textproduct: Portland

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.

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 18z Thursday

Late this morning conditions are quickly trending VFR across the region as broad high pressure remains overhead. Confidence is high 18-19z onward clear skies prevail through the remainder of the day and a good portion of the overnight hours. We'll once again have to be on the lookout for patchy fog/low cloud formation near sunrise Thursday morning, although continued weak offshore flow and a slightly drier airmass should further limit the proliferation of LIFR/IFR conditions, especially inland. Chances for IFR vis/cigs at the coast peak around 30-35% come 12-15z Thur with 10-20% chances for similar conditions at inland terminals (30-40% chance at KEUG). Winds increase of the north to northwest today with gusts around 20 kt along the coast, and to 8-12 kt inland before lightening this evening.

KPDX AND APPROACHES...VFR conditions remain in place through the remainder of the day and most of the overnight hours. We'll once again have to watch for patchy fog formation come sunrise Thursday morning (12-15z), however, chances for degraded conditions only sit at around 15-20% locally. -99

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

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 2 AM PDT Thursday for PZZ210- 251>253. Small Craft Advisory until 5 AM PDT Thursday for PZZ271>273.