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 Night

An upper level trough over the Gulf of Alaska will begin pushing into the Pacific NW on Friday, bringing an initial round of light rain associated with a weak cold front dropping south over the area. Rain likely begins early Friday along the coast before reaching 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. Average forecast rain totals during this period are 0.10-0.15 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 80% 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. DH/36

AVIATION...Through 00z Friday

VFR conditions are in place across the region thanks to high pressure remaining overhead. While confidence is high VFR cigs/vis persist through the evening, 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. Chances for IFR vis/cigs at the coast peak around 35-45% come 12-15z Thur with 15-25% chances for similar conditions at inland terminals (30-40% chance at KEUG). Given the latest HRRR, would lean towards high probability of fog in at least the vicinity of KEUG. Any fog/low stratus layer Thursday morning will be shallow, quickly dissipating by 17-18z. Winds peak late this afternoon along the coast with gusts around 20 knots before decreasing this evening.

KPDX AND APPROACHES...VFR conditions continue through this evening and most of the overnight hours. Come the sunrise hours we'll once again have to watch for patchy fog formation near the terminal (12-15z time period), however, chances for degraded conditions currently sit at around ~20%.-99

MARINE

High pressure at the surface will continue to linger over the eastern Pacific the rest of today through Thursday facilitating peak afternoon/evening wind gusts of 20-25 knots. As a result, Small Craft Advisories remain in effect through this evening and then again Thursday mid afternoon/evening for the waters south of Cape Falcon. During this time period, anticipate seas around 6-9 ft with an underlying long period west-northwest swell. 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.

Come Friday expect the development of more active marine conditions as troughing over the Gulf of Alaska pushing southward generates a reinforcing northwest swell in addition to the arrival of a frontal boundary. Winds out of the northwest associated with a cold front will increase on Friday into Friday evening as seas peak at 8-12 ft by Friday night and Saturday. At least this rise in seas is fairly short lived. Sunday winds ease and veer out of the north to north-northeast while seas fall back to the 5-7 ft range for the early to middle portion of next week. -99/36

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.