textproduct: Portland

SYNOPSIS

Mostly dry weather through Tuesday, except for chances for light rain/snow showers over the Cascades. A stronger frontal system arrives Wednesday, returning widespread rain, Cascade snow, and breezy southerly/southwesterly winds through Thursday. Accumulating snow with hourly snowfall rates likely over 1 in/hr at times along the Cascade passes Wednesday evening/night will bring hazardous travel conditions, especially along Santiam and Willamette Pass. High pressure re-builds Friday into the weekend, returning dry and sunny weather with warming temperatures.

SHORT TERM...Now through Wednesday night

Dry zonal flow aloft will bring sunnier skies on Monday with dry weather through Tuesday, mainly west of the Cascades. Surface high pressure builds, resulting in northerly winds along the I-5 corridor. Weak thermal troughing will also set-up along the coast and Cascades, leading to light offshore flow over the Coast Range and Cascade crest. In addition, a weak low pressure system over the southern Oregon and northern California border will maintain a 40-60% chance for rain/snow showers across the Lane County Cascades today, with chances spreading into the north Oregon and south Washington Cascades on Tuesday. Any accumulations will remain light with minimal impacts. Winds turn more southerly/southwesterly ahead of the next system.

The next robust system arrives on Wednesday as an upper level trough tracks southeastward from the Gulf of Alaska and swings a frontal system across the Pacific Northwest. This will return widespread rain, breezy southerly/southwesterly winds, and Cascade snow across the region. The majority of ensemble members from the high-resolution REFS suggests that precipitation from the initial warm front will begin along the coast early Wednesday morning (midnight-2 AM). Precipitation will gradually spread inland and persist through Wednesday afternoon as the trailing cold front follows.

Behind the cold front, precipitation turns showery and cooler air will filter in the area. Snow levels will drop to 2000-3000 ft, resulting in accumulating snow along the Cascade passes and potential travel impacts. A Winter Storm Watch has been issued for the northern and central Oregon Cascades from 5 AM Wednesday to 5 PM Thursday. Chances for 48-hour snowfall exceeding 1 foot ending 5 PM Thursday having increased to around 90% along Santiam and Willamette Passes, and 20-25% along Highway 26 at Government Camp. Ensemble guidance continue to suggest that the bulk of the precipitation will be south of Washington, with the heaviest snowfall over the Cascades from Marion to Lane County between 8 AM-4 PM Wednesday, except between 4 PM Wednesday and 8 AM Thursday as pass level as this is when snow levels will bottom out. This is when REFS guidance is showing a 50-75% chance for hourly snowfall rates exceeding 1 in/hr. Note if precipitation ends up tracking northward, then Highway 26 could see greater snowfall and impacts. 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.

West of the Cascades, minimal impacts expected from the rain and wind. The highest chances for 48-hour rainfall exceeding 1 inch between early Wednesday morning to early Friday morning are around 40-60% across the southern Willamette Valley and 70-85% for the coast south of Astoria. Southwesterly winds are forecast to gust up to 20-25 mph, with stronger gusts of 30-35 mph for higher terrain. Chances for stronger wind gusts along the coast (45+ mph) and across the Willamette Valley/southwest Washington lowlands (35+ mph) are around 10-25%. The main impact for the lowlands is a 10-20% chance for thunderstorms Wednesday afternoon/evening. Cooler air behind the cold front will result in a more unstable atmosphere, and model soundings support this with skinny CAPE profiles. Any passing thunderstorms may produce lightning, brief heavy downpours that reduce visibility, small hail, and erratic winds. -10/23

LONG TERM...Thursday to Sunday

Lowland rain showers and Cascade snow showers continue on Thursday as the system weakens and exits the region. Friday to Sunday, ensemble guidance is in agreement that upper level ridging will build over the Pacific Northwest, returning dry weather and sunnier skies. 500 mb heights also increase during this time, suggesting warmer temperatures and afternoon highs likely (70-90% chance) reaching the mid 60s by the weekend. -10

AVIATION

High clouds were beginning to fill back in over northwest Oregon and far southwest Washington from south to north late Monday evening as a weak frontal boundary over southwest Oregon continues pushing northward. Even with the increase in high clouds tonight into Tuesday morning, VFR flight conditions will likely prevail as cigs should generally stay above 6000-7000 ft. As this front lifts northward, there is a 20-30% chance of light rain at all terminals (except KAST) later tonight into Tuesday morning, along with clouds obscuring the high Cascades. Towards 06z Wednesday, a Pacific frontal system will be approaching the coast, bringing increasing chances for MVFR cigs (20% chance at KAST, 50% chance at KONP). Chances for MVFR cigs increase to near 90% by 12z Wednesday. Note this is not reflected in the 06z TAF cycle as conditions will most likely deteriorate shortly after 06z Wednesday as rain begins moving in from the coastal waters.

KPDX AND APPROACHES...VFR conditions are expected to continue through 06z Wednesday with increasing high clouds and cigs above 6000-7000 ft tonight into Tuesday morning. Light easterly winds around 4-8 kt will most likely continue through at least 03z Wednesday before becoming southerly around 5-10 kt by 06z Wednesday. -23

MARINE

High pressure offshore will maintain breezy northerly winds through this evening, gradually easing tonight as the pressure gradient weakens. Will maintain the Small Craft Advisory for the northern outer and central inner waters through midnight, while the advisory for the central outer waters south of Cape Falcon will continue through early Tue morning. Weak low pressure along the coast will drift offshore on Tuesday, bringing more benign conditions to the coastal waters. Seas of around 5 to 7 ft at 8 seconds today are expected to subside to around 3 to 5 ft on Tuesday.

A more robust frontal system is expected to move across the waters late Tuesday through Wednesday, bringing elevated winds and seas. Southerly winds begin increasing across the coastal waters later Tuesday through Tuesday night as an occluding front approaches. Guidance continues to suggest there is around a 70-90% chance of Gale Force wind gusts exceeding 34 kt with this system, most likely Wednesday morning. Have issued a Gale Watch from 2 AM through 2 PM Wednesday, as uncertainty remains in the timing of the frontal passage. Wind waves are expected to build on Wednesday becoming steep and choppy, with significant wave heights likely building to around 12 to 15 ft, highest over the southern waters.

Surface low pressure moves over the northern waters later Wednesday bringing more westerly winds, along with an increasing west swell which is expected to build seas into the lower to mid- teens on Thursday. Marine conditions are expected to ease later this week as high pressure returns northerly winds, and seas subside into the weekend. -DH/23

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Storm Watch from late Tuesday night through Thursday afternoon for ORZ126>128.

WA...None. PZ...Gale Watch from late Tuesday night through Wednesday afternoon for PZZ251>253-271>273.

Small Craft Advisory until midnight PDT tonight for PZZ252-253- 271.

Small Craft Advisory until 5 AM PDT Tuesday for PZZ272-273.