textproduct: Portland

SYNOPSIS

Building high pressure with periods of offshore flow will support warm and dry conditions through much of next week. Early morning valley fog will be limited to sheltered valleys, especially in the Coast Range.

DISCUSSION...Now through Thursday

A weak upper shortwave moving inland along the US-Canada border will act to flatten broad upper-level ridging centered over the desert Southwest through tonight. Any rainfall chances from this wave will remain to the north over the Olympic Peninsula and Puget South, with dry weather and above-normal temperatures continuing locally. As the wave exits eastward through this afternoon, passing high clouds will clear, leading to mostly sunny skies inland. The coastal marine layer will be more persistent, leaving those along the immediate coast and near the Columbia River and Willapa Bay estuaries largely overcast and slightly cooler.

Tonight through the weekend, the upper ridge amplifies over much of the West Coast in the wake of the departing shortwave, before shifting inland by early next week. Mid-level temperatures rising to near 20C will push surface temperatures well above seasonal norms through the weekend, likely peaking Sunday afternoon in the low to mid 80s within inland valleys. While most daily temperatures records appear to be out of reach, a record value could be tied or broken at Portland Int'l (84F set in 1952).

Thermal troughing developing along the coast is expected to remain fairly weak, with a mean pressure gradient between Portland and the Dalles of -2 mb and almost certainly remaining less than -4 mb per short-range ensembles. This magnitude of gradient favors a diurnally varying wind with offshore (easterly) flow developing nightly over the Cascades and their western slopes, as well as possibly in the highest reaches of the Coast Range, before daytime mixing sees winds decrease over terrain and increase out of the north within inland valleys. Offshore winds look to peak Saturday night into Sunday morning, with gusts of 20-35 mph in the Cascades and foothills, highest in typical areas within and near the Columbia River Gorge. This pattern will additionally promote nightly areas of valley fog thanks to light winds away from the Cascades, dry mid and upper levels limiting cloud cover, and robust nocturnal inversions during increasingly long nights. Coast Range valleys are the most likely to see areas of limited visibility, but the coverage of fog across the region and possibly including the Willamette Valley remains uncertain.

By early next week, the upper-level ridge looks to shift inland and broaden, yielding cooler but still above-normal temperatures locally. Long-range ensemble guidance supports high confidence in continued dry conditions through nearly the entire workweek, with the next widespread chances for rain holding off until next Friday at the earliest. -36

AVIATION...Through 18Z Saturday

Currently mostly VFR inland and a mix of IFR to VFR conditions along the coast. Coastal terminal conditions will gradually improve back to VFR in the afternoon as daytime mixing lifts clouds and decreases cloud cover. However, probabilistic guidance suggests a 40-50% chance of MVFR CIGs through the afternoon at all coast terminals. Around 02-06Z Sat, marine stratus will return to the coastal terminals and increase chances of deteriorated conditions. Specifically, chances of LIFR conditions will increase to around 30-50%, IFR around 50-60%, and MVFR around 60-70% overnight between 06-16Z Sat. While inland terminals will likely stay VFR through the period, chances of deteriorated conditions increase after 11Z Sat, to around 10-20% for LIFR/IFR conditions and 20-30% for MVFR conditions.

Light and variable winds will increase out of the northwest to around 3 to 6 kt this afternoon. Winds then ease tonight around 06Z Sat and shift easterly at terminals near the Cascade foothills and coastal gaps.

KPDX AND APPROACHES...VFR conditions likely through the TAF period, with a 20-30% chance of MVFR and 10-20% chance of LIFR/IFR conditions after 11Z Sat. Northwest winds 5 kt or less throughout the period. ~12

MARINE

A weak system moving into western Washington will see west to southwest winds north of Cascade Head today, veering out of the north by tonight. North winds otherwise remain in place through the next week as high pressure strengthens. Diurnal peak gusts Saturday and Sunday afternoon may exceed 20 kt, most likely within 20 NM of the coast. Seas continue at 3-6 ft through much of the next week with a persistent northwesterly swell at 8-9 seconds through Sunday, lengthening to 10-12 seconds early next week. -36

PQR WATCHES/WARNINGS/ADVISORIES

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