textproduct: Portland

SYNOPSIS

A weak disturbance at the Oregon-California border continues to maintain light rain within the Willamette Valley and a mix of rain/snow showers over the Cascades. A stronger frontal system arrives Wednesday, bringing widespread rain, Cascade snow, and breezy southerly to southwesterly winds through Thursday. Accumulating snow with periods of heavy snowfall rates along the Cascade passes Wednesday afternoon through Thursday morning will create hazardous travel conditions, particularly along Santiam and Willamette Passes. High pressure rebuilds Friday into the weekend, returning dry weather, increasing sunshine, and a warming trend.

MORNING UPDATE

The Winter Storm Watches have been upgraded to Winter Storm Warnings across the Marion, Linn, and Lane County Cascades. Lower snow amounts are forecast north of the Marion County Cascades; therefore, Winter Weather Advisories have been issued for the north Oregon and south Washington Cascades rather than warnings. There is high confidence (70-80% chance) for 1 foot of snow or greater along Santiam and Willamette Pass between 11 AM Wednesday to 11 AM Thursday. Based on REFS ensemble guidance, there is also 25- 50% chance for snowfall rates of 1+ inch per hour at any given hour during this time period along these passes, with the highest chances occurring between 11 PM Wednesday and 5 AM Thursday from heavier convective showers. -10

DISCUSSION...Now through Monday

A weak disturbance near the southern Oregon-northern California border will continue to bring a 20-40% chance of light rain to the Willamette Valley. There are higher chances of light rain to the east, with a 40-80% of light rain and snow showers over the Cascades. Any snow accumulations over the Cascades today will remain light with little to no impacts. Winds gradually turn southerly late today and tonight ahead of the next approaching weather system.

A more robust Gulf of Alaska trough arrives Wednesday, driving a frontal system across the Pacific Northwest. Precipitation associated with the warm front is expected to reach the coast early Wednesday morning, spreading inland through the morning hours. Rain will continue through Wednesday afternoon and evening as the trailing cold front sweeps through. Behind the cold front, cooler air lowers snow levels to 2000-3000 ft, bringing accumulating snow to the Cascade passes. A Winter Storm Watch remains in effect for the northern and central Oregon Cascades from 5 AM Wednesday through 5 PM Thursday. Probabilities for 48-hour snowfall 1+ foot have increased to around 70-90% along Santiam and Willamette Passes, with lower probabilities of 15-25% along Highway 26 at Government Camp. The heaviest snowfall is currently favored across the Marion, Linn, and Lane County Cascades, with the most impactful period expected Wednesday afternoon through late Thursday morning as snow levels drop quickly towards 2000 ft. Ensemble guidance indicates a 40-60% chance of hourly snowfall rates 1+ inch during this window. If the front were to shift more northward, expect an increase of snowfall along Highway 26 near Government Camp. While probabilities for 48-hour snowfall 1+ foot have gone down for Highway 26 near Government Camp and the south Washington Cascades, advisory-level snow is still possible from early Wednesday to early Friday, with a 50-70% chance of 6+ inches of accumulating snow. 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, impacts are expected to be more limited. Probabilities for 48-hour rainfall totals 1+ inch from early Wednesday through early Friday range from 40-60% across the southern Willamette Valley and 70-90% along the coast south of Astoria (30-50% at Astoria). South to southwest winds will become breezy Wednesday, with gusts of 20-25 mph expected and 30-35 mph possible over higher terrain. The probability of stronger advisory-level gusts remains low (<5% chance). There is also a 10-20% chance of thunderstorms Wednesday afternoon through early Wednesday night as colder air aloft increases instability. Current models show skinny CAPE with CAPE values only reaching as high as ~100 J/kg. Any thunderstorms that develop may produce lightning, brief heavy downpours, small hail, and locally erratic winds.

Showers continue Thursday as the system gradually weakens and shifts east. By Friday through the weekend, ensemble guidance supports building upper-level ridging over the Pacific Northwest. This will promote drying conditions, increasing sunshine, and a warming trend, with highs likely reaching the mid to upper 60s, possibly low 70s on Sunday and Monday. Ensemble clusters indicate uncertainty beyond Monday, with a 50/50 chance of the ridge either weakening or remaining moderately strong as it shifts eastward. ~12

AVIATION

West to southwest flow aloft with mid-level moisture will maintain mid to lower level clouds today. Weak low pressure will maintain chances for light rain showers inland, including cloud obscuration along the Cascades. Expect predominately VFR conditions through 08-10z Wed. Probabilities for MVFR increase across the area to around 50% afterwards as a robust frontal system approaches the region. Light rain is likely to begin spreading across the area by 08z Wed, with heavier rain arriving at the coast after 12z Wed. South to southeasterly winds are expected to increase into Wednesday morning as well, becoming gusty.

KPDX AND APPROACHES...Predominately VFR conditions expected with lower to mid level clouds persisting through this evening. Around a 30% chance of light rain showers will also continue across the area. A robust frontal system will approach later tonight into Wednesday bringing rain and increasing probs for MVFR (40-50%) after 08z Wed. Southeast winds around 5-10 kt expected to increase through Wed morning. Southerly winds with gusts to 20-25 kt will be possible by 18z Wed. /DH

MARINE

Breezy northerly winds continue to decrease early this morning as the pressure gradient weakens. The Small Craft Advisory for the outer waters south of Cape Falcon will remain in effect 5 AM this morning. Late this morning through early tonight, expect benign conditions over the coastal waters. Seas around 3 to 5 ft through today.

A more robust frontal system is expected to move across the waters late today through Wednesday, bringing elevated winds and seas. Southerly winds begin increasing across the coastal waters tonight as an occluding front approaches. Guidance continues to suggest there is around a 70-90% chance of max wind gusts exceeding 34 kt with this system, most likely Wednesday morning. A Gale Watch remains in effect 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 14 ft, highest over the southern waters.

Surface low pressure moves over the northern waters late Wednesday, bringing westerly winds and an increasing west swell which will likely build seas into the lower to mid-teens on Thursday. Marine conditions are expected to ease later this week as high pressure rebuilds, with seas subsiding to 6 to 8 ft during Friday and falling to 4 to 6 ft during the weekend. ~12/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory from 5 AM Wednesday to 5 PM PDT Thursday for ORZ126.

Winter Storm Warning from 5 AM Wednesday to 5 PM PDT Thursday for ORZ127-128.

WA...Winter Weather Advisory from 5 AM Wednesday to 5 PM PDT Thursday for WAZ211.

PZ...Gale Watch from late tonight through Wednesday afternoon for PZZ251>253-271>273.