textproduct: Portland

SYNOPSIS

Temperatures will be around ten degrees cooler than yesterday for inland valleys today due to persistent morning cloud cover and stronger onshore flow. Temperatures warm up a few degrees this weekend as an upper level shortwave ridge moves over the area. Trending a bit cooler again next week as an upper level trough slowly drops southward from the Gulf of Alaska. Dry conditions are heavily favored through Thursday with a possible pattern shift toward cooler and wetter conditions by next Friday, September 25th.

DISCUSSION..Now through next Friday

Satellite observations from early Friday morning showed a low marine stratus deck along the coast and Coast Range expanding into the lower Columbia and southern Willamette Valley. This cloud deck will most likely (60-80% chance) continue to expand towards sunrise, eventually filling in across the central and northern Willamette Valley, Cowlitz Valley, and greater Portland/Vancouver metro area. Morning clouds will eventually give way to afternoon sun as daytime heating progresses, except at the coast where skies will likely remain mostly cloudy through the day (>60% chance). Given the stronger onshore flow in place today and persistent low clouds in the morning, temperatures will be around ten degrees cooler than what was observed yesterday for inland valleys. Expect highs generally in the mid 70s Friday afternoon over the lowlands, except mid 60s at the coast.

Temperatures will warm a bit over the weekend. This is when model ensemble guidance depicts good agreement for an upper level shortwave ridge moving from west to east over the Pacific Northwest. It appears this ridge will move rather slowly with a closed upper low on the southwestern periphery of the ridge and another closed low on the eastern periphery of the ridge. Low-level onshore will favor at least some degree of morning cloud cover this weekend, however forecast soundings suggest the marine stratus deck will become more shallow. This means any cloud cover that does develop inland should dissipate quickly during the mid to late morning hours. Combined with warming temperatures aloft, this should allow for highs back into the upper 70s to lower 80s inland and upper 60s to around 70 degrees at the coast, warmest on Sunday.

The pattern trends relatively cooler next week as 500 mb flow becomes nearly zonal ahead of broad troughing approaching from the Gulf of Alaska. As this trough slowly approaches, temperatures will cool gradually Monday through Wednesday. However, cooling does not look overly significant as highs in the 70s are still favored inland with highs staying in the 60s at the coast. Confidence is high dry weather will continue Monday through Thursday afternoon, as 85-95% of the total ensemble space from the ENS/GEFS/GEPS shows no QPF at all. The remaining 5-15% showing very light rain amounts is mainly being driven by a few GEPS members. It appears a frontal system over the northeast Pacific will approach western Washington and western Oregon mid-week, which helps explain the small subset of ensemble guidance showing at least some light rain. However, the vast majority of ensemble guidance suggests this frontal system will decay over the coastal waters before pushing inland, hence the reason dry weather is the most likely outcome. That being said, chances for cooler and wetter weather increase more considerably come next Friday, September 25th. This is when roughly 40% of the total ensemble space from the ENS/GEFS/GEPS combined shows rain moving into the area. The remaining 60% keeps conditions dry. -23

AVIATION...Through 06Z Saturday

Satellite and surface weather observations from 10Z Friday depicted an expansive marine stratus deck extending along the coast and Coast Range, with ceilings around 1000-1500 ft from KAST to KONP. Confidence remains high this marine stratus deck will expand into the Willamette Valley and I-5 corridor by 12-15Z Friday at the latest. There is a 60-80% chance at any given hour between 12-18z Fri for MVFR ceilings around 1500-3000 ft across inland terminals, except for a 100% chance at KEUG where stratus has already developed. Stratus should gradually dissipate after 19-21z Friday across the Valley as daytime heating progresses, bringing a return to widespread VFR conditions inland. However, the coast will most likely remain socked in with MVFR ceilings through the day (>60% chance).

KPDX AND APPROACHES...VFR conditions with clear skies will most likely give way to MVFR ceilings with an incoming marine stratus deck between 13-17Z Friday (70-80% chance). Ceilings then trend back to VFR after 19-21Z Friday as low clouds gradually scatter out and skies become clear again. Confidence is moderate in regards to the timing for the morning stratus deck to lift in the afternoon. -23

MARINE

High pressure will strengthen over the waters on Friday and then remain in place through the weekend. Relatively light winds on Friday will strengthen out of the north Saturday afternoon and evening with gusts up to 15-20 kt, strongest over the central and southern waters. Northerly winds peak in strength Sunday afternoon and evening when there is a 70-90% chance for maximum wind gusts of 21 kt or slightly stronger across the waters, with the highest chances across the inner waters out to 10 NM offshore. Given wind gusts of this magnitude are not expected to be frequent or widespread, Small Craft Advisories are not anticipated at this time. Significant wave heights will remain around 4 to 6 ft through the weekend, mainly driven by a persistent northwesterly swell with a dominant wave period near 8 seconds.

Northerly winds are expected to linger Monday through Wednesday, with wind gusts generally staying under 20 kt. A long period west-northwesterly swell is set to arrive either Monday afternoon or evening, pushing seas up by one or two feet. However, seas will not be particularly steep as the dominant wave period with this swell will be around 11-13 seconds. -23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.