textproduct: Portland

SYNOPSIS

Dry weather prevails as high pressure continues through the middle of the next week. Breezy easterly winds continue through the Columbia River Gorge and eastern Portland/Vancouver Metro through the remainder of the weekend. Tonight and 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.

SHORT TERM...Now through Friday night

Sunny skies prevail across the majority of northwest Oregon and southwest Washington as strong high pressure sits over the Pacific Northwest. An exception would be the central Columbia River Gorge and Upper Hood River Valley, where low stratus has been persistent with little to no clearing. Easterly winds through the Gorge and eastern Portland Metro continues with gusts up to 35-40 mph. Will let the Wind Advisory drop off at 3 PM today as pressure gradients gradually weaken this evening. KTTD-KDLS pressure gradients around -9 to -10 mb this afternoon are forecast to fall to around -6 to -7 mb tonight. Easterly winds will remain breezy through the remainder of the weekend, however it does not appear that we'll have frequent wind gusts of 45+ mph across the majority of the eastern Portland Metro to warrant a continuation of the Wind Advisory. Based on high resolution guidance, chances for wind gusts 45 mph or greater fall below 5% this afternoon in this area, but can't rule out some brief, isolated gusts of 45-50 mph closer to the mouth of the Gorge. For higher terrain surrounding the Gorge and exposed ridgetops like Crown Point or Three Corner Rock, wind gusts up to 50-60 mph continue through this evening.

Tonight, clear skies and light winds will lead to efficient radiational cooling across the area, dropping overnight lows to near or below freezing across the Willamette Valley and southwest WA lowlands. High confidence that this would favor widespread frost development and/or freezing fog, especially over grasses and metal surfaces. Freezing fog could lead to locally slick road conditions. Meanwhile, there is moderate confidence for frost development in the Upper Hood River Valley due to today's persistent cloud cover. If clouds continue overnight, it may be more difficult for frost to develop. Temperatures right now are already in the mid to upper 30s across the Upper Hood River Valley, so if any clearing occurs, it won't take longer for temperatures to fall below freezing and lead to frost. Widespread frost remains in the forecast for the next several nights through Thursday morning as clear skies and light winds promote low temps in the low to mid 30s for many interior lowland valleys. Meanwhile, higher elevations above 1500-2000 ft but below 5000 ft 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 today into at least Tuesday. An Air Stagnation Advisory is in effect for most of the Willamette Valley due to this concern.

Beyond Tuesday, ensemble guidance indicates the strong high pressure will begin breaking down with ~75% of the GFS, Euro, and Canadian ensemble members indicating lowering 500mb heights over Oregon and Washington on Wednesday. However, the magnitude of lowering heights is not as strong as previous guidance, and the precipitation chances have been mainly eliminated on Wednesday. The main impact from the breakdown of the high pressure is the subsidence inversion beginning to break down along with temperatures beginning a return to near normal for January. By Thursday, nearly all ensemble members are on board with the breakdown of the high pressure, though not with the return of precipitation. There's around a 20-40% chance of precipitation returning sometime Thursday, mainly over the coast and terrain. Precipitation chances increase Friday to 40-60% chance for the interior lowlands and 60-70% for the coast and terrain. Onshore flow appears to return on Friday 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. -10/03

AVIATION

A persistent upper level ridge over the region will continue to yield widespread VFR conditions for most terminals through the period. Fog or low stratus is expected to reoccur in sheltered portions of the Cowlitz and southern Willamette Valleys through after 03-06z Sun, with a 40-50% chance of IFR cigs/vis at KEUG overnight tonight. Ongoing offshore flow will maintain east winds in the Portland area as well as along the coast, with northerly flow expected through the Willamette Valley. The gustiest east winds will be constrained to the western Columbia River Gorge including near KTTD where gusts of 35-45 kt are likely to continue through much of the period, up to 50-55 kts east of KTTD. Gap flows through the Coast Range will also yield areas of gusty east winds at favored locations along the coast, including near KONP. Where surface winds are sheltered from stronger easterlies to the west of the Columbia River Gorge, low-level wind shear of 35-40 kt in the lowest 2 kft will continue. Easing offshore pressure gradients should favor lighter winds as well as lowered chances of low-level wind shear later in the period.

KPDX AND APPROACHES...VFR flying conditions with largely clear skies expected. East-southeast winds of 10-15 kt with gusts up to 25 kt will ease to 10 kt or less after 00-03z Sun. Marginal low- level wind shear is possible in the lowest 2 kft when surface winds are lighter, however decreasing winds aloft will reduce the chances of terminal impacts later in the period. -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 as high as 20 kt. The most widespread area of these stronger gusts continues west of the mouth of the Columbia River, between Cape Foulweather and Cape Falcon beyond 10 NM, however easing gusts have resulted the cancellation of the Small Craft Advisory early this afternoon. While offshore pressure gradients will weaken slightly and these strongest east winds will continue to ease through tonight, lighter offshore flow will otherwise persist into early next week, generally at 10 kt or less. Seas remain below 10 ft around 10-13 seconds through the middle of next week. -03/36

PQR WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory from 7 PM this evening to 10 AM PST Tuesday for ORZ109-114>118.

WA...None.

PZ...None.