textproduct: Portland

SYNOPSIS

A weak front will bring light showers across the area through this afternoon. Drier weather returns tonight and persists through Tuesday, except for chances for light rain/snow across the Cascades. A stronger frontal system arrives Wednesday, returning widespread rain and Cascade snow through Thursday. Accumulating snow along the Cascade passes may lead to travel impacts. High pressure re-builds Friday into the weekend, returning dry and warm weather.

SHORT TERM...Now through Tuesday night

Radar imagery as of early Sunday afternoon depicts light showers across northwest Oregon and southwest Washington as a weak front swings through the region. Expect light showers to continue through this afternoon before drier conditions gradually return this evening. Cooler air will follow the cold front, dropping snow levels below 3000 ft across the north Oregon and south Washington Cascades by this evening. Any lingering showers across these Cascade areas will fall as snow, including over Highway 26 at Government Camp. Light snow amounts will result in minimal travel impacts.

Cloud cover decreases tonight behind the cold front passage, mainly west of the Cascades. Clearing skies and calm winds will support efficient radiational cooling and Monday morning lows in the low to mid 30s for interior valleys, Coast Range, and Willapa Hills. These temperatures will also support widespread frost development, especially over grasses and metal surfaces. An exception is the southern Willamette Valley, where there is lower confidence (20-40% chance) for frost due to minimal clearing of clouds and tighter pressure gradients maintaining a north wind above 5 mph.

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. A thermal trough will also set-up along the coast and Cascade foothills, leading to light offshore flow over the Coast Range and Cascade crest. A weak low pressure system over the southern Oregon and northern California border will maintain a 20-40% chance for rain/snow showers across the Lane County Cascades on Monday, and a 30-50% chance for rain/snow showers across the northern and central Oregon Cascades. Winds turn more southerly/southwesterly on Tuesday ahead of the next system. -10

LONG TERM...Wednesday to Saturday

The next robust system arrives on Wednesday, returning widespread rain, Cascade snow, and breezy southerly/southwesterly winds across the region. An upper level trough will drop down from the Gulf of Alaska, and the associated surface warm and cold fronts will swing through the Pacific Northwest. The majority of LREF ensemble members suggest that precipitation from the warm front will begin along the coast and spread inland between late Tuesday night to early Wednesday morning. Precipitation will persist through at least Wednesday night as the trailing cold front follows. Chances for 48-hour rainfall exceeding 1 inch between 4 AM Wednesday to 4 AM Friday are around 60-80% across the southern Willamette Valley and the coast from Cannon Beach southward, and 15-30% along the north Oregon/south Washington coast and the I-5 corridor from Salem northward. Southwesterly winds are forecast to gust up to 20-25 mph, with higher gusts of 30-35 mph for higher terrain ridgetops. Chances for stronger wind gusts along the coast (45+ mph) and across the Willamette Valley/southwest Washington lowlands (35+ mph) are around 15-25%.

Behind the cold front, cooler air will filter into the area and drop snow levels to 2000-3000 feet. This will result in accumulating snow along the Cascade passes and lead to potential travel impacts. Chances for 48-hour snowfall exceeding 1 foot from 4 AM Wednesday to 4 AM Friday are around 55-65% along Santiam and Willamette Passes, and 10-20% along Highway 26 at Government Camp. Based on LREF ensemble guidance, the heaviest snow appears to fall between 5 AM Wednesday to 5 AM Thursday. If you have plans to travel through the Cascades, make sure to pack an emergency supply kit and refer to ODOT/WSDOT for road conditions and restrictions.

Rain and Cascade snow transition to showers on Thursday as the system weakens and exits the region. Friday to Saturday, ensemble guidance is in agreement that upper level ridging will build over the Pacific Northwest, returning dry weather and sunny skies. 500 mb heights also increase during this time, suggesting warmer temperatures and afternoon highs potentially reaching the mid 60s by the weekend (45-65% chance). -10

AVIATION

Moist, southwest flow aloft will maintain mid to high level clouds today as a weak front pushing onshore brings light rain to northern portions of the forecast area through this afternoon. As of 19z Sunday, predominately VFR conditions are expected to continue across most of the airspace. By 20-21z this afternoon, chances for MVFR CIGs at the coast increase to around 50-60%. But, those probs drop off by 01-02z Monday, with conditions improving back to VFR behind the frontal passage. Inland terminals are less likely to see MVFR CIGs, with probs at around 30% between 22z Sunday and 03z Monday, with lower chances from KSLE southward. Light and variable winds are expected to increase out of the southwest this afternoon, then shift northwesterly by 00z Monday.

KPDX AND APPROACHES...VFR conditions expected to continue through much of the TAF period. There is around a 30% chance for MVFR CIGs later this afternoon between 22z Sunday and 03z Monday as a weak front pushes through with light rain. Light winds increase out of the southwest this afternoon to around 5-10 kt, then shift northwesterly by 02z Monday. /DH

MARINE

Breezy northwest winds may briefly gust up to 20-25 kt behind a frontal passage this afternoon. Northerly winds increase later tonight and remain elevated through Monday night as high pressure rebuilds over the waters. A Small Craft Advisory remains in effect for the outer coastal waters beginning 8 PM this evening. Northerly winds again increase Monday afternoon, as the stronger winds shift into the inner waters south of Cape Falcon, continuing through Monday night.

A more robust frontal system is expected to move across the waters late Tuesday through Wednesday, bringing elevated winds and seas. Guidance suggests there is around a 70-90% chance of Gale Force wind gusts exceeding 34 kt with this system. Wind waves are expected to build on Wednesday becoming steep and choppy, followed by a westerly swell building seas on Thursday into the lower to mid-teens. Marine conditions are expected to ease later this week as high pressure returns. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 2 PM Monday to 5 AM PDT Tuesday for PZZ252-253.

Small Craft Advisory from 8 PM this evening to 2 PM PDT Monday for PZZ271.

Small Craft Advisory from 8 PM this evening to 5 AM PDT Tuesday for PZZ272-273.