textproduct: Portland

SYNOPSIS

High pressure will deliver one more warm, dry day today, while the coast stays cooler at times under patches of marine clouds and drizzle. Confidence remains high in a sharp shift late tonight into Monday as a Gulf of Alaska system spreads widespread rain, breezy winds, and much cooler temperatures across the area. Showers linger into Tuesday with a low-end thunder chance, then conditions trend warmer and drier late next week.

DISCUSSION...Now through Saturday

Early this Sunday morning, the region remains on the warm side of the pattern beneath upper-level high pressure. Even so, onshore flow near the surface is still enough to bring marine clouds back to the coast and into some adjacent valleys at times. Any coastal drizzle/mist would be confined mainly to the morning hours. For today, temperatures will again run warm inland, but the final outcome will hinge on how much marine cloud cover hangs on near midday. Current expectations still favor the warmest highs of the weekend across interior valleys, generally upper 70s to lower 80s, while the coast stays cooler in the low 60s to low 70s. Cascades and foothills should stay in the mid 60s to mid 70s. If clouds push farther inland or linger longer than anticipated this morning, some locations could end up 5-10 degrees cooler than forecast.

The forecast turns much more active late tonight into Memorial Day. A broad low dropping out of the Gulf of Alaska will drive a frontal system into the Pacific Northwest, bringing widespread rain and thick cloud cover. Rain should begin along the coast around early Monday morning, then spread inland during late Monday morning/early Monday afternoon. Recent guidance now supports 24 hour rainfall totals (roughly 5 AM Monday to 5 AM Tuesday) in the 0.20 to 0.90 inch range, with the greatest totals and highest probabilities along the coast, Coast Range, Cascades, and Cascade foothills. Probabilities for at least 0.25 inch are generally 50-90%, while chances for 0.50 inch or more are lower in the Willamette Valley (5-35%) but remain much higher for the coast, Coast Range, and Cascades (around 60-90%). A corridor from Florence OR north toward Seaside OR continues to show the best odds for 1.00 inch or more (roughly 20-40%).

Winds will also pick up early Monday as the front moves through. Currently probabilities strongly favor gusts exceeding 25 mph across much of the area (around 80-99%). Gusts over 30 mph are more likely along the coast (about 70-90%) than inland (40-60%). Higher-end gusts around 35 mph or greater remain a lower-probability outcome inland (roughly 5-20%) but are more plausible along the coast (35-55%). Even without advisory-level winds expected inland, these speeds can still impact tents, canopies, and other unsecured items.

By Tuesday, the low shifts southeast with showers lingering and a more southerly flow developing. That setup is more supportive of isolated thunderstorms, and a 15% chance appears reasonable for the Willamette Valley and Cascades by late Tuesday morning into Tuesday afternoon. From midweek into late week, guidance trends toward a warming and drying pattern as weaker high pressure/zonal flow returns, though temperatures will likely remain cooler than this weekend's warm spell. While confidence remains low, ensemble guidance suggests that the weak high pressure ridge will weaken Friday as a broad trough over the Northeast Pacific nears the Pacific Northwest. ~12

AVIATION

High pressure and high clouds remain over the airspace, with VFR conditions inland and MVFR CIGs along the coast from KTMK to KAST as of 11Z Sun. Expect deteriorated conditions at the coast to improve to VFR by 17-19Z Sun. MVFR CIGs return to the coast around 06Z Mon as a frontal system approaches. Inland terminals will stay VFR through the TAF period, with a less than 10% chance of MVFR CIGs through at least 12Z Mon. Winds will stay north to northwesterly until around 00Z Mon, then shift west to southwesterly as the frontal system nears. Inland winds will stay under 8 kt through at least 12Z Mon, while coastal winds will increase to 8-12 kt with gusts up to 20 kt by 08-10Z Mon. Conditions at inland terminals will likely not deteriorate to MVFR or lower until after 12Z Mon. Rain will arrive at coastal and inland terminals a few hours after the end of the TAF period.

KPDX AND APPROACHES...High pressure, high clouds, VFR conditions. Less than 10% chance of MVFR conditions. North to northwesterly winds under 8 kt will shift west to southwesterly by 00Z Mon. ~12

MARINE

Breezy winds have weakened early this morning and will continue to weaken through today. Seas will remain around 6 to 8 feet through tonight. A frontal system will impact the waters Monday into Tuesday. Winds turn westerly and increase early Monday morning as the front approaches and moves through the waters. Latest guidance indicates widespread wind gusts up to 25-30 kts, strongest north of Cape Foulweather, with a 35-50% chance of gale force wind gusts, mainly isolated and right along the frontal passage from 2 AM to 8 AM Monday. Waves are forecast to rise behind the frontal passage Monday afternoon into Tuesday morning. There's high confidence (90+% chance) of significant wave heights exceeding 13 feet. Additionally, there's around a 25% chance significant wave heights could reach 17-19 feet for locations 20 nm and beyond west of the coast, and a 10% chance of 20 feet seas for locations 30 nm and beyond west of the coast. As a result, a Small Craft Advisory has been issued from 11 PM Sunday to 5 AM Monday for wind gusts around 25 to 30 kt. Additionally, a Hazardous Seas Watch has been issued from 5 AM Monday to 11 AM Tuesday for very steep and hazardous seas, while also highlighting possible isolated gusts up to 35 kt from 2 AM to 8 AM Monday for the waters north of Cape Foulweather OR. Waves forecast to decrease through Tuesday afternoon to around 10-12 feet. ~12/03

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 11 PM this evening to 5 AM PDT Monday for PZZ210-251>253-271>273.

Hazardous Seas Watch from late tonight through Tuesday morning for PZZ210-251>253-271>273.