textproduct: Portland

SYNOPSIS

Dry weather returns this evening and persists through the weekend as high pressure builds over the Pacific Northwest. Expect near to slightly above normal temperatures and increased offshore flow this weekend. Breezy east winds expected Saturday night through Sunday morning. Another shortwave trough returns chances for showers on Monday, with another potential system mid-to- late next week.

DISCUSSION...Tonight through Friday

Radar imagery as of early Friday afternoon shows light showers mainly from Salem northward. Expect any lingering showers to diminish by this evening as high pressure builds over the area with increasing cloud breaks. An upper level ridge will continue to build over the Pacific Northwest this weekend bringing a return to drier conditions. Forecast soundings for Saturday morning depict a weak surface inversion developing. With residual surface moisture and light winds expected, this would trend toward some fog and/or low stratus developing overnight into Sat morning. But, mid to high level clouds will likely not allow for completely clear skies, so still some uncertainty exists for sky cover and fog development Saturday morning.

Should see increasing sunny skies by the afternoon with afternoon high temperatures likely warming into the lower 60s. Offshore flow could make temperatures at the coast a degree or two warmer than the Willamette Valley this weekend. Similar, to slightly warmer conditions expected on Sunday as the ridge of high pressure amplifies into western Canada and persists over the region, shifting slightly eastward over the Great Basin. Speaking of offshore flow, east winds are expected to increase late Saturday into Sunday as the surface pressure gradient tightens through the Columbia River Gorge. Latest guidance suggests the gradient from KTTD to KDLS peaks around -6 to -8 mb, suggesting wind gusts up to 30-40 mph through the eastern Portland/Vancouver Metro, and up to 60 mph for exposed ridgetops (e.g. Crown Point).

Sunday night into Monday, the upper ridge begins to flatten as a shortwave trough pushes into British Columbia and NW Washington. Since most ensemble guidance suggest that this system will track further north of our area, precipitation amounts are not looking significant or impactful. The highest chances for rain are along the coast and southwest Washington (40-70%), while chances for rain are lower (20-40%) across interior northwest Oregon. Chances for 0.25" of rain or more in a 24 hr period from 4 AM Mon to 4 AM Tue are only 20-30% along the coast and 5-15% across the Willamette Valley and southwest Washington lowlands.

Another upper level ridge builds over the Pacific Northwest on Tuesday, bringing a brief return of dry weather. By Wednesday, this ridge quickly gets pushed eastward as the next frontal system approaches. The majority of ensemble members (70-80%) are in general agreement of light rain returning by Wednesday night and continuing into Thursday. Note that there is still some uncertainty with exact precipitation amounts.

AVIATION

Developing offshore winds will see a drying trend through the night, however sufficient cloud breaks in the near term may allow for periods of fog to develop at area terminals. With restricted vis already observed along the Lower Columbia River, including KKLS/KSPB/KVUO, KPDX may see brief drops to MVFR/IFR vis in the next 2-3 hours, a 20-30% chance, before increasing east winds aid in dissipating any mist/fog. To the east at KTTD, chances are much lower, less than 5%, as dry east winds will continue through the period, gusting over 25 kt after 16-18z Sat. To the south along the Willamette Valley, as well as other locales where terminals are more sheltered from east winds, chances for IFR/LIFR vis within fog/mist are higher; restrictions to vis are expected to develop by 08-12z Sat. Along the coast, east winds are likely to support continued VFR conditions, with less than a 10% chance of MVFR vis within mist. After any fog/mist dissipates by 16-18z Sat as east to northeast winds increase, VFR conditions are then expected through the remainder of the period as inland high pressure maintains offshore flow.

KPDX AND APPROACHES...Very marginal forecast for fog/mist versus increasing east winds through early Saturday morning. Terminals downstream on the Columbia River have already observed restricted vis to LIFR/IFR levels, including just across the river at KVUO. Brief restrictions are therefore likely to occur at KPDX, but it remains difficult to identify precisely when. Chances are the highest in the next 2-3 hours before easterly winds begin to increase, with even light east winds of 3-5 kt aiding to quickly lessen fog chances. If the onset of easterlies is delayed, fog chances will linger later in the overnight period. Once east flow of 5-10 kt is in place, VFR conditions will be favored to continue through the remainder of the period with only passing high clouds at 15-20 kft -36

MARINE

A Small Craft Advisory will begin across all waters including the Columbia River Bar from 4 PM today to 7 AM Saturday for seas around 10-14 ft at 12-13 sec. High pressure builds heading into the weekend, maintaining relatively benign marine conditions with winds remaining under 15 kt. Winds do begin to turn more easterly on Saturday as an offshore pressure gradient develops, then more southerly to southeasterly on Sunday. Minimal impacts expected as guidance suggests a less than 10% chance for frequent wind gusts greater than 21 kt and a >90% chance for seas under 10 ft this weekend. Into early next week, wind direction will be rather variable. Wednesday will likely be the next day of active weather, with low to moderate confidence for Small Craft winds over the outer waters. ~12/10

PQR WATCHES/WARNINGS/ADVISORIES

OR...None.

WA...None.

PZ...Small Craft Advisory until 7 AM PST Saturday for PZZ210- 251>253-271>273.