textproduct: Portland

SYNOPSIS

A slow-moving cold front will move southeastward across southwest WA and northwest OR late Friday afternoon through Friday night, spreading light rain southward from the Columbia River down to Lane County. Isolated post-frontal showers early Saturday morning will diminish quickly as surface high pressure builds into the area, bringing increasing sun on Saturday. This pattern will then hold steady for a few days, bringing cooler and drier conditions Saturday afternoon through Monday. A stronger Pacific cold front arrives sometime between Monday night and Tuesday evening, bringing widespread rain and breezy winds to the area. Snow levels fall behind the front late Wednesday, however the bulk of the precipitation associated with this system should be finished by the time snow levels fall below the Cascade passes. Minimal snow accumulation and travel impacts are expected at the passes.

LONG TERM...Tuesday through Thursday night

Confidence remains high for widespread rain and breezy winds Tuesday into Wednesday as a stronger Pacific cold front moves inland. Model ensemble guidance continues to show uncertainty on timing and exact precipitation amounts. However, confidence is high that river flooding is not expected, even in the wettest possible scenario. This front could arrive as early as early Monday night or as late as Tuesday evening, with the most likely arrival time being sometime between Tuesday afternoon and Tuesday evening. As a result of these timing differences, the NBM 10-90th percentile for snow levels continues to show large spread on Tuesday, ranging from 5000-10000 ft. Large model spread is evident for surface temperatures as well, with highs potentially ranging anywhere from the lower 50s to upper 60s. The outcome will be dependent on the cold frontal timing; if the front holds off until late in the day, surface temperatures will be well into the 60s with snow levels well above pass level. If the front arrives Monday night or Tuesday morning, temperatures will be in the lower 50s with snow levels approaching pass level. Regardless of the exact arrival time, confidence is high that snow levels will drop considerably behind the cold frontal passage, bottoming out Wednesday night into Thursday morning well below the Cascade passes. That being said, ensemble guidance for QPF clearly shows precipitation beginning to end by that time, meaning conditions will trend colder and drier with little accumulating snow and travel impacts at pass level. NBMv5.0 probabilities for 6 inches of snow or more in a 48-hour period ending at 5am PDT Thursday continue to back this expectation up, as chances are only around 5% at Willamette and Santiam Passes, and 15% along highway 26 near Government Camp. Note that lowland snow is NOT expected given conditions will be drying out by the time temperatures become colder.

By Thursday afternoon, the weather system is expected to move out of the area with upper level flow becoming more zonal. Expecting dry conditions with clearing skies by midday Thursday, which could allow for temperatures to warm into the upper 50s. -23/03

AVIATION

Radar imagery as of early Friday afternoon shows a front moving southward and spreading a light band of rain into the Willamette Valley and central Oregon coast. As this front moves through, chances for MVFR CIGs will increase to 40-60% at any given hour through 03z Sat, with higher chances of 60-80% around KEUG after 03-06z Sat. Meanwhile LIFR/IFR CIGs persist along the coast through this evening. Tonight, dry conditions return behind the front, returning VFR conditions to most places by 12z Sat. Winds will gradually turn more northerly to northwesterly this evening as high pressure re-builds behind the frontal passage and remain under 10 kt.

KPDX AND APPROACHES...30-50% chance for MVFR CIGs at any given hour through 03-06z Sat as a weak front moves through the area and brings light rain. Winds gradually turn northwesterly this evening, remaining under 5 kt. -10

MARINE

High pressure will gradually build tonight, turning winds more northerly over the waters and remaining under 20 kt. A brief northwesterly swell will move in tonight, bringing marginal small craft seas of 8-9 ft at 10 sec. Therefore, a Small Craft Advisory has been issued for all waters from 11 PM Friday to 8 AM Saturday. An exception is the Columbia River Bar, where a Small Craft Advisory is in effect between 5-10 AM Saturday for a very strong ebb current that will bring seas around 9 feet. Outside of the period with the very strong ebb, seas along the Main Channel of the bar are slightly lower (7-8 ft), so this zone was excluded from the Small Craft Advisory that starts late Friday evening. Benign marine conditions continue through Monday as high pressure persists. Seas fall to 5-7 ft at 10 sec by late Saturday night, but re-build to 8-9 ft heading into Monday.

The next robust weather system returns Tuesday into Wednesday. This system will bring a return of breezy southerly winds across all waters. Current guidance suggests high confidence (60-80% chance) for widespread and frequent small craft wind gusts of 21 kt or greater, and a 10-30% chance for widespread and frequent Gale force wind gusts of 34 kt or greater on Tuesday. Chances for isolated Gale force wind gusts are around 50-75%, with the highest chances beyond 20 NM offshore. Seas on Tuesday and Wednesday are also expected to build to around 14-16 ft in response to increasing wind waves along with an increasing westerly swell. There's a 30-60% chance seas could exceed 15 ft with a 5-15% chance for seas exceeding 20 ft. Chances increase from south to north, peaking across the waters north of Cape Falcon. -10

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 5 AM to 10 AM PDT Saturday for PZZ210.

Small Craft Advisory from 11 PM this evening to 8 AM PDT Saturday for PZZ251>253-271>273.