textproduct: Portland

SYNOPSIS

Persistent onshore flow will bring near-average temperatures with varying degrees of morning clouds and afternoon sunshine for much of the next week. A series of weak fronts will bring notable chances for light rain and drizzle to portions of the Washington and north Oregon coasts Tuesday and next weekend, while Friday is likely to be the warmest day.

DISCUSSION...Now through Sunday

The large-scale pattern remains fairly steady through the next week with the region wedged between upper-level troughing over the Gulf of Alaska and upper-level ridging over the Rockies and Central Plains. In general, the weather locally will be typical for the mid-summer: morning clouds followed by increasing afternoon sunshine, seasonable temperatures, and largely dry conditions, but with some variations as the strengths of these two dominant features change relative to one another.

Today, upper ridging wins out as heights aloft rise marginally, yielding clouds clearing all the way to the coast by the early afternoon with seasonably warm high temperatures in the 60s on the coast and mid 80s within the Willamette Valley. On Tuesday, an upper shortwave passing to the north will tilt the scales in favor of upper troughing, sending a weak front toward the coast resulting in cooler temperatures across the region and chances for light rain or drizzle along the coast and Coast Range, increasing from 40% at Florence to 60% at Newport, 75% at Tillamook, 95% at Astoria, and nearly 100% on the Long Beach Peninsula. Chances for accumulating rainfall are lower along the I-5 corridor, ranging from 20% at Salem to 30% at Portland, 60% at Woodland, and 75% at Kelso/Longview.

Through the latter half of the workweek, the upper-level ridge will strengthen and retrograde westward into the Great Basin, increasing heights aloft locally and again tipping the balance toward warm and dry weather. As such, temperatures will trend upward each afternoon, peaking on Friday in the upper 60s to around 70 degrees on the coast and in the mid to upper 80s within inland valleys. Despite this trend, onshore flow will continue, limiting the risk for hazardous heat. Minor HeatRisk is favored for the entire region on Friday, with 20-40% chances for Moderate HeatRisk in urban and suburban areas from Salem north to Vancouver, WA. By next weekend, there continues to be higher uncertainty in the evolution of the upper-level pattern, but many ensemble members depict more pronounced troughing returning to the Pacific Northwest, favoring cooler temperatures and further chances for rainfall. The details of timing and potential rainfall amounts remain fuzzy at this lead time, but those near the coast or within coastal terrain will be the most likely beneficiaries of any rainfall, with chances increasing to the north. -36

AVIATION...Through 18Z Tuesday

Mostly clear skies and VFR flight conditions will continue for inland TAF sites today. Low clouds with MVFR ceilings at the coast are beginning to scatter out, with VFR flight conditions expected at KAST and KONP by 18-19Z Monday. Winds at the coast will increase out of the southwest to near 10 kt with gusts to around 15 kt during the early afternoon, before easing below 5 kt and backing out of the south later this evening. A sea breeze front will bring increasing west to northwest winds for inland TAF sites between 01-06Z Tuesday, sustained between 5-10 kt. Winds become calm thereafter.

MVFR ceilings are expected to return to the coastal terminals by approximately 06Z Tuesday. There is some uncertainty regarding exactly how low ceilings will get at KONP; although low- end MVFR ceilings are the most likely outcome, there is around a 50% chance for IFR ceilings below 1000 ft. Inland TAF sites will most likely remain VFR tonight into tomorrow morning, however there is a 20-35% chance for a period of MVFR ceilings between 2000-3000 ft between 10-18Z Tuesday.

KPDX AND APPROACHES...Mostly clear skies will maintain VFR flight conditions today. Light and variable winds around 5 kt will become northwesterly around 5-10 kt around 02Z Tuesday with an incoming sea breeze. Although skies will most likely stay clear tonight, there is a 20-30% chance for a broken or overcast low stratus deck with ceilings between 2000-3000 ft between 10-18Z Tuesday. -23

MARINE

A weak front offshore will finally retreat northward today, bringing an end to light rain or drizzle chances off the south WA and for north OR coasts by this afternoon. Diurnal southerly winds around 10-15 kt and continue into Tuesday morning. Building high pressure behind another passing frontal boundary will see a wind shift to more seasonable northerly flow on Tuesday. Strengthening northerly flow through the latter half of the workweek will see 25-50% chances for peak wind gusts to reach 20-25 kt Wednesday through Friday afternoons, especially within 10 NM of the coast.

Seas continue at 4-6 ft through the next week with a dominant west to northwest swell at 8-9 seconds. -36

PQR WATCHES/WARNINGS/ADVISORIES

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