textproduct: Portland

SYNOPSIS

Showers and gusty winds will gradually diminish this evening as the low pressure system continues to move northeast. A cool and dry pattern settles in for the weekend and continues into early next week, with renewed chances for rain Tuesday through Thursday.

DISCUSSION...Now through Thursday

The surface low is moving northeastward through Washington this afternoon, with the strongest winds now shifted well northeast of the region. Scattered showers continue behind the departing front as cool air aloft maintains modest instability across the area. The potential for thunderstorms has decreased this afternoon, with the latest SPC outlook no longer highlighting any portion of the forecast area (SW Washington/NW Oregon) for thunderstorms. A stray thunderstorm cannot be ruled out given the lingering instability, but confidence is low and thunderstorms are not expected to be a significant concern. Any heavier showers could still produce brief gusty outflow. An isolated cold-core funnel is also possible, mainly near the coast, although significant surface impacts from these features are unlikely. Rainfall rates and totals remain well below levels of concern for flooding.

Showers will steadily diminish from west to east this evening, with most areas west of the Cascades drying out first and showers lingering longest in the Cascades. Clearing skies and a colder air mass will then allow temperatures to fall quickly tonight. Most lowland locations should drop into the 40s, while colder spots in the Hood River Valley could fall into the upper 30s. The potential for frost is greater in the Upper Hood River Valley, particularly around Dee and Parkdale, where current probabilities for temperatures at or below 36 degrees are around 40-50%. Probabilities are lower near Odell at 5-10%. Confidence still remains too low for frost headlines, especially when expecting cloud cover overnight to keep temperatures a tad warmer.

Shortwave ridging and surface high pressure build in behind the departing system, resulting in dry weather from Saturday through at least Monday. Temperatures will remain cool for mid- September, with afternoon highs generally in the mid to upper 60s across the lowlands. Overnight temperatures will also remain chilly, particularly under clear skies and light winds on Sunday and Monday, so additionally frost potential will need to be monitored in the Upper Hood River Valley. Confidence in frost impacts decreases beyond Saturday morning as the details of cloud cover and overnight winds become less certain.

The next meaningful change in the pattern arrives Tuesday through Wednesday as ensemble guidance increasingly favors another upper-level trough moving into the Pacific Northwest. 6 hourly rain chances rise to roughly 20-40% during this period, with higher probabilities generally favored along the coast, across northern portions of the forecast area (mainly southwest Washington), and in the higher terrain. Over a 48 hour period from Tuesday to Thursday morning, there is a 70-90% chance of measurable rainfall at Lincoln City and Portland northward, and a 30-60% chance southward (lowest chances to the south, such as 30% at Eugene). Considerable spread remains in the strength and position of the trough, so precipitation timing and amounts remain uncertain. ~12

AVIATION...Through 00Z Sunday

Radar imagery as of early Friday afternoon depicts scattered showers across northwest Oregon and southwest Washington as a low pressure system exits the region. There remains a 15-25% chance for thunderstorms at any given terminal through 00z Sat. Any passing thunderstorms may produce lightning, heavy rain that can reduce VIS, gusty and erratic winds, and small hail. CIGs remain predominately low-end VFR through this evening, but can't rule out some intermittent drops to MVFR due to heavier showers or thunderstorms. Expect breezy south/southwesterly winds through 00z Sat with gusts up to 30 kt at any given terminal. Winds further weaken this evening below 10 kt once the system exits, with return of drier conditions.

KPDX AND APPROACHES...Predominately low-end VFR with showers through this afternoon. Can't rule out intermittent drops to MVFR CIGs or VIS during periods of heavier showers or thunderstorms. Southwesterly winds with gusts up to 25 kt through 00z Sat, weakening below 10 kt after 02-03z Sat. -10

MARINE

Breezy west-northwesterly winds with gusts up to 25 kt continue through this afternoon as a robust low pressure system exits the region. Seas remain steep and choppy around 9-12 ft at 9 seconds through this evening. Therefore, Hazardous Seas Warnings remain in effect for all waters through 5 PM this afternoon for all waters including the Columbia River Bar, transitioning to Small Craft Advisories from 5-11 PM this evening. Tonight into early Saturday morning, winds weaken below 15 kt and seas gradually subside to 5-7 ft at 8-9 seconds.

High pressure re-builds over the waters on Saturday, returning breezy northerly winds by the afternoon. Small Craft Advisories have been issued beginning Saturday afternoon/evening through early Friday morning for steep and choppy seas of 6-8 ft at 8 seconds and northerly wind gusts up to 25 kt. Northerly winds remain in place through the weekend before another weak front shift winds more southwesterly Monday-Tuesday. Will also see a longer period west-northwesterly swell of 5-7 ft at 12-14 sec Monday-Wednesday, resulting in a moderate sneaker wave threat. -10

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Hazardous Seas Warning until 5 PM PDT this afternoon for PZZ210- 251>253-271>273.

Small Craft Advisory from 5 PM this afternoon to 11 PM PDT this evening for PZZ210-251>253-271>273.

Small Craft Advisory from 5 PM Saturday to 5 AM PDT Sunday for PZZ210-251-271.

Small Craft Advisory from 2 PM Saturday to 5 AM PDT Sunday for PZZ252-253-272-273.