textproduct: Portland

SYNOPSIS

Showery conditions continue today as the post- frontal system gradually shifts east of the region. Cascade snow will persist this morning, with hazardous travel conditions ongoing at the passes. Showers decrease in coverage this afternoon. High pressure builds Friday and persists through Monday, bringing dry weather, increasing sunshine, and warmer temperatures. A weak upper-level trough may return light precipitation chances Tuesday.

DISCUSSION...Now through Tuesday

In the wake of last nights frontal passage, a cool and unstable post-frontal pattern is in place early this morning. Scattered rain showers continue across northwest Oregon and southwest Washington with more persistent snow showers over the Cascades. Coverage remains greatest through the morning hours before gradually decreasing this afternoon as the upper trough shifts farther east.

Winter Storm Warnings remain in effect until 5 PM today for the Marion, Linn, and Lane County Cascades, with Winter Weather Advisories continuing for the north Oregon and south Washington Cascades. Snow levels are hovering between 2000 to 2500 ft this morning, supporting continued accumulating snow at pass level. Periods of moderate to briefly heavy snowfall remain possible through late morning. Ensemble guidance continues to support elevated probabilities of snowfall rates approaching or exceeding 1 inch per hour at Santiam and Willamette Passes early today before 9 AM, with lower but still impactful rates on Highway 26 near Government Camp. Travel conditions will remain hazardous through the morning hours before gradually improving later today. If you have plans to travel through the Cascades, make sure to pack an emergency supply kit and refer to ODOT/WSDOT for the latest road conditions and restrictions.

Shower activity will continue to diminish late this afternoon and evening as the upper trough shifts east. Clearing skies and lighter winds tonight into Friday morning will promote efficient radiational cooling. Temperatures are expected to fall into the low to mid 30s across most of the interior valleys, supporting areas of frost. Given the growing season is underway and the current overnight temperatures forecasted, a Freeze Watch has been issued from Thursday evening through Friday morning for the Upper Hood River Valley, South Willamette Valley, and Cascade foothills of Marion, Linn, and Lane Counties. Additionally, a Frost Advisory has been issued from 9 PM Thursday to 10 AM Friday for the Coast Range, Willapa Hills, the Cowlitz, Tualatin, and Central Willamette Valleys, the Clackamas Foothills, and the West Gorge above 500 ft. Probabilities for temperatures below 36 degrees range from 50-70% along much of the I-5 corridor and 70-90% across the Upper Hood River Valley. Chances of dropping below freezing are highest in the southern Willamette Valley (20-40% chance from Albany to Eugene) and higher elevations of the Hood River Valley (50-60% chance in Parkdale).

While guidance currently does not suggest very notable chances (mostly <10% with up to 20% in a few areas) of fog development Thursday night into Friday, clearing skies and wet ground are favorable conditions for fog. We will likely have to keep an eye out for fog development later Thursday night into Friday morning.

Friday through Monday, ensemble guidance remains in strong agreement that upper-level ridging will build over the Pacific Northwest. This will support dry conditions, increasing sunshine, and a notable warming trend. Highs climb into the upper 60s to low 70s by Saturday, then low to mid 70s on Sunday and Monday. There is high confidence that temperatures exceed 70 degrees across the interior lowlands Sunday and Monday. By Tuesday, a weak upper-level trough may approach, introducing slightly cooler temperatures and a low-end (20-40%) chance of light precipitation. With that said, each individual ensemble clusters have shifted to a more dry solution on Tuesday. Confidence regarding the weather pattern beyond Monday remains low.~12

AVIATION

Rain showers continue to circulate across the airspace, resulting in conditions bouncing between VFR and MVFR as of 11Z Thu. Expect these bouncing conditions to trend towards mostly VFR by 15Z Thu and persistent VFR by 18-19Z Thu, as showers within the lowlands begin to dissipate towards 17-21Z Thu. Rain showers over terrain features will likely continue until around 00-03Z Fri. Chances for thunderstorms around 5-10% remain over the airspace until 21Z Thu. Any showers passing directly overhead a terminal this morning will likely degrade conditions to MVFR. Southerly to westerly winds will likely remain around 8-14 kt with gusts up to 20-25 kt possible until 00-03Z Fri, decreasing thereafter.

While guidance currently does not suggest any notable chances of fog development Thursday night into Friday, clearing skies and wet ground are favorable conditions for fog. We will likely have to keep an eye out for fog development later Thursday night into Friday morning.

KPDX AND APPROACHES...Rain showers continue to bring a mix of MVFR/VFR conditions. As chances for rain showers decrease towards 18Z Thu, so will MVFR chances. Expect these bouncing conditions to favor VFR by 15Z Thu, then becoming persistently VFR by 18Z Thu. Southwesterly winds around 8-12 kt will shift more westerly towards 00-03Z Fri and decrease to below 8 kt thereafter. ~12

MARINE

Westerly swells will continue to push seas into the 9 to 13 ft range through the morning, then gradually decrease below 10 ft into tonight. Hazardous Seas Warnings for all the waters will continue through at least 8 AM today, then transition to Small Craft Advisories for elevated seas until tonight. The Columbia River Bar (PZZ210) will maintain a Small Craft Advisory for elevated seas and a strong ebb cycle.

As the latter part of the week approaches, conditions start to ease as high pressure rebuilds and northerly winds return to the coastal waters. Expect seas around 5 to 6 ft Friday and through the weekend. ~12/42

PQR WATCHES/WARNINGS/ADVISORIES

OR...Frost Advisory from 9 PM this evening to 10 AM PDT Friday for ORZ104>110-113>115-119-123.

Freeze Watch from this evening through Friday morning for ORZ116>118-121-124-125.

Winter Weather Advisory until 5 PM PDT this afternoon for ORZ126.

Winter Storm Warning until 5 PM PDT this afternoon for ORZ127- 128.

WA...Frost Advisory from 9 PM this evening to 10 AM PDT Friday for WAZ203>205-208.

Winter Weather Advisory until 5 PM PDT this afternoon for WAZ211.

PZ...Small Craft Advisory until 8 PM PDT this evening for PZZ210.

Hazardous Seas Warning until 8 AM PDT this morning for PZZ251>253-271>273.

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