Marine Weather and Tide Forecast for Westport, OR

SYNOPSIS
High pressure and dry weather continues through at least the middle of the week with breezy easterly winds through the Columbia River Gorge and eastern Portland/Vancouver Metro as well. As long as high pressure prevails, clear skies and light winds away from the Columbia Gorge will result in frost and/or freezing fog formation across most interior lowland valleys.
Near/below freezing temperatures in combination with any fog may lead to locally slick road conditions. Lower confidence for fog/frost for locations that remain windy and/or have persistent stratus. Chances for precipitation return late week but details on the exact timing and impacts remain nebulous.

DISCUSSION
Now through Sunday...Similar to the last several days a strong ridge of high pressure remains situated over the West Coast likely holding in place into Wednesday leading to dry weather and mostly clear skies across NW Oregon and SW Washington. The only exception is the central/eastern Columbia River Gorge and Hood River Valley where stratus has remained socked in for days, as well as portions of Cowlitz County and the southern Willamette Valley around and south of Eugene where diurnal low stratus and fog develop continues to take place.

Breezy easterly winds are still in place through the Columbia River Gorge, surrounding terrain, and eastern Portland Metro this afternoon due to a strong pressure gradient across the Cascades due in-part to our current weather pattern. The pressure gradient from KPDX to KDLS has peaked around -9.5mb this afternoon leading to observed wind gusts of 20-35 mph through the Portland metro area, 30-45 mph across the eastern Portland metro area around and east of Gresham/Troutdale, and 55-75 mph at the known windy locations east of Troutdale, including Crown Point, and Corbett. Gradients are expected to slowly decrease tonight before largely leveling off through Wednesday. Peak wind gusts will respond be lowering as well but still remain breezy/gusty, especially in the late afternoon through overnight hours.

Clear skies and light winds away from the winds from the Columbia River Gorge will lead to efficient radiational cooling across the area each night through the end of the week, dropping overnight lows to near or below freezing across the Willamette Valley, southwest Washington lowlands, and Upper Hood River Valley. High confidence in favorable conditions for widespread frost development, especially over grasses and metal surfaces. Freezing fog may also develop in some locations, particularly across the southern Willamette Valley and lower elevations of Cowlitz County where surface conditions are more moist and protected from the dry air filtering through the Cascade gaps. Any freezing fog that develops could lead to locally slick road conditions. Meanwhile, higher elevations above 1500-2000 ft but below 5000 ft will remain much warmer than the lowland valleys due to a strong subsidence inversion.

Additionally, stagnant air will be a continued concern under this subsidence inversion as the winds decrease, leading to air quality concerns through at least mid-week. Mixing heights will remain low and transport winds will be weak. Therefore, an Air Stagnation Advisory remains in effect for most of the Willamette Valley due to this concern through 4 PM Wednesday. The Portland Metro area from the West Hills eastward were not included in this advisory due to stronger winds from the Gorge maintaining a more mixed atmosphere.

Looking ahead, the GEFS, ECMWF, GEPS and their respective deterministic models continue to indicate high pressure could remain in place over the eastern Pacific and Pacific NW on Wednesday into Thursday and possibly into the weekend. The majority of ensemble members indicate a trough moving through the central Pacific that will pinch off into a cutoff low and move towards California, undercutting the ridge of high pressure instead of breaking it down completely, thus leaving us in a drier Rex Block pattern. There remains a 15-25% chance of precipitation returning late Thursday through Friday, mainly for the coast and higher terrain features, with only a 5-10% chance for precipitation across the inland valleys - all in all dry weather is favored to persist. This weekend, the latest ensemble 500 mb cluster analysis indicate about a 25% chance of a shortwave pushing southwest through the Great Basin and into the PacNW and a 30% chance of a weak weather system moving in from the Pacific and breaking down the ridge. Both solutions produce an increased chance for precipitation Saturday into Sunday. However, that still leaves ~45% of ensemble solutions continuing at least weak ridging through the weekend with mostly dry weather. At any rate, ridging that continues Wednesday and beyond would be weaker than earlier in the week, which could lead to a moderating of daytime temperatures and a potential breakdown of the strong subsidence inversion.
Additionally, about 15% of ensemble members suggest that overnight/early morning temperatures will remain cold enough to result in wet, non-impactful snow or rain/snow mix falling down to the Valley floor when precipitation returns. If this occurs, impacts would be minimal given air temperatures will be marginally freezing due to onshore flow and roads would be too warm to accumulate snow.

One location we need to keep a closer eye on when precipitation eventually returns is central Columbia River Gorge and Upper Hood River Valley. If easterly winds continue, then colder air may linger longer for these areas. This could result in wintery precipitation, especially if the incoming moisture overlaps at the same time as the freezing air near the surface. These areas can remain much colder than other interior valleys like the Willamette Valley and southwest WA lowlands, and model guidance is generally too quick scouring out the cold air. Specific details will become more clear as we get closer to precipitation returning. -99/03

AVIATION
A persistent upper level ridge over the region will continue to yield widespread VFR conditions for most terminals through the period. Pockets of IFR/LIFR conditions within low stratus/fog may redevelop in sheltered portions of the southern Willamette and Cowlitz Valleys late this evening into tonight, as well as MVFR cigs in valleys east of the Cascade crest including at K4S2. An offshore pressure gradient continues to support easterly winds at Portland-area and coastal terminals, and light northerly winds along the Willamette Valley. Stronger easterly gusts of 30-40 kt will continue at KTTD into tonight, with gusts up to 45-60 kt east of KTTD at the western end of the Columbia River Gorge. Winds will ease for a time overnight but gusts to 20-25 kt will remain possible at KTTD. Locally stronger gusts to 25 kt are also possible downwind of gaps in the coastal terrain, including at KONP. The pressure gradient is expected to ease tonight, reducing winds across the region before increasing again Tuesday morning.

KPDX AND APPROACHES...VFR flying conditions expected through the period. Diurnal east-southeast winds at 10-15 kt will ease to around 5 kt after 8-10z Tuesday before increasing again by 18-20z.
-19/36

MARINE
Strong ridging aloft continues to favor persistent offshore easterly flow through midweek. While winds are largely at 10 kt or less, areas downwind of gaps in the coastal terrain may see stronger flow with gusts up to 15-20 kt, mainly west of the mouth of the Columbia River. Seas remain below 10 ft at 10-13 seconds through the middle of the week. -03/36

PQR WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory until 4 PM PST Wednesday for ORZ109- 114>118.

WA...None.

PZ...None.