textproduct: Portland

UPDATE

A Dense Fog Advisory has been issued for the lowland and Cascade foothills of Lane County where available observations and cameras show widespread visibility of 1/4 mile or less, especially from Eugene/Springfield to the south and east including the I-5, OR-58, and OR-126 corridors. Morning commuters should use caution and expect to encounter rapidly changing roadway visibility. Conditions are expected to improve after 9 AM PDT.

SYNOPSIS

Unseasonably warm and dry weather with weak offshore flow will persist through Thursday, before a trough brings a return to cooler temperatures with chances for light rain Friday into Saturday. Cool nights will bring chances for morning frost in rural valleys away from urban cores this weekend into early next week.

SHORT TERM...Now through Thursday Night

Broad upper-level ridging centered over the West Coast will continue to amplify through Thursday, bringing a warming trend with continued dry weather. Very weak thermal troughing along the coast will only support a pressure gradient of around -1 mb between Portland and The Dalles, which in turn suggests overnight offshore winds in the Cascades and foothills only reaches 10-15 mph. Afternoon high temperatures will trend upward to the mid 70s to low 80s within inland valleys on Thursday, values some 10 or so degrees above normal for early October. Daily records on Thursday at most climate sites appear to be out of reach with less than 5% chances of tying or surpassing existing record values, except for slightly higher chances of 5-10% at Salem and Eugene. See the Climate section below for a full list of record temperatures for October 8.

Aforementioned light offshore flow at night will support dry mid-levels, which will allow efficient radiative cooling and morning low temperatures falling to the upper 40s to low 50s. While not as widespread as recent mornings, some areas of fog or low clouds may develop along the coast or in sheltered valleys. By Thursday night, high cloud cover will begin to increase ahead of an approaching weather system. -36

LONG TERM...Friday through Tuesday

Model consensus has continued to increase regarding the evolution of an upper-level trough over the Gulf of Alaska. The trough, with a leading shortwave impulse, will move inland over the Pacific Northwest from Thursday night into Friday, bringing an initial round of light rain showers associated with a weak front moving onshore. Rain may begin early Friday morning along the coast, but will more likely hold off until the late morning or early afternoon, then reach inland to the I-5 corridor by the afternoon to early evening. This will favor accumulations along the north Oregon and south Washington coast as well as in the Cascades and western foothills through the day on Friday, while areas along the central Oregon coast and within the Willamette Valley may tend to stay dry. Mean forecast rain totals during this period are 0.1-0.15 inches north of Tillamook Head and in the Mount St. Helens area, with lower amounts of 0.05-0.1 inches elsewhere in the Lower Columbia and Cowlitz Valleys, along the coast from Tillamook Head to Lincoln City, and in the north Oregon and south Washington Cascades. As the trough moves inland, a second shortwave impulse moving south on its western flank on Saturday will see the trough elongate and eventually pinch off into a closed upper low which will settle south over California by early next week. This feature will result in another chance for light rain on Saturday afternoon, with more widespread potential for up to 0.1 inches of rain, especially to the north of US-20, before trending drier by Saturday evening and night. All told, the chances for 0.25 inches of rain in 48 hours reach only 30-50% in the wettest areas north of Tillamook Head and in the Cascades at foothills north of OR-22. Elsewhere, those chances are 5-15% from Lincoln City to Tillamook Head and along I-5 north of Salem, and 5% or less south of Lincoln City and Salem.

As the initial front moves inland Friday evening though Friday night, peak gusts out of the west in the Cascades, Columbia River Gorge, and Hood River Valley could reach 30-40 mph. Behind the boundary and as the core of cooler air aloft moves overhead, temperatures will quickly fall back toward seasonal norms in the upper 50s and 60s Friday through the weekend. Overnight lows will similarly trend cooler, especially Saturday and Sunday nights when there are 70% or greater chances for frost in the Upper Hood River Valley, and 20-50% chances in Cascade valleys, north Oregon Coast Range Valleys, the Upper Cowlitz Valley, and the Willamette Valley south of Salem. Chances for a true freeze are 40-60% in the Upper Hood River Valley near Parkdale and Dee, and 20-40% along OR-58 between Eugene and Oakridge. Those with agricultural interests or home growers with sensitive outdoor plants should continue to monitor the forecast this week.

By early next week as the closed low drifts to the south, upper heights may briefly rise as ridging redevelops overhead. This will favor a modest warming trend Monday into Tuesday. There is much lower confidence in the evolution of the large-scale pattern by the middle of next week, but there are low chances for another trough to bring additional rain. -36

AVIATION...Through 12z Thursday

Broad high pressure will maintain similar conditions to recent days, however weak offshore flow will act to lessen the chances for fog and low stratus compared to mornings earlier this week. With largely clear skies supporting efficient radiative cooling, terminal observations across the region already show relative humidity of 90% or greater, meaning any further cooling may support fog/mist development. Latest satellite imagery depicts patchy coverage of low clouds along the coast, but recent trends seem to favor IFR vis/cigs from fog or marine stratus until after sunrise, while TEMPO groups through 16z Wed indicate conditions will likely continue to fluctuate. Inland, chances for IFR vis/cigs are much lower, 15-20% or less, but periods of MVFR vis from mist may occur at favored sites, most likely in the southern Willamette Valley but also central Willamette Valley and Cascade foothills sites.

Any restricted vis/cigs will tend to improve to VFR by 16-17z Wed as daytime heating and diurnal winds encourage vertical mixing. Light offshore flow early this morning will build out of the north to northwest to 10-15 kt with gusts around 20 kt along the coast, and to 5-10 kt inland by this afternoon.

KPDX AND APPROACHES...VFR conditions beneath clear skies favored throughout the period. Low chances, around 20%, for MVFR or worse vis/cigs from 14-17z Wed if fog/mist or low stratus is able to develop. High confidence in a return to VFR conditions after 16-17z Wed. Diurnal northwest winds rise to around 5 kt this afternoon, then ease this evening. -36

MARINE

High pressure offshore will continue to maintain diurnal northerly winds through the week with peak afternoon and evening wind gusts of 20-25 kt today through Thursday. Small Craft Advisories remain in effect across the waters from 1 PM PDT today through this evening, with additional advisories likely on Thursday. Expect largely wind-driven seas around 4-6 ft with a west-northwest swell of 3-4 ft at 10-12 seconds. High pressure aloft will also support areas of dense fog developing nightly through the morning hours as cloud bases are compressed down to the surface.

More active marine conditions are likely in the latter half of the week as troughing over the Gulf of Alaska generates a building northwest swell. Seas will increase to 7-10 ft Wednesday into Thursday, with the longer-period swell also supporting a moderate risk for sneaker waves at the coast. Winds out of the northwest associated with a cold front will increase on Friday as seas peak at 8-12 ft by Friday night, before winds ease and veer out of the north while seas fall back to 5-7 ft from Saturday into early next week. -36/03

CLIMATE

Daily record high temperatures for October 8

LocationValue (Year)

Astoria 81 (1951, 1971) Portland Int'l87 (2022) Vancouver, WA89 (1891) Hillsboro89 (2022) McMinnville88 (2022) Salem84 (1971, 2022) Eugene83 (1934, 1943)

PQR WATCHES/WARNINGS/ADVISORIES

OR...Dense Fog Advisory until 10 AM PDT this morning for ORZ118-125.

WA...None.

PZ...Small Craft Advisory from 1 PM this afternoon to 2 AM PDT Thursday for PZZ210-251>253.

Small Craft Advisory from 1 PM this afternoon to 5 AM PDT Thursday for PZZ271>273.