textproduct: Portland

SYNOPSIS

Onshore flow strengthens to start the week allowing for a return to slightly cooler temperatures and chances for light showers Monday to Tuesday, mainly along 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. Weather impacts appear minimal going forward.

DISCUSSION...Now through Saturday

Dry conditions continue to prevail across much of the region this afternoon as broad northerly flow remains aloft. Satellite and radar do show some cumulus build-ups and isolated showers initiating over our higher terrain features like the coast range and Cascades, but these should wane come the sunset hours with the loss of daytime heating. The Pop forecast was adjusted to match this shower threat - it was not captured well by the NBM. To start the work week the upper- level pattern shifts more northwesterly accompanied by 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 15-35% 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, close to normal for this time of year.

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. Due to better agreement among guidance regarding the amplitude and placement of the ridge, chances for exceeding 80 degrees on Thursday have increased further to 50-70% across the Portland/Vancouver Metro and 20-50% for the rest of the I-5 corridor.

Friday into the weekend, most ensemble guidance and their respective deterministic counterparts depict 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 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. -99/10

AVIATION

VFR conditions with high level clouds (20-25 kft) prevail for inland terminals through today. Isolated showers are developing over the Coast Range and Cascades as of 21z Sun. These should by 00-03z Mon. Along the coast, VFR conditions prevail north of KONP with MVFR ceilings at and south of KONP. Winds have turned due west along the coast, bringing lower confidence in a few hours of clearing to VFR or fluctuations between MVFR/VFR at KONP through 00z Mon. Westerly winds along the coast will push marine moisture inland, bringing a solid marine stratus deck back to the entire coast by 00-02z Mon. Expect a mix of MVFR and IFR ceilings along the coast. This will be a stronger marine push, so there is also a 60-80% chance for marine stratus moving into the Willamette Valley after 09-12z Mon. Expecting MVFR cigs at all inland terminals by 12-14z Mon, with conditions improving to VFR by 18-21z Mon. West to northwesterly winds less than 10 kt become southerly for inland terminals after 06z Mon.

KPDX AND APPROACHES...High confidence for VFR conditions with west to 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

Winds are beginning to shift more west to northwest over the waters Sunday afternoon as an approaching weak front from the north is breaking down the high pressure over the coastal waters. Winds remain west to northwest into Tuesday with gusts expected to remain below 15-20 kts as the 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 60-80% chance for at least isolated small craft northerly wind gusts between 20-25 kt Wednesday into Friday. Gusts will be strongest each late afternoon and evening hours when pressure gradients are tightest. Seas of 5-7 ft persist through the week with a northwesterly swell. -03

PQR WATCHES/WARNINGS/ADVISORIES

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