textproduct: Portland

SYNOPSIS

A strong and persistent area of high pressure will remain locked in place over the Pacific Northwest through 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 fog and low stratus during the overnight and morning hours over the next few days. Late in the week, cold easterly winds will flow through the Columbia River Gorge, with sub-freezing overnight temperatures likely in the Upper Hood River Valley and much of the Willamette Valley.

UPDATE...Tuesday morning

Observed surface temperatures over the southern Willamette Valley this morning are a few degrees colder than expected, falling to 28-30 degrees. At the same time, this area continues to see areas of dense fog with surface visibilities around 1/4 mile or less. Given the sub-freezing temperatures in place and the potential for light riming of ice on surfaces, have replaced the Dense Fog Advisory for this area with a Freezing Fog Advisory. As such, a few slick spots will be possible this morning, mainly for locations within the Willamette Valley to the south of Salem that continue to observe dense fog. Still expecting fog to lift by noon, with temperatures warming above freezing. -23

DISCUSSION...Now through Monday

Little has changed in the overall forecast since yesterday afternoon. Weather conditions now through Jan. 20th will be fairly consistent each day due to an anomalously strong upper-level ridge centered over the Pacific Northwest and northeast Pacific. Models still suggest 500 mb heights will peak around 585-590 mb along the ridge axis today into tonight in western OR/WA, highest to the south. These values are around the 99th percentile compared to climatology, signifying the unusual strength of this high pressure system. Ensemble guidance remains in strong agreement that this ridge will persist through the upcoming weekend, resulting in calm, dry, and relatively mild conditions for mid-January, especially for elevations above 1000 feet where fog and low stratus will not be a factor. 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 guidance (around 75%) suggests a potential breakdown of the ridge and a return of at least light precipitation sometime between January 21 to 23. Until then, dry weather will continue each day from Tuesday through at least Monday.

One exception to the otherwise calm conditions will 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 the airmass originating from the Columbia Basin, expect cooler temperatures to advect into the Upper Hood River Valley, Columbia River Gorge and Willamette 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). Low temperatures are forecast to drop to near freezing across most of the Willamette Valley as well.

Otherwise, dense fog and low stratus remains the main concern for the interior lowlands this week, especially during the overnight and morning hours today through Thursday when reduced visibilities will pose a travel hazard for many commuters. Note webcam and surface weather observations from 3 AM Tuesday showed widespread fog over most of the Willamette Valley with visibilities falling below 1/2-1 mile. Fog was also being observed in the Coast Range lowlands. Therefore, a Dense Fog Advisory remains in effect for these areas through this morning. In contrast, areas above the valley inversion, including the Coast Range, Cascade foothills, and higher terrain in the Cascades 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 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. -23/12

AVIATION

High pressure builds over the region through the next several days, bringing dry and calm conditions with valley fog and low stratus. Fog and low stratus are expected to lift by 20-23z Tuesday, however there is a 20-30% chance low stratus lingers through the day Tuesday despite improving surface visibilities. The central and southern Willamette Valley from KSLE to KEUG have the highest chance of low stratus continuing through the day, or with clearing only lasting a few hours in the late afternoon and early evening. Regardless of the outcome, low stratus and fog will quickly expand again Tuesday night after 03-06z. Fog may be dense at times Tuesday morning and again Tuesday night through Monday morning, with surface visibilities dropping to 1/4-1/2 SM or less. Note dense freezing fog is occurring at KEUG with surface visibilities at 1/4 SM and temperatures around 28-29 degrees. This is cold enough for a thin layer of rime ice to form on surfaces, which can result in slick spots.

Confidence is increasing that KTTD and KONP will stay fog free this morning through this afternoon with clear skies, however there is still a 10-20% chance fog does briefly form at KTTD towards 15z Tuesday.

KPDX AND APPROACHES...While fog has not formed at the KPDX terminal yet this morning, probabilities increase to near 60% for fog and surface visibilities at 1/2 SM or lower by 15z Tuesday. While not guaranteed to happen, suspect this will occur as fog is currently being observed around 5-10 miles northwest of the KPDX terminal and winds remain calm. Assuming fog does develop at the terminal by 15z Tuesday, expect conditions to improve to VFR by 20-21z Tuesday, however there is a 20% chance low stratus will continue through the daylight hours. Regardless of any clearing that does or does not occur, expect fog and low stratus to return by 06z Tuesday with LIFR to IFR flight conditions thereafter. -23

MARINE

Winds have become relatively light this morning as high pressure strengthens over the region, with seas hovering between 10-12 ft this morning before lowering to 8-10 ft this afternoon. Northerly winds 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 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. -23

PQR WATCHES/WARNINGS/ADVISORIES

OR...Dense Fog Advisory until 11 AM PST this morning for ORZ104-105- 109>111-114-115.

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

Freezing Fog Advisory until 11 AM PST this morning for ORZ116>118.

WA...Dense Fog Advisory until 11 AM PST this morning for WAZ205-206.

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

PZ...Small Craft Advisory until 10 AM PST this morning for PZZ210-251-252-271-272.