textproduct: Portland

SYNOPSIS

Expect a cool and unsettled weather pattern this weekend thanks to an upper-level low pressure system moving into the Pacific Northwest. Showers and a slight chance (10-20%) for thunderstorms develop Saturday and continue into Sunday. A much warmer and drier pattern takes hold by the early to middle portion of next week as high pressure builds offshore. Confidence is beginning to increase in a window of easterly winds around the middle of next week as well helping to aid this warm-up.

DISCUSSION...Now through Thursday

Overall rather benign weather conditions are in place this afternoon into the evening as we await an upper-level low pressure system currently deepening over the eastern Pacific. Come Saturday models are in excellent agreement showing this feature moving overhead with rain showers reaching the coast Saturday morning and spread inland through the afternoon. Rain and showers will continue Saturday night, followed by additional showers through most of Sunday. At this point confidence remains high the vast majority forecast area will see measurable precipitation and temperatures stay well below normal during this period.

Saturday through Sunday, total rainfall amounts should generally remain on the lighter side, with around 0.10-0.40 inch expected for most locations. The Cascades will likely receive more, with the southwest Washington and north Oregon Cascades currently favored for the highest totals around 0.50-1.00 inch (highest values in the southwest Washington Cascades). These latter two areas is where we'll have to keep an eye out for a mid-level deformation band forming on the north to northeast periphery of the low pressure center which will act to consolidate shower bands and lead to higher rainfall amounts. Should it shift a touch further south, the Portland Metro would see rainfall totals notably increase during the Saturday night/Sunday morning time period. Once the low pressure begins to shift east of the Cascades as Sunday progresses showers become more isolated/scattered and terrain driven, especially in the afternoon, while the inland valleys and immediate coastline revert back towards mostly dry weather by the late afternoon.

It's worth highlighting there is a slight chance for thunderstorms (10-20%) Saturday mid to late afternoon into the evening as the cold air associated with the upper-level low moves overhead. How much instability develops will depend partly on the timing of the system as best daytime heating and coolest air aloft current don't overlap for more than a couple hours (if that). But, current deterministic model soundings show a deep enough layer of CAPE to produce lightning. If the arrival of the coldest air aloft pushes back further into the evening thunderstorm chances will be lower. Any weak Thunderstorms could produce infrequent lightning, locally erratic winds, and moderate to heavy rainfall. Cold-core funnel clouds could also develop in more robust convective elements. These are generally short lived and way more often than not remain above the ground.

A warmer pattern is then expected to develop early next week as high pressure strengthens over the eastern Pacific. Some lingering troughing overhead will help keep Monday and Tuesday from becoming completely clear and too warm, but the overall trend is toward drier weather and rising temperatures. Interior valley high temperatures should climb into the mid 70s by Tuesday afternoon.

By Wednesday, ensemble guidance favors the offshore ridge moving closer to the Pacific Northwest, with about a 70-75% chance of this scenario playing out. If the ridge continues to build as expected, temperatures could reach well into the 80s across the interior lowlands. The other 25% of the guidance keeps some troughing near the region and would result in slightly cooler temperatures. Increasing easterly flow is also looking increasingly likely by midweek as the ridge strengthens coupled with a low pressure feature over northern CA helping to drive broad NE winds aloft. Current guidance suggests the strongest winds from Wednesday into Thursday morning, with sustained winds of 10-20 mph and gusts 25-35 mph (isolated gusts up 40 mph) for our more wind prone locations in the Columbia River Gorge, Cascades and adjacent foothills, and to a lesser extent the tops of the Coast Range (higher uncertainty here). With rising temperatures and lowering relative humidity values Tuesday through Thursday coupled with the increasing winds out of the east, this will definitely be a time period to monitor in the coming days. -99/12

AVIATION...Through 06z Sunday

VFR conditions remain in place across the airspace ahead of an approaching area of low pressure which will bring increasing rain chances after 18-24z Sat. Bkn- ovc largely high clouds will continue to stream overhead tonight, with bases lowering from 20-25 kft down to 8-10 kft by 18z Sat. Low marine stratus arriving on a southerly surge will favor IFR cigs along the central OR coast after 09z Sat, while terminals farther north maintain VFR cigs at 5-8 kft overnight. There is also a 20-40% chance for a period of MVFR cigs in the hours after sunrise at inland terminals, most likely from 15-18z Sat. Fairly light winds overnight will begin to rise out of the south by Saturday morning.

Low-end VFR cigs at 3-6 kft and rain showers arrive from the west by 21-24z Sat, initially along the coast and spreading inland. VFR cigs remain favored during rain showers, while restricted vis is most likely along the coast. South to southwest winds rise to 8-12 kt at coastal terminals and to 5-10 kt inland as the rain showers approach. There are low chances, 15-20%, for an isolated thunderstorm across the region Saturday afternoon and evening.

KPDX AND APPROACHES...VFR conditions expected through the period. Increasing low clouds Saturday as an area of low pressure approaches, but rain may hold off until after 00z Sun. Modest 40% chances in MVFR cigs in the mid to late morning, 15-19z Sat, before increasing mixing again favors VFR conditions by midday. Light winds tonight will back out of the south to southwest at 6-10 kt by Saturday afternoon. -36

MARINE

An area of weak low pressure near Vancouver Island will strengthen slightly was it drops south over the waters tonight into Saturday. Relatively light winds tonight become southerly by Saturday morning, and increase to around 10-15 kt with gusts up to 20 kt. Heavier showers or thunderstorms that develop with this low pressure system over the waters on Saturday could produce stronger wind gusts and lightning. The low is expected to push inland by Sunday with onshore southwesterly winds persisting through Sunday night. High pressure rebuilds over the waters by Monday, returning northerly winds to the coastal waters early next week.

Somewhat choppy seas around 6-7 ft continue this afternoon, but are expected to subside slightly to around 4-5 ft during the weekend as a northwest swell persists at 9-10 seconds. /DH

PQR WATCHES/WARNINGS/ADVISORIES

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