textproduct: Portland

SYNOPSIS

High pressure continues to build over the Pacific Northwest today, establishing a dry and mild pattern that is expected to persist through at least Friday. Clear skies, light winds, and persistent low-level inversions will support areas of fog and frost across interior lowland valleys through much of the week. In addition, stagnant air conditions may develop in some interior valleys. Breezy easterly winds through the Columbia River Gorge will help limit air stagnation impacts across the Portland and Vancouver metro area and portions of the west-central Willamette Valley.

DISCUSSION...Now through Sunday

Upper-level ridging becomes firmly established over the region today, marking the start of an extended period of dry weather and above-normal temperatures. Mid to high-level cloud cover may linger through portions of the day as a Pacific system tracks well north into British Columbia along the northern periphery of the ridge. However, clouds are expected to gradually thin and clear, with sunnier conditions becoming more widespread Wednesday and Thursday. Afternoon high temperatures through the end of the work week are forecast to reach the upper 50s to lower 60s, roughly 10 degrees above climatological average for early February. Wednesday looking to be the warmest day of the work week by a degree or two.

Despite the mild daytime temperatures, clear nights and light winds will promote strong nocturnal inversions across interior valleys. These inversions will favor the development of fog and low stratus during the overnight and morning hours, particularly within the Willamette Valley and other sheltered lowland locations. There remains some uncertainty regarding the low-level moisture, as model soundings indicate easterly to east-southeasterly flow around 1000 to 2000 feet above ground level may inhibit stratus formation in some areas while surface winds remain calm. This scenario would result in a favored fog pattern rather than stratus, especially in locations removed from the influence of the Columbia River Gorge outflow. Patchy frost will also be possible during the overnight and early morning hours where skies remain clear and winds decouple.

Winds during the ridging period will generally be light across most of the forecast area, though notable exceptions are expected in and near the Columbia River Gorge. Strengthening east-west pressure gradients will lead to breezy easterly gap winds, particularly on Wednesday and Thursday. Deterministic and ensemble guidance suggests pressure gradients between Troutdale and the Dalles will range from approximately -5 to -6 mb on Wednesday and increasing overnight to around -7 to -8 mb by Thursday morning. Few members indicate gradients as strong as -9 mb during Thursday morning. Probabilities for wind gusts exceeding 35 mph are around 15 to 25 percent in the far eastern Portland and Vancouver metro area on Wednesday and 25 to 35 percent on Thursday. Additionally, there is a 70 to 90 percent chance for gusts to exceed 35 mph within the western Gorge and adjacent terrain. Exposed ridgetops such as Crown Point and Three Corner Rock have a 30 to 50% chance of experiencing gusts exceeding 50 mph.

Away from the Gorge, persistent ridging, light winds, and low mixing heights will contribute to stagnant air conditions. An Air Stagnation Advisory remains in effect from early Tuesday morning through Friday afternoon for the southern Willamette Valley, east- central Willamette Valley, north and central Oregon Cascade foothills, north Clark County lowlands, and the lower Columbia and Cowlitz River valleys. The Portland and Vancouver metro area and the west-central Willamette Valley were excluded from the advisory due to better low-level mixing associated with easterly Gorge winds, which gradually veer more northerly farther south into the west- central valley. In addition to air quality concern, the stagnant pattern will continue to support fog and frost potential across interior valleys throughout the week, with front potential most notable on Thursday and Friday.

Attention then turns to the weekend, when confidence decreases regarding the timing of the next precipitation event. Ensemble guidance indicates a return to wetter conditions is likely, though timing varies. Approximately 75 percent of ensemble members bring precipitation back into the region by Saturday afternoon, while others delay the onset until Sunday morning. Most ensemble guidance also suggest a transition toward cooler temperatures through the weekend and into next week. While details remain uncertain, the overall pattern suggests a cooler and wetter regime developing late this weekend into early next week. ~12

AVIATION

Fog/low stratus formed across portions of the Willamette Valley and lower Columbia and Cowlitz County valleys this morning, creating pockets of IFR/LIFR conditions. Lower vis and cigs are beginning to improve as of 17z Tue, and all locations should improve to VFR conditions by 19-20z Tue. Then VFR conditions through the rest of the day before fog formation is possible again tonight into Wednesday morning across the inland valleys. Conditions expected to begin deteriorating to MVFR/IFR between 06-09z Wed with more widespread IFR/LIFR vis by 10-12z Wed. Chances for fog formation are much lower (10-15%) around KPDX due to continued east winds from the Columbia River Gorge. Any lowered conditions should improve to VFR by 18-21z Wed.

Winds will remain easterly through the Columbia River Gorge. stronger than yesterday with gusts around KTTD reaching 25-30 kt. Within the Willamette Valley, winds above 1000 ft AGL will be southerly while at the surface winds will be variable.

KPDX AND APPROACHES...Fog at KPDX is expected to improve to VFR by 18-19z Wed. Winds become easterly around 18z Wed less than 10 kts. After 06-09Z Wed, there's a 10-15% chance of fog reformation. If winds remain easterly then there fog likely won't reform. However, if winds shift northerly or become variable, chances increase for fog formation. -03

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, and a Small Craft Advisory will need to be considered. -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 until 7 AM PST this morning for PZZ210- 251>253-271>273.