textproduct: Portland

SYNOPSIS

Today will be the warmest day of the week as weak offshore flow gives way to onshore flow during the afternoon. A few showers and an isolated thunderstorm are possible in the Lane and Linn County Cascades, while the remainder of the area stays dry. Temperatures warm again over the weekend before a cooler pattern becomes increasingly likely early next week.

DISCUSSION..Now through Wednesday

The upper-level low over northern California will begin shifting northeast toward the Great Basin today as ridging builds eastward across the Pacific Northwest. This will allow the surface thermal trough to weaken through the day, but not before another warm afternoon across the interior lowlands. Highs are expected to reach the low to mid 80s, with the warmest readings from the Portland metro to Corvallis. There is a 30-70% probability of temperatures reaching 85 degrees or higher from the Portland metro to Corvallis, with the highest chances in the Tualatin Valley. As for 90 degrees or higher, only a 10-20% chance in the Tualatin Valley and less than 5% elsewhere. Areas south of Corvallis should remain mainly in the low 80s, while coastal and higher terrain locations stay considerably cooler with highs in the upper 60s to mid 70s.

Offshore flow will linger through the morning, particularly across the Cascades, foothills and western Columbia River Gorge, before the thermal trough breaks down and onshore flow returns this afternoon. Northerly winds will continue elsewhere, with breezy conditions possible through the morning before the wind pattern gradually becomes more westerly. The return of onshore flow should also help moderate temperatures later in the day, particularly closer to the coast.

The upper low's northeastward movement will bring a modest increase in moisture into the southern Cascades today. As a result, shower chances increase across the Lane and Linn County Cascades, with probabilities reaching 20-60%. A stray thunderstorm cannot be ruled out, with around a 10% chance along the higher terrain. The remainder of the forecast area is expected to remain dry, with little support for convection elsewhere.

Friday marks a return to a more typical onshore-flow pattern, with marine stratus likely developing along the coast and spreading into portions of the interior lowlands during the morning, such as through the coastal gaps and up the Columbia River. Temperatures will depend in part on how quickly the low clouds erode, as well as the evolving upper-level pattern. Ensemble guidance still shows some spread, with a cooler outcome favoring interior highs in the lower 70s and warmer solutions supporting upper 70s to possibly 80. Regardless, precipitation chances remain low and conditions should stay dry across the area.

Forecast confidence improves slightly heading into the weekend, with temperatures expected to climb into the mid 70s to low 80s Saturday and mid 70s to upper 80s Sunday under generally dry conditions. Attention then turns toward a developing trough dropping south from the Gulf of Alaska early next week. There remains considerable uncertainty in the eventual strength, position and timing of this feature, but the overall pattern increasingly favors cooler temperatures and stronger onshore flow. At this time, there is around a 20% chance for measurable rainfall Tuesday through Wednesday across southwest Washington and northwest Oregon, but again, much of the forecast beyond the weekend remains uncertain. ~12

AVIATION...Through 12Z Friday

East to northeast flow aloft continues through most of the TAF period as an upper level low over northern California begins to head northeast. Expect VFR conditions at inland terminals through most of the period, with a 10-30% chance of MVFR CIGs from 06-12Z Fri. These chances will continue to increase as marine stratus is increasingly likely to push inland Thursday night. At the coast, there is a 30-40% chance of IFR and 20-30% chance of LIFR conditions until 17-18Z Thu. Coastal conditions then improve back to VFR by 19Z Thu (80-90% chance of VFR). Between 00-12Z Fri, the return of onshore winds will bring chances for deteriorated conditions at the coastal, around 70-90% for MVFR, 30-40% for IFR, and 10-20% for LIFR (highest chances between 6-12Z Fri at KAST and 00-06Z Fri at KONP). Otherwise, light winds this morning will increase to 5-10 kt during the afternoon, with rather low confidence in wind direction. Most terminals look to shift from northwesterly to southerly through the period.

KPDX AND APPROACHES...VFR conditions expected through most of the TAF period with mostly clear skies, then a 10-30% of MVFR CIGs after 06Z Fri as marine stratus attempts to push inland. Northwest to westerly winds around 4-6 kt through most of the period, then shifting southerly by 07-09Z Fri. ~12

MARINE

North winds ease today and shift more west to southwest over the waters about 40-50 NM from the coast tonight into Friday. A persistent northwest swell around 4 to 6 ft at 7 to 8 seconds is expected to continue through at least Sunday. Northerlies strengthen again over the weekend. The waters south of Cape Falcon have around a 50-80% chance of at least occasional wind gusts exceeding 21 kt. The waters north of Cape Falcon have a 15-30% chance of wind gusts exceeding 21 kt on Saturday, increasing to 50-70% by Sunday. There is then a 50/50 chance of continued Small Craft winds into early next week. A north to northwest swell of 5 to 7 ft at 8 to 10 seconds arrives early next week. ~12/03

PQR WATCHES/WARNINGS/ADVISORIES

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