textproduct: Portland

SYNOPSIS

Mainly dry and mild conditions are expected across the region today, with periods of light rain confined to southwest Washington and far northwest Oregon. Confidence remains high for a prolonged stretch of dry weather and above-normal temperatures from Tuesday through at least Friday as a strong upper-level ridge builds over the Pacific Northwest. Uncertainty increases toward next weekend regarding the eventual breakdown of the ridge, though most guidance suggests precipitation may return by late Saturday.

DISCUSSION...Now through Sunday

A weak warm frontal boundary begins to lift northward across southwest Washington and far northwest Oregon this morning, bringing a period of light rain mainly north of a Lincoln City to Portland line. Locations south of this corridor are likely to remain dry, with probabilities of staying precipitation-free ranging from 80 to 95%. Elsewhere, dry conditions prevail under mild temperatures, with afternoon highs generally reaching the mid 50s to lower 60s.

Beginning Tuesday, deterministic and ensemble guidance remains in strong agreement that upper-level ridging becomes firmly established over the Pacific Northwest. This pattern will support an extended period of dry weather, above-normal temperatures, and generally light winds through at least Friday. One notable exception will be in the western Columbia River Gorge and far eastern portions of the Portland and Vancouver metro area, where increasing easterly pressure gradients will lead to gusty winds, particularly Wednesday and Thursday.

Ensemble guidance suggests surface pressure gradients between Troutdale and The Dalles will be around -5 to -7 mb during this time, possibly peaking up to -8 mb. Under stable low-level conditions, this setup would favor strong easterly winds through the Gorge, with potential wind gusts reaching up to 70 mph near Crown Point, 50 to 55 mph near Corbett, and 35 to 40 mph around Troutdale. In a possibly stronger but less likely scenario (peak of -8 mb), wind gusts could be an additional 10 mph stronger than the aforementioned potential wind gusts. Overall, these values (-7 and -8 mb) assume the stronger end of the projected gradient range; weaker gradients would result in correspondingly lower wind speeds. While some localized tree damage and isolated power outages are possible, impacts are not expected to be widespread.

Away from the Gorge, lighter winds and persistent ridging will promote stagnant conditions, particularly across the central and southern Willamette Valley. Calm winds and low mixing heights persisting for several consecutive days may lead to gradual air quality degradation. Forecast confidence is also lower regarding sky cover and temperatures in these valley locations, as inversion-driven low stratus and fog should be slow to clear, resulting in cooler daytime temperatures compared to surrounding higher terrain, the Coast Range, and the Cascades. That said, guidance suggests improved clearing and greater sunshine potential by late week, even within the central and southern valley.

Forecast uncertainty increases heading into Saturday and Sunday. Some model solutions maintain dry conditions under lingering ridging, while others introduce precipitation as the ridge begins to weaken. Current probabilities suggest a 20 to 40% chance of rain returning Saturday afternoon, and a 50 to 80% by late Saturday night. Regardless of the timing, rain will likely continue through Sunday and into the start of next week. ~12

AVIATION

Currently a drastic mixed bag of VFR and LIFR throughout the airspace as fog develops under clear skies in the south Willamette Valley and somewhat develops in the rest of the Valley under broken cloud cover. With a warm front approaching the northern part of the airspace (generally north of a line from KTMK and KUAO), conditions will vary by location. For the northern part, any terminals with LIFR fog conditions should improve back to MVFR/VFR by 14Z Monday as the front brings rain and improved mixing. For terminals south of the aforementioned line, the front will likely miss the area and LIFR fog conditions will likely persist until 18Z Monday, then improve to VFR thereafter. As for the coastal terminals, expect a mix of VFR/MVFR CIGs through most of the TAF period, with the exception of KAST more susceptible to IFR conditions after 18Z Monday (20-30% chance of IFR conditions). Around 06-09Z Tuesday, clearing skies and wet ground will again bring the potential for LIFR fog at most inland terminals. Otherwise, expect generally southerly to southeasterly winds between 5-10 kt throughout the airspace, with the exception of gusts up to 20-25 kt at KAST between 18-23Z Monday.

KPDX AND APPROACHES...A weak frontal system moves northeastward towards southwest Washington, bringing the potential for rain at the terminal between 15-23Z Monday. In the meantime, there is a 10-20% chance for deteriorated conditions as fog tries to develop near the terminal (recent obs of LIFR and MVFR). When the front arrives and provides rain and mixing, expect mostly VFR CIGs with a 10-20% chance of MVFR CIGs through most of the TAF period. Around 06-09Z Tuesday, clearing skies and wet ground will bring another chance for fog formation. ~12

MARINE

A weak front moves northeasterly through the waters between Cape Shoalwater and Cape Falcon this morning, bringing increased south winds with gusts 20 to 25 kts. Winds will decrease below 20 kts again late today and remain 5 to 10 kt through the rest of the week. Expect wave heights of 10 to 13 feet at 11 to 13 seconds through this evening. Due to a combination of winds and seas, a Small Craft Advisory is in effect through 1 AM PST on Tuesday. High pressure builds over the waters Tuesday with southerly winds less than 10-15 kts and seas less than 10 ft. By Wednesday, an increasing westerly swell is forecast to push into the waters, increasing seas to around 10-12 feet around 12-16 seconds through the end of the week. -12/03

BEACH HAZARDS

A long period westerly swell will result in a high threat for sneaker waves persisting through this afternoon. Waves can run up significantly farther 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. -42

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 1 AM PST Tuesday for PZZ210-251>253- 271>273.