textproduct: Portland

SYNOPSIS

A major pattern shift is underway early this morning as a Gulf of Alaska system approaches. Expect widespread rain to spread inland today, along with breezy winds and a sharp drop in temperatures. Showers persist Tuesday with a low-end (10-20%) thunderstorm chance, then a gradual return to warmer and drier weather midweek into next weekend.

DISCUSSION...Today through Saturday

Early this Monday morning, the remaining influence of last nights ridge is quickly giving way to a broad upper low over the northeast Pacific. Satellite trends support an eastward progression of this system, and the associated cold front will be the driver for todays wetter, cooler weather.

Rain is expected to reach the coast around daybreak, then expand inland through late morning into early afternoon. Current timing supports onset along the coast roughly 7-9 AM, with the I-5 corridor and interior lowlands seeing steadier rain develop around 12-2 PM. Widespread cloud cover will keep temperatures well below this past weekends temperatures.

Rainfall totals through early Tuesday remain in a beneficial but non-hazardous range. Guidance supports 48-hour amounts (roughly 5 AM Monday to 5 AM Wednesday) around 0.15 to 0.80 inches, heaviest along the coast and Coast Range. Probabilities for at least 0.25 inch are about 35-75% across the Willamette Valley and 70-95% across the coast/Coast Range/Cascades. For 0.50 inch or more, odds are lower in the Willamette Valley (around 5-35%) but much higher for the coast and Coast Range (60-90%), with the Cascades generally 25-75%. The best signal for 1.0 inch or more remains along portions of the coast from Florence north toward Seaside, where probabilities run about 10-30%.

Winds increase as the front passes, especially near the coast and exposed headlands. Probabilities remain high for gusts at or above 25 mph across much of the area (roughly 70-99%). Gusts exceeding 30 mph are more likely along the coast (about 60-90%) than inland (15-30%), and gusts around 35 mph or higher are still more favored along the coast (30-60%) compared to inland (5-10%). These winds are not expected to meet advisory criteria, but could still be disruptive for tents, canopies, and other unsecured outdoor items.

By Tuesday, the upper low drops southeast with showers continuing and winds turning more southerly aloft. That change increases instability modestly, and a 10-20% chance of thunderstorms remains for the Willamette Valley and Cascades from late Tuesday morning into Tuesday evening.

From Wednesday into next weekend, guidance favors a drying and warming trend as weaker high pressure and more zonal flow return. Even with that warm-up, daytime highs likely stay somewhat cooler than the recent warm spell, with ensemble guidance suggesting a 85-99% chance temperatures remain on the cooler side relative to the weekend (below 80 degrees). There are early hints that the ridge could weaken again late week as another trough approaches the northeast Pacific, but recent ensemble guidance looks to trend toward the trough weakening and the ridge remaining over the region. While confidence has increased regarding a trough late week, confidence for the weakening of this upcoming midweek ridge has decreased as the late week trough looks significantly weaker. ~12

On a slightly personal note, with Monday being Memorial Day, it is time to remember those who served that are no longer with us. I want to remind everyone that if you are a fellow veteran or know a veteran that needs help, you can always call the VA Crisis Line by dialing 988 and pressing 1 or text 838255 24/7/365. We must take care of and look out those who stood besides us, who came before us and who will follow in our footsteps. Semper Fi. /42

AVIATION

Currently VFR inland and isolated LIFR conditions at the coast (mainly KONP) as of 11Z Mon. A frontal system will move through the airspace this morning and afternoon, currently located just off the coast. Specifically, expecting the front to bring rain and deteriorated conditions to the coast by 14-16Z Mon and persisting through the TAF period. With rain expected to become weaker and showery by 22-23Z Mon for coastal terminals, conditions at KTMK and KAST may improve back to VFR around then. As for inland terminals, the front will bring rain and high-end MVFR / low-end VFR conditions by 20-22Z Mon (10-30% chance of MVFR conditions). While rain will become weaker and showery by 01-03Z Tue, low chances for MVFR conditions will continue until at least 12Z Tue.

Current southerly winds will shift a bit more southwesterly by 17-18Z Monday as the front moves through the airspace. Expect continued winds around 14-17 kt with gusts up to 20-25 kt at the coast until 03-06Z Tue, while inland will see winds increase to around 10-14 kt with gusts up to 20 kt between 17Z Mon and 03Z Tue. Some areas within the Columbia River Gorge may be even windier with some models suggesting as high as 30 kt at 3000 ft.

Additionally, there is a 15-30% chance of isolated thunderstorms after 18Z Mon, with most terminals at the low end of this probability range. Expect an isolated thunderstorm or two rather than widespread thunderstorms with this system.~12

KPDX AND APPROACHES...Expect VFR conditions through the TAF period with increasing SCT/BKN high clouds as a frontal system approaches. Affiliated with the front, southerly winds will increase with gusts up to 20 kt between 17Z Mon and 03Z Tue. CIGs will gradually fall to low-end VFR after 20Z Mon with around a 10-30% chance of MVFR CIGs. Rain in the area with gusty winds may temporarily reduce VIS at times. 15-20% chance of isolated thunderstorms after 18Z Monday, with low confidence for anything widespread. Expect more of an isolated thunderstorm or two.~12

MARINE

Southerly winds have increased since last night, with now widespread gusts up to 30 kt across all waters as the front approaches. Peak winds are expected between 5-11 AM this morning with a 50-70% chance for isolated Gale force gusts up to 35 kt for the inner waters (from shore to 10 NM out) north of Cape Foulweather. After 11 AM today, winds turn more west- southwesterly behind the front with gusts up to 25 kt. Small Craft Advisories are in effect until 5 PM Monday for all waters including the Columbia River Bar. Seas around 6 to 8 ft through early Monday morning.

Following the frontal passage, a westerly swell will move in a build seas above 13 ft (>90% chance). There is also a 5-10% chance for seas exceeding 20 ft tonight into Tuesday morning, with the highest chances for the outer waters beyond 30 NM offshore. Given the potential for seas between 15-20 ft, there remains a Hazardous Seas Watch for all waters including the Columbia River Bar between 5 PM today and 11 AM Tuesday. Seas subside to 10-12 ft by Tuesday evening and gradually subside into Wednesday. Will also note that there is a 10-20% chance for thunderstorms across the waters tonight. Any passing thunderstorms may produce lightning, brief heavy downpours that reduce visibility, erratic winds, and/or small hail. ~12/10

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 5 PM PDT this afternoon for PZZ210- 251>253-271>273. Hazardous Seas Watch from this afternoon through Tuesday morning for PZZ210-251>253-271>273.