textproduct: Portland

SYNOPSIS

Dry weather prevails as high pressure continues through at least the middle of the week with breezy easterly winds through the Columbia River Gorge and eastern Portland/Vancouver Metro. As long as high pressure prevails, clear skies and light winds 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.

DISCUSSION...Monday morning through Sunday

The forecast for Monday into Tuesday will be a rinse and repeat of conditions observed over the weekend. A strong ridge of high pressure remains situated over the West Coast through at least Tuesday, with increasing confidence through Wednesday, leading to dry weather and mostly clear skies across NW Oregon and SW Washington. The only exception is the central Columbia River Gorge and Hood River Valley where stratus has remained socked in for days, as well as the southern Willamette Valley around and south of Eugene where low stratus and fog develop in the late night through early morning hours.

Breezy easterly winds continue through the Columbia River Gorge, surrounding terrain, and eastern Portland Metro through Monday due to increased pressure gradients over the Cascades under the current pattern. The pressure gradient from KPDX to KDLS has increased from -8 to -9 mb to -9 to -10 mb late Sunday night into early Monday morning, leading to an increase in wind gusts compared to Sunday. Ultimately, the pressure gradient is expected to fluctuate through Monday between -8 to -10 mb leading to peak wind gusts of 25-35 mph through the Portland metro area, 35-45 mph across the eastern Portland metro area around and east of Gresham, and 50-70 mph at the known windy locations east of Troutdale, including Crown Point and Corbett. Gradients are expected to begin decreasing Monday night to around -6 to -8 mb, remaining in this range through at least Tuesday and now likely into Wednesday. Peak wind gusts would decrease but still remain elevated, 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 Wednesday night, dropping overnight lows to near or below freezing across the Willamette Valley, southwest Washington lowlands, and Upper Hood River Valley. High confidence remains that this would favor widespread frost development, especially over grasses and metal surfaces. Freezing fog may also develop in some locations, particularly across the southern Willamette Valley where surface conditions are more moist. 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 and has been extended 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.

The past couple of GFS, Euro, and Canadian ensemble runs have begun indicating high pressure could remain in place over the eastern Pacific and Pacific NW on Wednesday into Thursday and possibly into the weekend. This is because 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. This has caused a significant decrease in precipitation chances returning late this week, though it doesn't completely eliminate this possibility. There is now a 15-25% chance of precipitation late Thursday through Friday, mainly for the coast and terrain. For the weekend, 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 20% 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 ~55% of ensemble solutions continuing 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 would keep a closer eye on when precipitation returns would be the 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. These areas can remain much colder than other interior valleys like the Willamette Valley and southwest WA lowlands. Specific details will become more clear as we get closer to precipitation returning. -03/10

AVIATION

A persistent upper level ridge over the region will continue to yield widespread VFR conditions for most terminals through the period. Pockets of low stratus/fog continue to erode in sheltered portions of the southern Willamette and Cowlitz Valleys this morning, with a recurrence of IFR/LIFR cigs/vis favored beginning late this evening and continuing into tonight in the same areas. MVFR cigs east of the Cascade crest at K4S2 are favored to trend to VFR by 21z Mon. An offshore pressure gradient remains in place, producing 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 through much of the period, with gusts up to 45-60 kt east of KTTD at the western end of the Columbia River Gorge. 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.

KPDX AND APPROACHES...VFR flying conditions expected through the period. Diurnal east-southeast winds around 5 kt will build to 10-15 kt by 21-22z Mon, then ease to around 5 kt tonight. -36

MARINE

Strong ridging aloft continues favoring persistent easterly offshore 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 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.