textproduct: Portland

SYNOPSIS

Unseasonably warm and dry weather will give way to cooler temperatures and chances for light rain as an upper trough moves over the region Friday into Saturday. Cool nights will bring chances for morning frost in rural valleys away from urban cores this weekend into early next week. Dry conditions with seasonable temperatures are expected Sunday and Monday before another round of light rain is possible on Tuesday.

SHORT TERM...Now through Saturday Night

Sunny skies under broad upper level ridging will maintain warm and dry conditions through this evening. A bit of a pattern change is forthcoming as the weather transitions toward more typical fall-like conditions. Water vapor satellite imagery reveals the upper level trough currently over the Gulf of Alaska that is expected to drop south into the Pacific Northwest on Friday. As this upper trough approaches, it will push a decaying cold front across the area later Friday morning through the afternoon. Expect clouds cover to increase tonight with light rain pushing onto the south WA/north OR coast early Friday morning. Best chance for rain along the I-5 corridor will be later Fri morning into the afternoon. Rain accumulations will be favored along the north OR and south WA coasts as well as the Cascades and their western slopes, while areas along the central Oregon coast and within the Willamette Valley are more likely to stay drier.

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 produce a second chance of light rain later Friday night into Saturday as scattered showers develop within the cooler air aloft. Mean forecast rainfall totals during this period are 0.10-0.25 inch north of Lincoln City and in the Cascades and foothills north of US-20, with 0.10 inch or less elsewhere. The chances for 0.25 inches of rain in 48 hours reach 40-60% in the wettest areas from Tillamook County north on the coast and north of OR-22 in the Cascades and foothills. Elsewhere, those chances are 15-25% in the Lower Columbia and Cowlitz Valleys, and 5-15% within the Willamette Valley and along the coast south of Pacific City.

As the initial front moves inland Friday through Friday night, peak gusts out of the west in the Cascades, central Columbia Gorge, and Hood River Valley could reach 35-45 mph. Behind the boundary as the core of cooler air aloft moves overhead, temperatures will quickly fall back toward seasonal norms in the upper 50s and 60s Friday and Saturday. Overnight lows will similarly trend cooler, especially Saturday night. On Friday night, there is high confidence that higher elevation portions of the Upper Hood River Valley near Dee and Parkdale will see temperatures in the mid 30s and an elevated potential for frost. A Frost Advisory is in effect for portions of the Upper Hood River Valley County late Friday night into Sat morning.

By Saturday night, the chances for a true freeze in the Upper Hood River Valley reach 35-65% in those same areas. There are additionally greater than 50% chances for temperatures in the mid 30s and frost on Saturday night in the Cascade foothills and north Oregon Coast Range Valleys, and 25-50% chances in the southern Willamette Valley and higher elevation portions of Clark and Cowlitz County lowlands in southwestern Washington. Frost Advisories and/or Freeze Watches may therefore be needed, and those with agricultural interests should continue to monitor the forecast. DH/36

LONG TERM...Sunday through Wednesday Night

Ensemble guidance is in good agreement that the elongated upper trough drops south over California on Sunday and eventually splits into a cutoff upper low early next week. Weak upper level ridging of the NE Pacific will nudge back over the region, leading to dry and more seasonably warm temperatures into the mid to upper 60s. Sunday morning will feature a near-repeat of Saturday morning, if not cooler under more clear skies, with widespread chances for frost in sheltered valleys away from urban areas and chances for a freeze within the Upper Hood River Valley. There is increasing consensus among long-range ensemble members in additional upper-level troughing moving into the region Tuesday into Wednesday. At this point, low to moderate (35- 55%) chances for rain are assessed during this period until confidence in the evolution of this feature increases. DH/36

AVIATION...Through 18Z Friday

Currently VFR conditions throughout the airspace, although KONP could still experience moments of LIFR stratus this afternoon as marine stratus lingers near the coastline. Looking ahead, coastal terminals will likely drop back down to LIFR/IFR conditions by 02-04Z Fri, with chances of LIFR conditions around 20-50% (highest at KONP) and chances of IFR around 30-60%. Additionally, KEUG has a 15-25% chance of LIFR and 25-35% chance of IFR conditions after 06-08Z Fri.

Into early tomorrow, a trough dips into the region, bringing rain to KAST around 12Z Fri and then interior lowland terminals around 15-20Z Fri as the front moves southward (KPDX: around 15Z, KEUG: around 19Z). Additionally, this trough will bring a 20-40% chance of MVFR CIGs.

Otherwise, expect north to northwest winds around 5-10 kt inland and 10-15 kt with gusts near 20 kt along the coast this afternoon. Winds will then ease to 5 kt or less by this evening. Expect similar winds tomorrow afternoon, although possibly a little stronger with the passing weather system.

KPDX AND APPROACHES...VFR with high cloud cover through most of the period. Northwest winds around 4 to 6 kt this afternoon, then easing into the evening. As a trough approaches tomorrow morning, expect rain to arrive around 16Z Fri and a 30-40% chance of MVFR CIGs. Additionally, winds will likely increase tomorrow afternoon, to around 8 to 10 kt. ~12

MARINE

High pressure at the surface over the northeastern Pacific today continues to bring afternoon/evening wind gusts of 20-25 kt. As a result, Small Craft Advisories for the waters south of Cape Falcon remain in effect until 11 PM tonight. Anticipate largely wind-driven seas of 6-9 ft with an underlying west-northwest swell during the advisories. While weakening high pressure aloft would continue to support areas of fog in the morning hours tomorrow, an approaching weather system that will bring rain and wind will likely inhibit fog from becoming dense.

This weather system on Friday and through the weekend, which is a trough over the Gulf of Alaska pushing southward, will generate a northwest swell in addition to the arrival of a frontal boundary. Winds out of the northwest associated with the frontal boundary will increase to around 20 to 25 kt on Friday, especially north of Cape Falcon, while seas peak at 8-12 ft Friday evening into Saturday. By Sunday, winds ease and veer out of the north to north-northeast while seas fall back to 5-7 ft through early next week. ~12/36

PQR WATCHES/WARNINGS/ADVISORIES

OR...Frost Advisory from midnight Friday night to 9 AM PDT Saturday for ORZ121.

WA...None. PZ...Small Craft Advisory from 11 AM to 6 PM PDT Friday for PZZ210.

Small Craft Advisory from 8 AM to 2 PM PDT Friday for PZZ251.

Small Craft Advisory until 11 PM PDT this evening for PZZ252-253- 272-273.

Small Craft Advisory from 5 AM to 2 PM PDT Friday for PZZ271.