textproduct: Portland

SYNOPSIS

Dry weather prevails as high pressure continues through the middle of the week. Breezy easterly winds continue through the Columbia River Gorge and eastern Portland/Vancouver Metro through early this week. For the few nights, clear skies and light winds will result in frost 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 Thursday night.

DISCUSSION...Now through Saturday night

Sunny skies prevail across the majority of northwest Oregon and southwest Washington as strong high pressure remains overhead. An exception is the central Columbia River Gorge and Upper Hood River Valley, where low stratus is being held in place due to the low level subsidence inversion. High confidence that this pattern continues through at least Tuesday, maintaining dry weather and chilly nights.

Breezy easterly winds continue through the Columbia River Gorge, surrounding terrain, and eastern Portland Metro through Monday. Observations as of early Sunday afternoon show wind gusts of 25 to 40 mph across the eastern Portland Metro with locally higher gusts of 40-50+ mph for exposed ridgetops like Crown Point. KTTD-KDLS pressure gradients are around -8 to -9 mb, and most guidance suggests that these gradients will hold through Monday afternoon and maintain similar wind gusts. By Monday night into early Tuesday morning, pressure gradients ease to around -4 to -6 mb and winds gradually weaken.

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 at least Tuesday night, dropping overnight lows to near or below freezing across the Willamette Valley, southwest WA 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 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.

Ensemble guidance indicates the strong high pressure will begin to slowly break down Wednesday into Thursday. Conditions are expected to remain dry on Wednesday with the subsidence inversion likely to begin to break down and temperatures will begin a return to near normal for January. There's around a 20-40% chance of precipitation returning sometime Thursday, mainly over the coast and terrain. Precipitation chances increase late Thursday night into Friday to 30-50% chance for the interior lowlands and 40-60% for the coast and terrain, continuing into Saturday. Onshore flow appears to return Thursday as well, bringing more mild air. However, about 15% of ensemble members suggest that temperatures will remain cold enough to result in wet, non-impactful snow or rain/snow mix falling down to the Valley floor. This would mainly be Friday night and Saturday night when temperatures are forecast the coldest. 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 out on 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 would result in wintery precipitation, especially if the incoming moisture overlaps 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. -10/03

AVIATION

Persistent upper-level ridging over the region will continue to yield largely VFR conditions throughout the period. Pockets of fog within sheltered valleys, most notably the far southern Willamette and northern Cowlitz Valleys, are possible tonight, though with lower confidence than previous days. Through the Columbia River Gorge, including at K4S2, MVFR stratus is likely to persist through tonight. A modest offshore pressure will be maintained through the period, supporting continued east winds at Portland-area terminals and along the coast, with light northerly flow expected through the Willamette Valley. Strong gusts of 35-40 kt continue at KTTD, with gusts up to 45-55 kt east of KTTD at the western end of the Columbia River Gorge.

KPDX AND APPROACHES...VFR flying conditions expected beneath largely clear skies. Diurnal east-southeast winds at 10-15 kt will ease below 10 kt late this evening, by 06-08z Mon, then rise again to 10-15 kt after 18z Mon. -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.