textproduct: Portland

SYNOPSIS

Expect cooler and unsettled conditions through the weekend as an upper-level low pressure system progresses overhead. Conditions then turn progressively warmer and drier early next week as high pressure builds offshore. Increasing easterly flow by Wednesday and Thursday could bring locally breezy/gusty winds to the Gorge, Cascades and nearby foothills, while also aiding the warmer and drier weather.

DISCUSSION...Now through Friday

This afternoon an upper- level low pressure system continues to move towards the coastline with rain and showers expected to further develop headed into the evening hours. This precipitation likely persists at times overnight into Sunday morning as the upper low gradually shifts eastward. From this afternoon through Sunday evening most locations from the coast into the Willamette Valley should see around 0.10-0.30 inch of rainfall; measurable amounts but not much more. However, there's decent confidence noticeably higher amounts fall across the southwest Washington foothills/Cascades into the north Oregon Cascades where closer to 0.50-1.25 inch of rain is anticipated. This is due to a deformation band developing on the northern periphery of the upper-level low helping to consolidate showers leading to locally moderate sustained rainfall. There still remains a slight bit of uncertainty among high-resolution models as to exactly how far south this area of steadier precipitation could wander. A slight southward nudge would increase rainfall totals across the Portland Metro, particularly Sunday morning. As the upper low moves well east of the Cascades by Sunday afternoon, showers should become more scattered and increasingly focused over higher terrain features. The inland valleys and immediate coast likely trend mostly dry by the mid to late afternoon hours, while the Cascades continue to see lingering showers into the evening.

It's worth quickly highlighting a ~15% chance of thunderstorms was maintained in the forecast for this evening as cold air moving in aloft may provide just enough instability for a stray short-lived T-storm. Cold-core funnel clouds are also possible with the stronger showers through Sunday. These are typically short-lived and largely remain above the ground.

Come Monday and Tuesday the low pressure system ejects further eastward with large scale troughing and northwesterly flow holding in place across the Pacific Northwest thanks to a secondary weak shortwave feature sliding down the BC coastline late Monday. Temperatures will warm day to day compared to this weekend, although only a couple degrees each afternoon. Expect highs in the low to mid 70s by Tuesday afternoon - mid 60s along the coast.

Then a more noticeable warm-up begins Wednesday into Thursday as a ridge of high pressure which had been forming over the eastern Pacific begins to shift eastward into the region. The bulk of deterministic models and ensemble systems have now converged on this solution. High temperatures to soar well into the 80s across the interior lowlands as a result of this more summer- like pattern. For Thursday afternoon specifically, the latest NBM shows a 15-35% chance for highs to exceed 90s degrees across the central/northern Willamette Valley, 0-10% elsewhere. Easterly flow is also becoming more likely midweek. The strongest winds currently appear on Wednesday into Thursday morning, with gusts of 25-35 mph and isolated gusts near 40 mph possible in more wind-prone areas of the Columbia River Gorge, Cascades and adjacent foothills. The tops of the Coast Range could also become breezy, although confidence here is lower. Currently, there is around a 10-15% chance gusts could exceed 45 mph in the most exposed locations near/within the Columbia River Gorge. The combination of warmer temperatures, lower humidity and increasing east winds Wednesday and Thursday may provide a window of elevated fire-weather concerns. -99/12

AVIATION...Through 00z Monday

VFR conditions prevail across the airspace ahead of an approaching weak area of low pressure. Scattered to numerous rain showers moving onshore the coast will continue to spread inland into this event, arriving at Willamette Valley terminals by 01z Sun. 15-20% chances of an isolated thunderstorm will also persist through 06z Sun across the region. While confidence is low in the occurrence of terminal impacts at any particular time, showers and storms will be capable of producing gusty, erratic winds and locally heavy rain which could briefly restrict vis. The likelihood of MVFR cigs generally increases through the evening as rain showers assist in moistening the lower atmosphere. MVFR chances reach 20-40% at inland terminals from 06-15z Sun and 40-60% from 15-21z Sun, while along the coast they continue at 30-60% into tonight, rising to 60-80% by 15z Sun. As showers end from west to east after 15-18z Sun, conditions may begin to improve late in the period.

South to southwest winds of 8-12 kt with gusts up to 20 kt at coastal terminals and 5-10 kt inland will ease to around 5 kt out of the south tonight, then rise to 5-10 kt out of the southwest by Sunday afternoon.

KPDX AND APPROACHES...VFR conditions favored through at least 06-12z Sun. Rain showers will spread inland through the coming hours as an area of low pressure near the coast, with a 15-20% chance for an isolated thunderstorm through 06z Sun. While impacts are unlikely, gusty and erratic winds or heavy downpours briefly restricting terminal vis cannot be ruled out. Chances for MVFR cigs otherwise generally increase as showers continue from 20-30% this evening and into tonight to 35-50% by 12-15z Sun. Conditions may improve late in the period as shower coverage dwindles. South to southwest winds 4-8 kt continue through the period. -36

MARINE

A weak surface low centered off the central Oregon coast as of early Saturday afternoon will traverse the waters this afternoon before moving inland over the north Oregon coast by early Sunday morning. Ahead of this system, southerly winds of 8-15 kt will continue, with the strongest winds and gusts to 20 kt limited to areas within 20 NM south of Cape Foulweather. Numerous rain showers are also expected during this time, as well as 15-20% chances of isolated thunderstorms capable of producing gusty and erratic winds and locally heavy downpours through this evening. Onshore flow at 5-10 kt out of the west to southwest continues on Sunday while rain shower coverage decreases through the day. A persistent northwesterly swell at 8-9 seconds will maintain overall seas around 5 ft through the weekend.

Surface high pressure rebuilds over the waters through the first half of next week, with northerly flow returning late Monday. Strengthening northerlies through midweek will see 50% or greater chances of winds 20 kt or higher on Wednesday, especially beyond 10 NM south of Cape Falcon. Seas around 5 ft with a dominant northwesterly swell at 8-9 seconds otherwise continue. -36

PQR WATCHES/WARNINGS/ADVISORIES

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