textproduct: Portland

SYNOPSIS

High pressure continues to linger over the region through the evening, but confidence remains high that a significant pattern change is on deck starting tonight. The cause of this cooler and wetter pattern is a Gulf of Alaska low that looks to bring widespread rain, breezy winds, and much cooler temperatures across the area. Showers linger into Tuesday along with a slight chance (10-20%) for thunderstorms. Conditions trend warmer and drier through the middle of the week and into next weekend.

DISCUSSION...Now through Saturday

Clear skies this evening will soon be replaced by widespread cloud cover starting tonight. GOES full disk, satellite observations shows a broad upper level low covering a vast area of the northeastern Pacific Ocean. Putting the satellite on a loop, this low is moving eastward which will result in a cold front being pushed across the Pac NW. This front will bring widespread rain and thick cloud cover across our CWA. Rain should begin along the coast around 5AM-7AM Monday morning, with rain starting to impact inland locations around 12PM-2PM Monday. Recent guidance supports 24 hour rainfall totals (roughly 5 AM Monday to 5 AM Tuesday) in the 0.15"-0.80", with the greatest totals along the coast and Coast Range. Probabilities for at least 0.25 inch are generally 35-75% for the Willamette Valley and 60-95% for the coast, Coast Range, and Cascades. Probabilities for 0.50 inch or more are generally 5-30% for the Willamette Valley and 60-95% for the coast and Coast Range, and generally 15-45% for the Cascades. A corridor from Florence OR north toward Seaside OR continues to show the best chances, around 20-40% for 1.0" or more.

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 (60-90%) than inland (25-60%). Higher-end gusts around 35 mph or greater remain a lower-probability outcome for inland locations (5-15%) but are more favored along the coast (30-50%). Even without advisory- level winds expected inland, these speeds can still impact tents, canopies, and other unsecured items.

By Tuesday, the low shifts southeastward with showers lingering and a more southerly flow developing. That setup is more supportive of thunderstorms, which is resulting in a 10-20% chance for the Willamette Valley and Cascades by late Tuesday morning through late Tuesday evening.

From midweek through next weekend, guidance trends toward a warming and drying pattern as weaker high pressure/zonal flow returning. For reference, daytime highs will likely (70-85%) remain slightly cooler than this weekend. 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. /42~12

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

A frontal system remains just offshore this evening which will be the weather maker over the next 24 hours. Most locations remain VFR except a few isolated areas along the central Oregon coast. A layer of marine stratus is working it's way northward and should impact KONP in the next few hours. 50% chance of IFR conditions and greater than an 80% chance of MVFR along coastal terminals. The north coast though will likely stay out of a bulk of that stratus due to mixing from the frontal passage after 12Z Mon. Winds will continue to shift through the period becoming southwesterly in the afternoon with the front. As they do, expect gusty winds with many airports seeing gusts in excess of 20 kt. Some areas within the Columbia River Gorge will be even windier with some models suggesting as high as 30 kt at 3000 ft.

Rain expected over the region with a 15-30% chance of thunderstorms after 18Z Mon. Trending on the lower end of that range, but cannot rule out an isolated thunderstorm or two - though confidence is much lower than in previous forecasts. Have included PROB30 in the TAF for KAST, KONP, KHIO, KPDX, and KTTD. This is not looking like a widespread thunderstorm system but rather a few one-hit-wonders.

KPDX AND APPROACHES...Predominately VFR conditions through the TAF period with increasing SCT/BKN high clouds ahead of the next system. After 13-15z Mon, southerly winds increase with gusts up to 20 kt as the front approaches. CIGs also gradually fall to low- end VFR after 19Z Mon with around a 15% chance of MVFR CIGs. Rain in the area with gusty winds may temporarily reduce VIS at times. PROB30 included late in the forecast for thunderstorms but confidence is low. At this point, there is a high level of uncertainty in regards to whether or not we will see any kind of thunderstorm activity so those probs could be removed in later forecasts. -27

MARINE

Northerly winds will gradually turn more southerly this afternoon and evening ahead of the next frontal system. Tonight, southerly winds increase with widespread gusts up to 30 kt across all waters as the next front moves through. Peak winds are expected between 5-11 AM Monday 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 Monday, winds turn more west-southwesterly behind the front with gusts up to 25 kt. Small Craft Advisories are in effect from 11 PM Sunday through 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 Monday night 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 Monday to 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 Monday night. Any passing thunderstorms may produce lightning, brief heavy downpours that reduce visibility, erratic winds, and/or small hail. -10

PQR WATCHES/WARNINGS/ADVISORIES

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

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