textproduct: Portland

SYNOPSIS

A brief period of high pressure will maintain dry conditions through early Wednesday before the next frontal system approaches from the Pacific. This will return cooler temperatures and wet weather by Wednesday night. Snow levels are expected to fall Thursday into early Friday, allowing for accumulating snow across the Cascade passes. The active and unsettled pattern is expected to persist through the weekend and likely into early next week.

DISCUSSION...Now through Monday

Todays weather remains quiet under a weak shortwave ridge extending across the Pacific Northwest. Conditions will stay dry with seasonable temperatures and light northerly flow. Mid and high-level clouds will gradually increase through the afternoon as the next system draws closer from the Pacific. Winds turn southerly overnight and into Wednesday as the ridge slides east and the approaching trough deepens offshore.

By Wednesday afternoon, ensemble guidance continues to suggest a broad trough advancing from the northeast Pacific toward the region. There remains moderate confidence in the surface low tracking offshore near the southern Oregon and northern California border. The GFS and Canadian solutions remain consistent with this southern track, while the ECMWF continues to entertain a slightly more northern track. Should the ECMWF solution verify, gustier southerly winds would become more likely along the central Oregon coast-particularly from Cape Foulweather south to Florence, and possibly extend into portions of the southern Willamette Valley. Localized impacts such as downed tree limbs or isolated power outages would be possible. That said, given current ensemble trends aligning more closely with the GFS and Canadian guidance, this scenario appears increasingly unlikely.

An occluded front tied to the system will move through northwest Oregon and southwest Washington late Wednesday night into early Thursday, bringing a return to widespread rain. The heaviest precipitation should remain displaced farther south toward far northern California and southwest Oregon. Between 4 PM Wednesday and 4 PM Thursday, probabilities for rainfall exceeding one inch sit around 10-20% for interior valleys and 50-60% along the coast, Coast Range, Willapa Hills, and north Oregon/south Washington Cascades. Winds will also strengthen Thursday while turning more southerly to southwesterly, though probabilities for gusts exceeding 45 mph remain limited to around 10-15% in interior valleys and 20-30% along the coast.

Behind the front, a cooler post-frontal air mass arrives Thursday. Forecast confidence decreases late Thursday into Friday as model solutions diverge regarding the degree of cooling and snow levels. Current models suggest snow levels could range from 3000 and 5000 feet, placing the Cascade passes near the transition zone where precipitation type could vary. While there is high confidence that snow will occur at pass level, impacts are likely to remain limited given the marginal temperatures aloft. Between 4 AM Thursday and 4 AM Saturday, there is a 30-40% chance of six inches or more of snow at Santiam and Willamette Passes, and around a 20-30% chance near Government Camp.

Rain continues Thursday as the low lingers over the region, and a few embedded thunderstorms cannot be ruled out, though instability appears minimal as CAPE values are expected to be marginal. The broader pattern stays active through the end of the week and into the weekend. A warm front lifting north on Saturday will raise snow levels and transition precipitation back to rain in the Cascades, followed by another cold front on Sunday that will lower snow levels once again, returning snow chances to the passes. Elsewhere, periodic rain is expected to persist through Monday under continued southwest flow aloft. ~12

AVIATION

As of 22z Tue, VFR conditions have returned to the coast and inland locations south of KSLE. A stratus shield remains in place in the Willamette Valley south of KSLE causing IFR/MVFR CIGs and VIS. Confidence is increasing that this stratus deck will remain in place through most of the night and into Wednesday morning. There could be a period of a few hours after 00z where it lifts to VFR before filling back in as temperatures cool overnight. Elsewhere, there is a 20-30% chance of MVFR/IFR CIGs returning after 09-12z Wed. Any lowered conditions should become VFR by 18-21z Wed. Winds will generally light and variable under 6 kt across all terminals. A weather system approaching the region Wednesday will bring a slight chance of -SHRA or -DZ to the coast after 09-12z Wed and inland after 18z Wed.

KPDX AND APPROACHES...Predominately VFR conditions with high- level BKN/OVC clouds. There's a low (20-30%) chance of MVFR CIGs returning after 09-12z Wed. Northwesterly winds under 5 kt turn easterly after 02z Wed. -03

MARINE

High pressure over the waters will maintain calm marine conditions through early Wednesday morning. Winds will generally remain northerly with gusts less than 10 kt and seas of 5-8 ft at 10-13 sec.

The next low pressure system in the NE Pacific will return breezy southerly winds and building seas Wednesday into Thursday, with high confidence for another period of conditions hazardous to small craft. As this system swings a front through the waters, there's a 60-80% chance of frequent southerly wind gusts exceeding 21 kt and a greater than 90% chance that seas build above 10 ft by late Wednesday night/early Thursday morning. Therefore, a Small Craft Advisory continues for all marine zones including the Columbia River Bar from 10 AM Wednesday to 10 AM Thursday. During this time, there is also a 40-60% chance for isolated gale-force wind gusts greater than 34 kt, with the highest chances across the outer waters beyond 20-30 NM. Active weather continues into the end of the week with additional systems moving through the waters. -10/03

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 10 AM Wednesday to 10 PM PST Thursday for PZZ210-251>253-271>273.