textproduct: Portland

SYNOPSIS

Unseasonably warm and dry weather will persist through Thursday. Weak offshore flow will bring slightly warmer temperatures Wednesday and 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.

LONG TERM...Friday through Monday

Model ensemble guidance is in better agreement on Friday and Saturday in regards to the synoptic-scale pattern and the potential for light rain. WPC's cluster analysis for 500 mb heights and anomalies suggests a trough will move over B.C. and western WA Friday afternoon before becoming more amplified and forming a potential closed cut-off low on Saturday over the Pacific Northwest. This will bring two separate chances for rain. The first chance will be with a weak Pacific cold front associated with the incoming trough on Friday; rain chances begin Friday morning for much of southwest WA and the north OR Coast/Coast Range before spreading inland Friday afternoon/evening. Model ensemble guidance suggests eastern slopes of the Coast Range and locations to the west of the I-5 corridor within the Willamette Valley will be mostly rain-shadowed, with only a trace to a few hundredths of an inch of rain falling, if any at all. The NBM backs this up, as PoPs only peak around 10-15% in these areas. Meanwhile, PoPs and forecast rain amounts are relatively higher for the coast, Willapa Hills, Cowlitz Valley, western slopes of the Coast Range and Cascades, Cascade foothills, and locations east of the I-5 corridor within the Willamette Valley and Portland-Vancouver metro. That said, even these areas have low forecast rain amounts, with only a 15-35% chance for 0.10 inches of rain or more, except a 30-50% chance along and near the south WA/north OR coast, and in the south WA/north OR Cascades and adjacent foothills. Therefore, no impacts are expected with this frontal passage. QPF guidance suggests minimal post-frontal showery activity Friday night, except for the Cascades and foothills where orographic lift will allow showers to linger for a bit longer.

Chances for rain then increase again on Saturday as the cut-off low moves over the area, especially for locations north of Florence, Eugene-Springfield, and Oakridge. Northern zones have a 20-40% chance of rain Saturday afternoon and around a 10-25% chance for 0.10 inches of rain or more. Rain chances fall below 5-10% on Sunday and Monday as dry low-level offshore flow develops, bringing breezy easterly winds to the Cascades, foothills, western Columbia River Gorge, and eastern Portland-Vancouver metro. Meanwhile, breezy north winds will develop in the Willamette Valley.

The set-up this weekend will finally bring noticeably cooler temperatures, with highs in the 60s to around 70 degrees on Friday, lowering to the upper 50s to lower 60s on Saturday. Sunday and Monday trend a touch warmer as upper level heights rise again and offshore flow strengthens, however guidance is confident temperatures will be much closer to seasonal normals compared to what has been observed thus far this month. Morning lows will similarly cool, yielding increasing chances for frosty temperatures in the low to mid 30s from Friday night through Monday night. Frost may occur as early as Friday night in the Upper Hood River Valley, with 30-40% chances of falling to 36F or lower near Parkdale and Dee. Saturday and Sunday nights look to be the coldest, with nearly 10% chances for frost in Hood River proper, 50% near Odell, and 90% in Parkdale and Dee. Elsewhere, frost chances reach 15-35% in north Oregon Coast Range valleys, the central and southern Willamette Valley away from urban cores, and the Cascade foothills from Cowlitz County south to Lane County on Sunday night. Chances for a true freeze peak Sunday night at 50-60% near Parkdale and Dee and 15-20% near Odell in the Upper Hood River Valley, and at 20-35% along the OR-58 corridor between Eugene and Oakridge. Those with agricultural interests or home growers with sensitive outdoor plants should continue to monitor the forecast this week. -23/36

AVIATION...Through 06z Thursday

A broad ridge of high pressure remains over the region with light southwest flow aloft. Satellite imagery shows mostly clear skies across the area as a marine stratus deck offshore pushes south along the central OR coast. However, there is a 40-50% chance that VIS remains reduced at KONP to LIFR due to mist or fog overnight. VFR conditions are expected inland through at least 10-12z Wed. Light offshore flow developing later tonight may mix in enough dry air to disrupt the low stratus and fog that has developed these past few nights throughout much of the interior lowlands. There remains a 20-30% chance for MVFR/IFR conditions developing at inland terminals, except for around a 30-50% chance within the southerly Willamette Valley near KEUG, between 12-18z Wed. North to northwest winds again increase along the coast with gusts to around 20 kt near KONP by 20z Wed.

KPDX AND APPROACHES...VFR conditions are expected to persist under mostly clear skies through at least 12z Wed. There is a low chance (20-30%) of MVFR/IFR stratus returning between 13-17z Wed. Northwest winds become light overnight. /DH

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 2 PM PDT this afternoon into tonight, and another round of Small Craft Advisories have been issued for 2 PM PDT Wednesday through Wednesday evening. Expect largely wind- driven seas around 4-6 ft through midweek, 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. Chances for rain return to the region Friday into the weekend, though rain amounts are not expected to be impactful. -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...None. WA...None. PZ...Small Craft Advisory until 2 AM PDT Wednesday for PZZ210-251>253.

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

Small Craft Advisory until 5 AM PDT Wednesday for PZZ271>273.

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