textproduct: Portland

SYNOPSIS

A strong and persistent area of high pressure continues to dominate the Pacific Northwest this afternoon and is expected to remain firmly in place through much of the week. This pattern will support an extended period of dry weather, light winds, and temperatures running above seasonal norms. In contrast, portions of the interior lowlands, particularly the Willamette Valley, will likely experience cooler daytime temperatures due to recurring fog and low stratus beginning early Tuesday and persisting at times through midweek. Later in the week, cold easterly winds will flow through the Columbia River Gorge, possibly bringing overnight freezing conditions to the Upper Hood River Valley.

DISCUSSION...Now through Monday

Little has changed in the overall forecast since early this morning. Any light precipitation that occurred earlier this morning over parts of the southwest Washington coast and the far north Oregon coast has ended, and dry conditions are now firmly established across the forecast area. Looking ahead, weather through the next 7 days or so will continue to be governed by an anomalously strong upper-level ridge centered over the Pacific Northwest and northeast Pacific. Ensemble guidance remains in strong agreement that this ridge will persist through the work week and into the weekend, resulting in calm, dry, and relatively mild conditions for mid-January.

The upper-level pattern remains highly amplified, with the ridge positioned between troughing well offshore to the west and another trough over the central to eastern United States. This configuration reflects a classic blocking regime, initially resembling an omega block and potentially evolving toward a Rex block later in the week. Forecasted 500 mb heights continue to peak around 585 to 590 dm over southern Oregon and northern California, falling within the upper end of the historical distribution for this time of year and underscoring the unusual strength and longevity of the ridge.

While some guidance indicates weak shortwave energy may rotate around the periphery of the ridge this weekend, these features are expected to remain moisture-starved and should not disrupt the dry pattern. The timing of a more meaningful pattern change remains uncertain, through a growing subset of ensemble members suggests a potential breakdown of the ridge and a return of precipitation sometime around January 21 to 23. Until then, dry weather is favored each day from Tuesday through at least Sunday. One exception to the otherwise calm conditions may be increasing easterly winds in the western Columbia Gorge, Cascades, and portions of the east Portland and Vancouver metro area late Thursday into Friday as offshore pressure gradients strengthen. With these winds originating from the Columbia Basin, expect much cooler temperatures as a result for areas around the Gorge such as the Upper Hood River Valley. The Upper Hood River Valley could see overnight minimum temperatures fall below freezing in the mornings Thursday through at least Sunday (20-40% chance for overnight temps to fall below 32 F).

Fog and low stratus remain a key concern for the interior lowlands. Clear skies and light winds tonight will again promote efficient radiational cooling, with widespread fog development expected late tonight into Tuesday morning. Probabilities for surface visibilities of one-half mile or less remain high, especially across inland valleys. While some fog may lift or thin during the afternoon hours, low clouds are likely to persist through much of the day Tuesday and possibly into Wednesday. With moderate to high confidence for widespread fog, a Dense Fog Advisory has been issued from 10 PM tonight to 11 AM Tuesday. Otherwise, high temperatures in valley locations below roughly 1000 feet are expected to remain relatively cool, generally staying below 50 degrees. In contrast, areas above the valley inversions, including the Coast Range, Cascade foothills, and higher terrain will experience significantly warmer and sunnier conditions, with highs commonly in the upper 50s to mid 60s. Coastal areas will also trend mild, with afternoon temperatures well into the 50s and locally near 60 degrees.

The prolonged period of high pressure will also favor stagnant air conditions with limited vertical mixing. These conditions are conducive to the accumulation of air pollutants near the surface, particularly in valley locations prone to inversions. An Air Stagnation Advisory remains in effect for the Willamette Valley and may be expanded as the week progresses. State air quality agencies continue to recommend avoiding outdoor burning and minimizing the use of residential wood-burning devices. Residents are encourages to monitor local air quality information and comply with any burn restrictions issued by their local agencies. ~12/23

AVIATION

High pressure builds over the region through the next several days. High clouds remain over the region with MVFR CIGs along the north Oregon and south Washington coast. Around KAST, flight categories have been variable bouncing between IFR, MVFR and VFR due to a sea breeze and lowered CIGs. In general will see this similar pattern, but have incorporated a stronger trend towards MVFR conditions. Further inland and south, VFR conditions prevail. Overnight, we will experience diurnal cooling due to clear skies. Widespread fog is expected in the lowlands which may become dense at times. Areas most at risk will be terminals within the Willamette Valley and Vancouver-metro. Areas that will experience easterly winds (KTTD, K4S2, etc) have significantly lower probability of fog formation.

KPDX AND APPROACHES...Expect VFR flight conditions to give way to fog and low stratus after 06z Tuesday, resulting in an abrupt drop to LIFR/IFR cigs and visibilities (50-60% chance for surface visibilities of 1/2 SM or lower by 12Z Tuesday). -27

MARINE

Steep and hazardous seas through the remainder of the day with seas as high as 14 ft at 14 seconds. Around the Columbia River Bar, waves of 10-14 ft have been observed. Elsewhere, a Small Craft Advisory remains in effect over the southern waters where seas are relatively lower compared to the northern waters, but still high enough to be hazardous for small craft.

Winds become relatively light Monday night into Tuesday as high pressure strengthens over the region, with seas hovering between 8-10 ft. Northerly winds then develop late Wednesday into Thursday as a thermal trough builds northward up the south Oregon coast, with seas increasing by a foot or two. Northerly winds will likely peak in strength on Thursday with gusts up to 25 kt over the central and southern outer waters.

Very calm conditions are expected Friday through Sunday 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

BEACH HAZARDS

There is a high threat of sneaker waves at the coast through this afternoon due to a continued long-period southwesterly to westerly swell. Sneaker waves can create potentially life-threatening conditions as waves may run farther up the beach than expected. These waves can easily catch people off guard and cause beachgoers to be knocked off their feet and pulled out into cold ocean waters. Never turn your back to the ocean. Caution should be used when in or near the water, and those with children should be especially watchful. Sneaker waves can lift or roll large, heavy logs or rocks which can lead to serious death or injury. Keep off rocks, logs, and jetties near the waterline on beaches. -23

PQR WATCHES/WARNINGS/ADVISORIES

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

Dense Fog Advisory from 10 PM this evening to 11 AM PST Tuesday for ORZ109>111-114>118.

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

Dense Fog Advisory from 10 PM this evening to 11 AM PST Tuesday for WAZ205-206.

PZ...Hazardous Seas Warning until 6 PM PST this evening for PZZ210.

Small Craft Advisory from 6 PM this evening to 10 AM PST Tuesday for PZZ210.

Hazardous Seas Warning until 4 PM PST this afternoon for PZZ251- 252-271-272.

Small Craft Advisory until 10 AM PST Tuesday for PZZ251-252-271- 272.

Small Craft Advisory until 4 AM PST Tuesday for PZZ253-273.