textproduct: Portland

SYNOPSIS

Dry weather continues today with mostly sunny skies and morning stratus along the coast. Weak shortwave troughs will return slightly cooler temperatures and chances for light showers Monday to Tuesday, with the highest chances along the coast, Coast Range, and Cascades. Locations along the I-5 corridor remain mostly dry. Drier and warmer weather return Wednesday through the end of the week as high pressure builds.

DISCUSSION...Today through Saturday

Satellite imagery as of early Sunday morning depicts mostly clear skies across northwest Oregon and southwest Washington except along the coast where marine stratus is increasing. Given the clear skies, efficient radiational cooling will allow morning temperatures to drop into the upper 30s to low 40s for most locations. An exception is the Upper Hood River Valley, where colder temperatures in the low to mid 30s will support frost development this morning. A Frost Advisory remains in effect through 8 AM this morning for the Upper Hood River Valley. Expect dry weather with mostly sunny skies today across the region with afternoon highs forecast in the upper 60s to low 70s across interior valleys and upper 50s to low 60s along the coast.

The upper-level pattern turns more northwesterly Monday through Tuesday with weak upper-level shortwave troughs rotating into the Pacific Northwest from a parent low located over south- central Canada. These weak upper-level features combined with an influx of low to mid level moisture (surface to 800mb) will increase chances for light showers, mainly over high terrain which can provide orographic lift. Chances for showers on each of these days remain less than 15% along the I-5 corridor and 20-40% across the Cascades and Coast/Coast Range. Due to the onshore flow and increased cloud cover, daytime temperatures early this week cool down a few degrees compared to this weekend with 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 LREF ensemble members show upper-level ridging building over the northeast Pacific and shifting into the Pacific Northwest. Most ensemble members also show above-average 500 mb heights over the area, suggesting high confidence (60-80% chance) that temperatures will rebound into the upper 60s to low 70s by Wednesday and mid 70s by Thursday. Chances for exceeding 80 degrees on Thursday have increased to 50-70% across the Portland/Vancouver Metro and 20-40% for the rest of the I-5 corridor.

Friday into the weekend, most ensemble guidance show the 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 10-20% 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. -10/99

AVIATION

VFR conditions with high level clouds (20-25 kft) prevail for inland terminals through today. At 1730z Sun, marine stratus is clearing north of KTMK along the coast, with VFR conditions expected through the afternoon. South of KTMK, marine stratus is likely to continue through the TAF period with MVFR ceilings, though there could be a few hours of clearing to VFR or fluctuations between MVFR/VFR from 20z Sun - 00z Mon. Onshore flow increases late this afternoon, bringing a solid marine stratus deck back to the entire coast by 00z Mon. Expect a mix to MVFR to IFR ceilings. This will be a stronger marine push, so there is also a 60- 80% chance for marine stratus moving into the Willamette Valley around 09-12z Mon. Expecting MVFR cigs at all inland terminals by 12- 14z Mon, with conditions improving by 18-20z Mon. Variable winds under 5 kt through 18-21z Sun turn more west- northwesterly less than 10 kt. Winds become southerly inland after 06z Mon.

KPDX AND APPROACHES...High confidence for VFR conditions with high clouds through Sunday. Northwesterly winds under 10 kt. An onshore marine push shifts winds southerly after 06z Mon and introduces a 60- 80% chance for marine stratus producing MVFR conditions between 09- 12z Mon. Expect conditions to improve to VFR by 18-21z Mon. -03

MARINE

High pressure remains over the waters into Sunday, maintaining northerly to northwesterly winds. Winds shift more westerly on Monday into Tuesday morning as a weak front moves over the waters. By Tuesday evening, high pressure returns over the waters through the end of the week, returning northerly winds. Winds expected to remain under 20 kts through at least Tuesday. Seas of 6-7 ft persist through next week with a northwesterly swell. By mid to late week, there is a 60-80% chance for at least isolated small craft northerly wind gusts between 20-25 kt, strongest each afternoon when pressure gradients are tightest. -10/03

PQR WATCHES/WARNINGS/ADVISORIES

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