Portland, OR 97252 Weather

Synopsis

Dry weather is expected through much of Saturday, before rain chances increase through the latter half of the weekend. Persistent seasonable temperatures through the period.

Discussion

Now through Thursday...A weak upper shortwave passing overhead this afternoon will reinforce onshore flow, allowing for continued seasonable temperatures through the coming days, but is unlikely to produce rainfall. Meanwhile, a cutoff low over California is transporting moisture northward on its eastern flank toward eastern/central Oregon, which may produce light rain showers through this evening in the north and central Oregon Cascades. Much of the precipitation evident on area radar is evaporating before it reaches the surface, with the majority of observed rainfall east of the Cascade crest measuring 0.1 inches or less. If any rain does reach the ground west of the crest, it will likely also only manage to add up to a few hundredths of an inch. As skies clear west of the Cascades tonight, efficient radiative cooling will allow temperatures to fall into the low to mid 30s away from the coast and from urban areas, yielding widespread chances for frost and, within the southern Willamette Valley and sheltered Coast Range valleys, local area where a freeze is expected. Freeze Warnings have therefore been issued from 11 PM PDT this evening until 9 AM PDT Friday for portions of Lane, Benton, and Linn Counties within the Willamette Valley as well as interior valleys of the northern Coast Range including near Vernonia and Grand Ronde. Frost Advisories also remain in effect for the same times elsewhere in the Coast Range, central Willamette Valley, and interior south Washington lowlands. Those with agricultural interests or sensitive outdoor plants should take action to prevent damage.

There is then high confidence that dry weather returns tonight through much of Saturday as transient upper ridging traverses the region. As the ridge axis shifts inland on Friday night, light offshore flow, peaking overnight in the Cascades and Coast Range, will see daytime temperatures warm slightly on Saturday, but will generally remain near seasonal norms with afternoon high temperatures in the 50s on the coast and in terrain, and in the low to mid 60s within inland valleys. Overnight temperatures will again fall into the low to mid 30s for many away from the coast on Friday night, with Freeze Watches and Frost Advisories remaining in effect; Freeze Watches may be upgraded to Freeze Warnings if confidence increases in temperatures falling to near or below freezing.

There remains good consensus among long-range ensembles that another cutoff low will develop over the Gulf of Alaska on Saturday and track to the southeast offshore of Vancouver Island. By Sunday, there is less agreement on how the low will evolve as it nears the Pacific Northwest. For now, There are modest chances, near 50%, for rain on Sunday across the region, decreasing through the first half of next week as troughing may linger overhead. Cool air aloft will also likely continue to support seasonable temperatures for late October. -36

Aviation

Through 18Z Friday...A weak southwest-northeast oriented cold front was approaching the north coast as of 17Z Thursday, bringing MVFR ceilings to KAST. Expect conditions to improve back to VFR behind the front later this afternoon. This front will eventually reach the central Oregon coast around 20-21Z Thursday, bringing a 50-60% chance for MVFR ceilings below 3000 feet at KONP for a 2-3 hour period of time before conditions likely improve back to VFR. This front will also bring gusty north to northwest winds to KAST and KONP with gusts up to 20-25 kt. Winds will decrease this evening behind the front.

Predominately VFR flight conditions are expected for inland TAF sites through this evening with scattered to broken high clouds above 15-20 kft. Northwest winds will develop this afternoon for inland TAF sites, but should generally stay under 10 kt the vast majority of the time. Heading into tonight, clearing skies and decreasing winds will support at least some patchy fog development within the Willamette Valley, particularly from KSLE to KEUG. That said, surface visibilities should stay above 6 SM at both TAF sites the majority of the TAF period, if not the entire TAF period. Probabilities for visibilities below 1 SM are less than 5-10%. Therefore, have decided not to include visibility restrictions in the TAFs, but will acknowledge there is a non-zero chance for brief visibility reductions towards 12Z Friday.

KPDX AND APPROACHES...High pressure over the region will lead to light northwest winds around 5-10 kt and VFR conditions. Winds become light and variable less than 5 kt tonight. -23

Marine

Buoy observations from 2 PM PDT Thursday showed northerly winds increasing behind a cold frontal passage, with sustained winds between 15 and 25 kt and gusts up to 30 kt. Seas were beginning to gradually increase as a northwest swell continues to build over the waters. Seas are expected to continue building this evening, becoming steep and hazardous shortly after sunset. Seas will then remain steep and hazardous through early Friday morning with significant wave heights ranging between 8 and 12 ft and wave periods between 8 and 10 seconds. Small Craft Advisories remain in place for all marine zones until 8 PM PDT Thursday, before ramping up to a Hazardous Seas Warning lasting from 8 AM PDT Thursday until 8 AM PDT Friday for all marine zones, except the main channel of the Columbia River Bar where a Small Craft Advisory is in effect now through 8 AM PDT Friday. Seas will be relatively less steep over the Columbia River Bar with significant wave heights of 7 to 9 ft at 8 to 10 seconds, but will still be hazardous to small craft, especially during ebb currents. It is also worth noting that brief and isolated wind gusts as high as 35 kt are possible through 10 PM PDT Thursday over the Columbia River Bar, but wind gusts of this magnitude are not expected to be frequent enough to justify the issuance of a Gale Warning.

North winds weaken late Friday morning into the afternoon with a continued northwest swell and seas becoming less steep. North winds will then persist through at least Saturday afternoon as an approaching low stalls west of the coastal waters. There is still uncertainty on the exact strength and track of this low heading into Saturday night and Sunday. Despite this uncertainty, confidence is high wind speeds will stay below gale-force, as probabilities for wind gusts of 34 kt or stronger are now less than 10% this weekend. Probabilities for seas over 10 ft reach 10-20% Sunday morning and afternoon. -23