textproduct: Portland

SYNOPSIS

Onshore flow will bringing increased cloud cover and cooler temperatures today and Tuesday, along with chances for light drizzle along the coast and light showers across the Cascades. Drier and warmer weather returns Wednesday through the weekend as high pressure builds. Weather impacts appear minimal going forward.

DISCUSSION...Today through Sunday

Satellite imagery as of early Monday morning depicts marine stratus along the coast and mostly clear skies inland. Some of the marine stratus is beginning to filter through the Coast Range gaps as well as the Lower Columbia River into the Cowlitz Valley. Expect this marine stratus to gradually spread into the Willamette Valley this morning. Will maintain mostly cloudy skies today as onshore flow increases due to a passing shortwave trough. However, increased mixing due to daytime heating could lead to afternoon clearing along the I-5 corridor. In addition, the aforementioned shortwave will provide enough lift to result in chances for light drizzle along the coast and light showers across the Cascades. Interior valleys should remain dry. Winds will generally be light and westerly today, except for the Columbia River Gorge and Upper Hood River Valley where gusts are forecast to peak around 25-35 mph this afternoon. A similar pattern continues into Tuesday. Since we'll have more cloud cover, temperatures today and Tuesday will be a few degrees cooler than this past weekend, with afternoon highs forecast in the low to mid 60s across interior valleys and mid to upper 50s along the coast.

Wednesday to Thursday, the majority of deterministic and ensemble members show a heavily tilted upper-level ridge building over the northeast Pacific and eventually shifting into the Pacific Northwest. Most ensemble members also show above- average 500 mb heights over the area, suggesting high confidence (70-90% chance) that temperatures will rebound into the upper 60s to low 70s by Wednesday and upper 70s to near 80 by Thursday. Chances for exceeding 80 degrees on Thursday are around 50-70% for the Portland/Vancouver Metro Area, and 20-40% for the rest of the I-5 corridor.

Friday through Saturday, most deterministic and ensemble guidance depicts the upper-level ridge flattening as a weak trough moves into the Pacific Northwest from Canada. It appears that there is limited moisture with this trough, so precipitation chances remain around 20-40% across the Cascades and less than 10% elsewhere. In addition, most ensemble members maintain above-average 500 mb heights over the area, meaning seasonably warm temperatures would continue through the end of the week with high temperatures forecast in the 70s for interior valleys. By Sunday, the majority of ensemble members have the trough moving southward toward California while upper-level ridging re-builds over the Pacific Northwest. Even this far out, NBM guidance is already suggesting higher confidence (60-80% chance) that temperatures exceed 80 degrees along the I-5 corridor on Sunday. -10

AVIATION

Satellite imagery and terminal observations as of early Monday morning depicts MVFR conditions along the coast due to marine stratus. This stratus is making its way up the Lower Columbia River and Coast Range gaps, and will gradually move into the Willamette Valley this morning. There is also some stratus already developing in the far east Portland/Vancouver Metro. High confidence for at least low-end VFR (3-4 kft) stratus across the Willamette Valley with a 40-60% chance for MVFR CIGs at any given hour between 12-18z Mon.

After 18z, increased mixing from daytime heating will help lift CIGs, but remain low-end VFR. Along the coast, high confidence for MVFR stratus holding throughout the day at KAST, while there is a 40-60% chance for improvement to VFR at KONP. After 03z Tue, a renewed marine push will support CIGs dropping to MVFR again at KONP. Winds generally west-southwesterly across the region and remaining under 10 kt. An exception would be breezy west winds through the Columbia River Gorge into the Upper Hood River Valley with gusts up to 25-30 kt.

KPDX AND APPROACHES...VFR through 11-12z Mon. After 12z Mon, high confidence for high-end MVFR/low-end VFR stratus. Increased mixing after 18z Mon will improve CIGs to predominately VFR. Westerly winds under 10 kt through 21z Mon, turning more northwesterly in the afternoon and evening. -10

MARINE

Winds remain westerly to northwesterly into Tuesday with gusts expected to remain below 15-20 kts as a weak front moves through the waters Monday. By Tuesday evening, high pressure returns over the waters, remaining through the end of the week. Winds become northerly with a 70-90% chance for at least isolated small craft northerly wind gusts up to 20-25 kt from Wednesday through Friday. Gusts will be strongest each late afternoon and evening hours when pressure gradients are tightest. Seas of 6-7 ft persist through the week with a northwesterly swell. -10

PQR WATCHES/WARNINGS/ADVISORIES

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