textproduct: Portland

SYNOPSIS

Strong high pressure remains firmly established over the Pacific Northwest through at least early next week. This will maintain an extended stretch of dry weather through at least Jan. 20-21. For elevations below 1000 ft, a persistent low-level inversions will support widespread dense fog, low stratus and cool temperatures today through tomorrow morning with any improvement expected to be brief this afternoon. Elevations above 1000 ft will remain much warmer with clear skies. The stagnant pattern will also promote areas of degraded air quality through Thursday. The pattern becomes less stagnant Thursday night into Friday as offshore flow strengthens, bringing breezy east winds to the Cascades, western Columbia River Gorge and eastern Portland metro. This should also bring an end to fog and low stratus in the lowlands, except for the southern Willamette Valley which could see fog linger overnight.

DISCUSSION...Now through Tuesday night

Anomalously strong high pressure will remain locked in place over the region now through at least Jan. 20-21, bringing dry weather each day with a persistent low-level temperature inversion in place. This temperature inversion will result in vastly differing weather conditions for elevations below 1000 feet compared to elevations above 1000 feet. Expect clear skies and warmer temperatures above 1000 feet and cooler, cloudier and foggier conditions below 1000 feet. For the lowlands below 1000 feet, calm winds and residual low-level moisture will result in widespread dense fog and low clouds this morning and again Wednesday night into Thursday morning. While fog is likely to lift by the early to mid afternoon, there is a 40-50% chance low clouds will linger through the day and evening before lowering back to the surface as fog Wednesday evening/night. The other potential outcome is for both fog and low clouds to scatter out by the mid to late afternoon, before quickly redeveloping in the evening. Regardless of the outcome, gloomy conditions will be predominant most of the time, which will keep temperatures chilly in the lowlands (upper 20s to mid 30s in the morning, 40-50 degrees during the afternoon). Note freezing fog is occurring this morning across portions of the southern and central Willamette Valley where surface temperatures are below freezing, which means a thin layer of rime ice is possible on exposed surfaces where freezing fog is most dense. With widespread surface visibilities at or below 1/4 to 1/2 mile this morning and likely again Wednesday night/Thursday morning, be prepared to drive slowly if you must commute as fog will pose a travel hazard. Pedestrians and bikers should wear reflective clothing. Note with the continued stagnant airmass in place, air quality may become degraded at times over the lowlands through Thursday night.

As mentioned earlier, elevations above 1000 feet will be warmer as these elevations will be poking above a low-level temperature inversion and will see plenty of sunshine. In fact, high temperatures in the mid 50s to mid 60s are in the forecast for the Cascade foothills, Coast Range lowlands and central Oregon coast, warmest in the Lane County Cascade foothills.

By Thursday night/Friday morning, models and their ensembles continue to suggest low-level offshore flow will strengthen, bringing breezy east winds to the Cascades, western Columbia River Gorge and eastern Portland metro. Light east winds are forecast to reach the coast as well as a surface thermal trough along the southern/central OR coast begins to shift slightly offshore. This will bring an end to fog and air stagnation for these locations. Meanwhile, the central and southern Willamette Valley may see at least some patchy fog occur again Thursday night/Friday morning as this wind-sheltered area will have minimal influence from the increasing offshore flow. Offshore flow will most likely remain in place Friday night through the upcoming weekend, albeit in a weaker state. This should maintain mostly clear conditions over the area, aside from the central and southern Willamette Valley where low stratus and fog concerns will likely persist. Note overnight temperatures are expected to remain chilly each night through at least Monday night, particularly in the Willamette Valley and Upper Hood River Valley where below freezing low temperatures are expected. That said, apparent temperatures are not forecast to be cold enough to warrant any cold weather advisories at this time. -23

AVIATION

High pressure will persist over the region over the next several days, with widespread dense fog and low stratus across the lowlands through at least 20z Wednesday. Surface temperatures are below freezing at KSLE, KEUG, and KUAO this morning, which means fog droplets will have the potential to freeze onto exposed surfaces as a thin layer of rime ice.

While surface visibilities will most likely lift from LIFR to MVFR/VFR by 22-23z at the latest, there is 50% chance low stratus will persist beyond that time, continuing into Wednesday evening. There is currently a 40-50% chance this outcome occurs at all inland terminals, with the highest chance at KSLE and KEUG. The other potential outcome would be fog and low clouds both clear out by the mid to late afternoon, before quickly redeveloping early in the evening. Regardless of any clearing that does or does not occur Wednesday afternoon, expect another round of widespread dense fog and low stratus across the interior lowlands from 06z-12z Thursday (60-80% chance).

KPDX AND APPROACHES...Fog will continue to reduce surface visibilities down to 1/4 SM or less through 18-20z Wednesday. Visibilities should improve to either MVFR or VFR by 22z Wednesday, with low clouds attempting to scatter out. However, there is a 40% chance low clouds do not scatter out at all today, with cigs staying below 2000 ft. Even if low clouds do scatter out this afternoon, expect fog and low stratus to quickly redevelop by 06-09z Thursday. -23

MARINE

High pressure will remain in place through the week with seas hovering close to 10 ft today and tonight. A surface thermal trough develops along the southern and central Oregon coast on Wednesday before pushing slightly offshore on Thursday. This will bring increasing northerly winds late Wednesday, with gusty offshore gap winds on Thursday for the inner waters and the Columbia River Bar. Small craft advisories are in effect for all marine zones today as northerly winds begin to increase. The northern waters drop off Wednesday night as seas and winds at least temporarily fall below small craft advisory criteria to the north of Cape Falcon. Meanwhile, breezy north winds up to 25-30 kt will be in place over the central and southern waters, where a small craft advisory is in effect through Thursday evening.

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 into early next week with seas around 4 to 6 ft. -23

PQR WATCHES/WARNINGS/ADVISORIES

OR...Dense Fog Advisory until noon PST today for ORZ101-102-104-105- 108>112.

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

Freezing Fog Advisory until noon PST today for ORZ114>118.

WA...Dense Fog Advisory until noon PST today for WAZ201-202-204>207.

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

PZ...Small Craft Advisory from 11 AM this morning to 5 PM PST this afternoon for PZZ210.

Small Craft Advisory until 1 AM PST Thursday for PZZ251-271.

Small Craft Advisory until 10 PM PST Thursday for PZZ252-253-272-273.