTTD will limit chances for fog to only 10-15% and near 0%, respectively. If easterly winds become light and variable or shift northerly at KPDX, vis restrictions in fog will become more likely. Otherwise, skies clear after 18-19z Wed. -36

MARINE

Variable winds with gusts up to 15 kt will be the trend through Friday. Wave heights this morning are around 7-9 ft at 10-12 seconds, and will remain here through this evening. An increasing westerly swell will push into the waters late tonight, increasing seas again to around 10-12 feet around 12-16 seconds. Seas will remain elevated above 10 feet through the end of the week. A Small Craft Advisory has been issued beginning at 9 PM PST tonight for zones PZZ251, 252, 271, and 272 and at 4 AM PST for zones PZZ253, 273, and 210. The Small Craft Advisory extends through 4 PM PST Friday. -03

BEACH HAZARDS

A moderate threat for sneaker waves continues today before another long-period westerly swell pushes into the waters tonight through Wednesday night, causing another high threat for sneaker waves. A Beach Hazards Statement has been issued for 4 AM PST Wednesday through 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. -03

PQR WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory until 4 PM PST Friday for ORZ108- 115>118-123>125.

WA...Air Stagnation Advisory until 4 PM PST Friday for WAZ204-205.

PZ...Small Craft Advisory from 4 AM Wednesday to 4 PM PST Friday for PZZ210-253-273.

Small Craft Advisory until 4 PM PST Friday for PZZ251-252-271- 272.