textproduct: Portland

SYNOPSIS

The region is transitioning from the cool pattern of the past several days toward a much warmer and drier regime. Monday and Tuesday will remain on the cool side with areas of morning low clouds and fog, but temperatures begin a sharper climb Wednesday as high pressure builds inland. Interior highs are expected to reach the 80s Wednesday and Thursday, accompanied by strengthening easterly winds in the Columbia River Gorge, Cascades and adjacent foothills. Confidence decreases toward the end of the week as the ridge begins to weaken, although dry weather remains favored.

DISCUSSION...Now through Sunday

The most notable change in the forecast is the transition in the large-scale pattern from the recently cool and unsettled regime to a warm, dry setup by midweek. The weekend upper low has moved east of the area, and precipitation will no longer be a significant concern across the lowlands. Some residual moisture combined with light winds and partial clearing will favor low clouds and areas of fog this morning, like more widespread than dense. Visibility could still become locally poor in sheltered locations.

Temperatures will be slow to recover at first. Interior highs Monday and Tuesday should only reach the upper 60s to low 70s, remaining roughly 5-7 degrees below the seasonal average. Another round of morning fog is possible Tuesday, particularly in protected valleys where a stronger low-level inversion is expected. Otherwise, dry weather and gradually increasing sunshine will set the stage for the much warmer pattern arriving midweek.

By Wednesday, the offshore ridge will begin exerting more influence on the Pacific Northwest. Meanwhile, the shortwave dropping south from British Columbia is expected to close off over northern California. This evolving pattern favors both rapid warming and increasing easterly flow. Interior temperatures should reach the upper 70s to low 80s Wednesday through Friday, with Thursday currently favored to be the warmest day. The latest NBM gives a much lower probability of reaching 90 degrees compared to yesterday, with now a 5-10% chance across the north Willamette Valley and less than 5% elsewhere on Thursday and Friday, and little to no chances on Wednesday. This change continues to suggest widespread 90-degree heat unlikely, but highs should still run approximately 5-10 degrees above the seasonal average.

Easterly winds will become increasingly important across elevated terrain Wednesday into Thursday. A combination of east- northeasterly flow aloft and a northward-surging thermal trough near the surface should strengthen easterly winds Wednesday into Thursday morning. While the latest NBM has trended towards slightly weaker winds, gusts around 20-30 mph are still expected in the more wind-prone portions of the Columbia River Gorge, Cascades and adjacent foothills, with isolated gusts up to 35 mph. The higher terrain of the Coast Range could also become breezy, although confidence is lower. Relative humidity may fall into the 30-35% range during the warmest part of the period. Taken together, the warmer temperatures, lower humidity and increasing east winds could create a period of elevated fire- weather concerns.

The degree to which the warm pattern persists beyond Thursday is less certain. Ensemble guidance currently splits into two general solutions for Friday, with roughly 40-50% of members keeping the ridge close enough to maintain mid 80s temperatures across the interior lowlands, while 50-60% favor a quicker breakdown and highs back in the 70s. In short, current guidance suggests high temperatures anywhere between 72 and 86 on Friday. Saturday carries similar uncertainty, although there is a general trend toward cooler temperatures even among the warmer solutions (high temperatures anywhere between 68 and 83 degrees). Despite this temperature spread, the precipitation signal remains much more consistent, with a 90% chance of dry weather continuing through Friday and into the weekend. ~12

AVIATION...Through 12Z Tuesday

Dry, northwesterly flow aloft an upper level low departs the region and high pressure builds offshore. Expect primarily VFR conditions throughout the airspace through the TAF period. The only exception will be a 20-50% chance of MVFR CIGs at coastal terminals and 10-30% chance of MVFR CIGs at inland terminals until 18-20Z Mon. Expect persistent VFR conditions thereafter. Some guidance suggests a 20-50% chance of deteriorated conditions along the coast beginning around 03-06Z Tue, but confidence remains too low to include in the TAFs at the moment. Otherwise, current light and southerly winds will rise to 5-9 kt and veer out of the north to northwest by mid-afternoon.

KPDX AND APPROACHES...VFR conditions through the period, with the exception of a 10-20% chance of MVFR CIGs until 18-20Z Mon. Light southerly winds will increase to 4-6 kt and veer out of the northwest by 18-20Z Mon. ~12

MARINE

Current west winds will veer out of the north by late this afternoon as high pressure builds offshore. The ridge will continue to strengthen through midweek, intensifying northerly winds to 15-20 kt with gusts to 25 kt by Wednesday afternoon. There is a 60-90% chance sustained winds will peak above 21 kt on Wednesday to the south of Cape Falcon, with lower chances, 10-40%, on both Tuesday and Thursday. Additionally, there is a 10-35% chance of wind gusts of 30 kt or greater on Wednesday. Winds then ease into next weekend as high pressure weakens.

Seas around 5 ft early this week will similarly build to 6-7 ft as winds strengthen. A persistent northwesterly swell at 8-9 seconds otherwise continues through the week, with seas easing into next weekend as high pressure and winds weaken. ~12/36

PQR WATCHES/WARNINGS/ADVISORIES

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