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 next several 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.

DISCUSSION...Sunday morning through Saturday

A strong ridge of high pressure continues over the West Coast with a thermally induced trough remaining along the OR and WA coast at the surface. There is high confidence from ensemble guidance that this pattern will continue through Tuesday. This pattern is leading to dry weather, mostly clear skies, and warmer than normal daytime temperatures. Additionally, the surface thermal trough has caused a strong pressure gradient over the Cascades. Observations indicate the gradient from KPDX to KDLS has remained around -8.5 to -9 mb since last evening instead of falling to around -6 to -7 mb as previous guidance suggested. Latest CAMs now indicate the gradient will remain elevated around -7 to -9 mb through at least Monday and possibly into Tuesday. Now this gradient is still just slightly weaker than the previous two days, so peak gusts in the far eastern Portland metro are mainly around 30-40 mph, generally around and east of Troutdale. However, around the mouth of the western Columbia Gorge, wind gusts are still howling with gusts of 45-55 mph, locally up to 60-70 mph at the known windy locations of Crown Point and Corbett. Expecting wind gusts to generally remain at these level into Monday night, though if the pressure gradient does slacken some, could see a bit of a decrease in gusts.

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 Tuesday night, dropping overnight lows to near or below freezing across the Willamette Valley and southwest WA lowlands as well as the Hood River Valley. High confidence remains that this would favor widespread frost development and/or freezing fog, especially over grasses and metal surfaces. Freezing fog is not expected to be widespread as the cold temperatures will likely limit fog development in most areas except for typical fog prone areas. Freezing fog 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 is increasingly a concern under this subsidence inversion as the winds decrease, leading to air quality concerns into at least Tuesday. An Air Stagnation Advisory is in effect for most of the Willamette Valley due to this concern.

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 40-60% chance for the interior lowlands and 60-75% for the coast and terrain, continuing into Saturday. Onshore flow appears to return Thursday as well, bringing more mild air. However, if easterly winds continue through the Columbia River Gorge, cold air may linger longer across the central Gorge and Upper Hood River Valley. This would result in wintery precipitation for these areas, especially if the incoming moisture overlaps the same time as the freezing air. Specific details will become more clear as we get closer to precipitation returning. -03/10

AVIATION

Persistent upper-level ridging over the region will continue to yield largely VFR conditions throughout the period. Pockets of nightly fog in sheltered valleys, most notably the far southern Willamette and northern Cowlitz Valleys, continue to slowly erode this morning as insolation and mixing increase, with a return to VFR conditions expected by 21z Sun. Similar conditions 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 much of today. 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. East-southeast winds will rise to 10-15 kt by 20-21z Sun, then ease below 10 kt late this evening. -36

MARINE

Strong ridging aloft continues through the weekend favoring persistent easterly offshore flow. 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. This offshore flow will persist into midweek. Seas remain below 10 ft around 10-13 seconds through the middle of the week. -03

PQR WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory until 10 AM PST Tuesday for ORZ109- 114>118.

WA...None.

PZ...None.