textproduct: Portland

SYNOPSIS

Warm and dry conditions through Thursday. Low pressure will return cooler temperatures and chances for showers Friday into Saturday, with a 5-10% chance for thunderstorms. Winds out of the north/northwest through Thursday, shifting to the west/southwest on Friday. Seasonable weather continues Sunday into early next week.

DISCUSSION...Today through Tuesday

Dry and warm weather continues today and tomorrow as an upper-level ridge amplifies over the Rockies and Great Basin. Expect high temperatures near or slightly above average for this time of year, with mid 80s this afternoon across inland valleys and 60s along the coast. Highs for inland valleys increase slightly into the upper 80s as the upper ridge reaches its peak strength. Onshore flow continues as broad upper-level troughing sits over the northeast Pacific with light winds from the north/northwest.

On Friday, the aforementioned trough over the northeast Pacific will progress toward the Pacific Northwest, bringing slightly cooler temperatures, increased cloud cover, and a 20-30% chance for showers in the afternoon and evening. Will note that this upper-level pattern on Friday looks like a typical set-up for thunderstorm chances, at least over the Cascades. Southerly flow aloft will bring increased moisture and instability into the mid-levels of the atmosphere (5-10 kft). Vorticity from the upper-level trough will also provide the necessary forcing for convection. Currently, NBM guidance shows low chances (5-10%) for thunderstorms on Friday, however, in the past we've seen models struggle with capturing the potential for high-based thunderstorms. There is also uncertainty among deterministic/ensemble guidance with how much instability (SBCAPE/MUCAPE) there will be, however, we will continue to evaluate the thunderstorm threat today. Those with outdoor plans on Friday should monitor the forecast closely these next couple days.

By Saturday, ensemble guidance shows the upper-level trough moving into the Pacific Northwest with chances for showers continuing through the morning. As the associated surface front moves through, will see winds shift and come out of the west/southwest. The strongest winds are expected through the Central Columbia River Gorge and Hood River Valley where winds could gust to 25-35 mph. There is also a 30-60% chance for at least isolated gusts exceeding 40 mph for these areas, especially for higher terrain and exposed ridgetops.

Late this weekend into early next week, the trough weakens and most ensemble members suggest a return of zonal flow before another upper-level trough drops down from the Gulf of Alaska and approaches British Columbia. At this point, there still remains high uncertainty with the exact magnitude and placement of the trough next week. However, this would suggest that onshore flow continues across the region resulting in seasonable temperatures for late August. -10

AVIATION...Through 12z Thursday

Satellite imagery and terminal observations as of early Wednesday morning depicts marine stratus along the coast bringing IFR conditions to KAST. KONP has been VFR, however, expect the stratus to fill in and return IFR/MVFR CIGs this morning. Some stratus has made its way up the Lower Columbia River Valley and down toward the eastern Portland Metro Area, so expect low-end MVFR CIGs (1-2 kft) at KPDX and KTTD after 12-13z Wed. Low confidence for stratus to make it into the central and southern Willamette Valley this morning as guidance shows a 15-25% chance at any given hour between 13-17z Wed for MVFR CIGs. After 17-18z Wed, daytime heating will dissipate stratus and return VFR conditions. VFR prevails across the Valley through the TAF period. Along the coast, guidance suggests brief improvement to VFR between 21z Wed- 01z Thu before IFR/MVFR stratus returns in the evening. Light west/northwest winds around 5-10 kt along the coast and 4-7 kt across the Valley.

KPDX AND APPROACHES...Low-end MVFR CIGs (1-2 kft) through 17-18z Wed. Afterwards, VFR conditions prevail through the TAF period. Northwest winds 4-6 kt today. -10

MARINE

High pressure will continue to support northerly winds across the waters through Thursday with gusts remaining below 20 kt. Friday to Saturday, weak low pressure moving over the northeast Pacific will shift winds out of the south to southwest across the waters. As the surface front moves through Saturday morning, there will be a 40-50% chance for maximum wind gusts up to 25 kt for the inner waters north of Cape Falcon, including the Columbia River Bar. For the rest of the waters, chances are around 5-10%.

High pressure and northerly winds return by Sunday with a 50-60% chance for northerly wind gusts exceeding 21 kts in the afternoon and evening hours south of Cape Foulweather. Seas of 4-6 ft persist through the period. Periods around 12 sec today may result in a moderate risk for sneaker waves along the coast. Caution should be used when in or near the water. -10

PQR WATCHES/WARNINGS/ADVISORIES

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