textproduct: Portland

SYNOPSIS

High pressure will maintain dry and sunny weather over the region with chances for fog/frost overnight for most interior lowland valleys. Breezy easterly winds continue through the Columbia River Gorge and eastern Portland/Vancouver Metro Area through this afternoon, weakening tonight. Air stagnation concerns continue through Friday for portions of the Willamette Valley and southwest WA where there is little wind. The next frontal system arrives Saturday, returning widespread rain through Sunday. Rain turns into post-frontal showers early next week, with light snow showers returning to the Cascades.

DISCUSSION...Today through Wednesday

High pressure continues over the Pacific Northwest, maintaining dry, warm, and sunny conditions through Friday. Temperatures are forecast the peak in the upper 50s to low 60s across most areas, while places like the southern Willamette Valley and the Lower Columbia/Cowlitz Valleys remain a bit cooler due to fog and stratus that lingered into the late morning. On the contrast, some locations along the coast and Cascade foothills may reach the mid to upper 60s. A strong inversion around 1000-2000 feet is resulting in warmer conditions for the Cascade foothills, while light easterly winds downsloping from the Coast Range are bringing warmer temps to the coast. Overall, these high temperatures are around 10 to 15 degrees above normal for this time of year.

Offshore pressure gradients between Troutdale (KTTD) and The Dalles (KDLS) will maintain breezy easterly winds through the Columbia River Gorge, surrounding terrain, and eastern Portland/Vancouver Metro. The pressure gradient between KTTD- KDLS as of early Thursday afternoon is -8.6 mb, with guidance suggesting gradients of -6 to -8 mb continuing through this afternoon. Deterministic and ensemble high resolution guidance (HRRR/REFS) suggests pressure gradients will ease to around -3 to -5 mb after 5-7 PM this evening with winds weakening and frequent wind gusts falling below 40 mph. Wind Advisory for the eastern Portland/Vancouver Metro has been extended through 6 PM this evening.

For areas away from the influence of the easterly Gorge winds, winds will generally remain light. Calm winds and clear skies across interior lowland valleys will support another night of foggy and/or frosty conditions. Air stagnation will also be a concern through Friday afternoon for parts of the Willamette Valley and southwest WA lowlands. The Air Stagnation Advisory remains in effect through 10 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.

A notable pattern shift begins Saturday as an approaching Pacific frontal system pushes inland. Cloud cover will increase early Saturday, followed by a return of rain along the coast by late morning or early afternoon, then spreading inland through the evening. Mild temperatures from the initial warm frontal passage will keep snow levels above 6000-7000 feet, resulting in rain through the Cascade passes. Widespread rain continues Saturday night into Sunday as the trailing cold front moves through.

Behind the cold front Sunday night through Tuesday, rain transitions into post-frontal showers and cooler air will filter into the area. This will bring snow levels down to 3000-4000 feet and finally return some snow showers at pass-level in the Cascades. This upcoming Cascade snow appears sub-advisory, as chances for 6 inches of snow or greater in a 48 hour period ending 4 PM Tuesday are only 5-10% through the passes. Uncertainty remains in the forecast heading into the middle of next week, but most ensemble members suggests relatively drier conditions with some continuing chances for showers. -10/12

AVIATION

Dry southerly flow aloft as an upper level ridge of high pressure remains over the region into Friday morning with predominately VFR conditions as of 05z Friday. Another round of LIFR to IFR fog is likely (50-60% chance) across the southern and parts of the central Willamette Valley, as well as around KHIO, after 06- 12z Friday. The southern Willamette Valley will see LIFR/IFR conditions begin earlier around 06-08z Friday while lowered conditions could hold off until closer to 10-12z Friday in the central Willamette Valley. Elsewhere, chances for reduced vsbys due to mist or fog early Friday morning is lower, around 10-20%. A weakening offshore pressure gradient through the Columbia River Gorge will maintain breezy east winds into the east Portland metro through tonight with gusts up to 20-25 kts at KTTD. The upper ridge weakens and shifts east on Friday bringing increasing clouds after 00z Saturday with a 50-70% chance of MVFR ceilings forming along the coast after 02-04z Saturday.

KPDX AND APPROACHES...VFR conditions expected with scattered high clouds. Southeast winds around 10 kt decrease to below 6 kts and become variable after 08-10z Friday. East approaches will see stronger east winds with gusts up to 20-25 kt near KTTD through 12- 15z Friday. /DH/03

MARINE

Light southeast winds become more southerly later today as a weak area of low pressure drifts north. Southerly winds around 10-15 kt gradually increase on Friday as a front approaches the waters by Saturday. There is around a 40-60% chance for maximum wind gusts to exceed 34 kt on Saturday, but chances for widespread gales remains low. Winds are expected to ease Saturday night into Sunday.

A persistent southwest to westerly swell at around 13-15 seconds will maintain seas around 9 to 11 ft through the weekend. Seas likely build to around 11 to 13 ft on Saturday with the frontal passage. A more westerly swell arrives early next week with seas again around 9 to 11 ft. Will maintain the current Small Craft Advisory across all coastal waters including the Columbia River Bar through Friday afternoon. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory until 10 PM PST Friday for ORZ108- 115>118-123>125.

WA...Air Stagnation Advisory until 10 PM PST Friday for WAZ204-205.

PZ...Small Craft Advisory until 4 PM PST Friday for PZZ210-251>253- 271>273.