textproduct: Portland

SYNOPSIS

Dry conditions return across northwest Oregon and southwest Washington today. High pressure will maintain dry and seasonable weather through Tuesday, aside from isolated light precipitation over the Cascades. A stronger Gulf of Alaska system arrives Wednesday, bringing widespread rain, Cascade snow, and breezy winds through Thursday. Accumulating snow over the Cascade passes may lead to travel impacts. High pressure rebuilds Friday into the weekend, returning drier and warmer conditions.

DISCUSSION...Now through Saturday

Dry conditions return today across northwest Oregon and southwest Washington as surface high pressure begins to build inland. Cooler air remains in place behind the weak front that moved through the region yesterday, keeping snow levels around 1000 to 3000 ft across the Cascades. Any lingering precipitation is confined to the higher terrain, of which would likely fall as snow, with no travel impacts expected today.

Partial clearing overnight combined with light winds has allowed temperatures to fall into the low to mid 30s across much of the interior lowlands, Coast Range, and Willapa Hills. These conditions support areas of frost development, particularly on exposed and elevated surfaces. Frost coverage may be more limited across the southern Willamette Valley where cloud cover thinned later and boundary layer winds remained slightly elevated.

High pressure continues to strengthen today and Tuesday, supporting dry weather west of the Cascades. Zonal flow aloft transitions toward weak ridging, while a surface thermal trough develops along the coast and Cascade foothills. This pattern will promote light northerly flow through the interior valleys and weak offshore flow through the Coast Range gaps and Cascade crests. Highs today rise into the mid to upper 50s inland and lower 50s along the coast, with similar or slightly warmer temperatures Tuesday. Meanwhile, a weak disturbance lingering near the southern Oregon-northern California border may bring a 40-70% chance of light rain and snow showers to portions of the Oregon Cascades through Tuesday. Any accumulations remain light and primarily confined to higher elevations.

Attention then turns to a more robust system expected to arrive Wednesday. An upper trough digging southeast from the Gulf of Alaska will drive a frontal system across the Pacific Northwest. Precipitation is likely to spread inland early Wednesday morning, continuing through Wednesday night as the cold front moves through. Ensemble guidance supports high confidence for widespread rainfall, but only moderate confidence for 48-hour totals from early Wednesday through early Friday to exceed one inch across portions of the southern Willamette Valley and most of the Oregon coast. Current guidance suggests a 30-50% chance for 1+ inches of rain in the southern Willamette Valley, less than 15% for the rest of the Valley, and a 60-80% chance for much of the coast (south of Astoria, as Astoria only has a 20-40% chance of 1+ inches of rain over 48 hours).

Breezy south to southwest winds are expected Wednesday, with gusts of 20 to 25 mph expected in the lowlands and stronger gusts possible over exposed terrain. While advisory-level winds are not the most likely outcome at this time, localized higher gusts remain possible, particularly along the coast. Current guidance suggests a 20-30% chance of 35+ mph wind gusts inland and a 10-20% chance of 40+ mph wind gusts for the inland coastal communities.

Behind the cold front, colder air will lower snow levels to around 2000 to 3000 ft, bring accumulating snow to the Cascade passes. Ensemble probabilities indicate a moderate chance of significant snowfall (12+ inches) along Santiam and Willamette Passes through Thursday night, with the most intense snowfall rates currently favored from Wednesday morning into early Thursday. Travel impacts are increasingly likely during the period, with current guidance suggesting a 40-60% chance of 12+ inches of accumulating snow over Santiam and Willamette Passes from 11 AM Wednesday to 11 AM Thursday. With this front likely pivoting more south than earlier forecasted, chances for 12+ inches of snow over Government Camp have gone down, only 5-10% now.

Showers gradually decrease Thursday as the trough shifts east. By Friday into the weekend, ensemble guidance favors rebuilding upper-level ridging of the Pacific Northwest, supporting a transition back to drier conditions and a warming trend heading into the weekend, possibly warming up into the high 60s. ~12

AVIATION

High to moderate clouds over the airspace continue to thin out through the day. Expect widespread VFR conds through 12Z Tue as high pressure builds over the area. With clouds thinning, temperatures are expected to dip close to freezing this morning at all terminals, most likely allowing for frost development. Otherwise, expect northwesterly to northerly winds through 12Z Tue, generally light before 18Z Mon for most terminals and then 5-10 kt until around 06Z Tue. KTTD and KONP will be the exceptions, with gusts up to 18-20 kt possible between 20Z Mon and 03Z Tue. Additionally, KTTD will likely become mostly easterly to northeasterly after 18Z Mon.

KPDX AND APPROACHES...Expect VFR conds through 12Z Tue with cloud cover clearing out through the day. Expect temperatures to fall to near freezing with frost development likely by sunrise. Northwesterly winds will be light before 18Z Mon, then increase to 5-10 kt until around 06Z Tue. ~12

MARINE

Breezy northwest winds remain elevated through Monday night as high pressure rebuilds over the waters. A Small Craft Advisory remains in effect for all the outer coastal waters until 2 PM Monday. Northerly winds again increase Monday afternoon, as the stronger winds shift into the inner waters south of Cape Falcon, therefore another Small Craft Advisory will be in effect for the inner and outer waters south of Cape Falcon from 2 PM Monday to 5 AM Tuesday.

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. ~12/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 2 PM this afternoon to 5 AM PDT Tuesday for PZZ252-253. Small Craft Advisory until 2 PM PDT this afternoon for PZZ271. Small Craft Advisory until 5 AM PDT Tuesday for PZZ272-273.