textproduct: Portland

SYNOPSIS

High pressure sits across the region into at least the middle of next week with dry weather prevailing. Gusty easterly winds continue through the Cascade and Coast Range gaps, and along the Columbia River Gorge into portions of the Portland metro this weekend as well, strongest today. Expect overnight/morning low stratus and fog to remain limited to valley areas more protected from east winds like the southern Willamette Valley this morning before coverage gradually increases day to day going forward due to a decrease in offshore winds. Overnight lows near/below freezing in combination with any fog may lead to locally slick road conditions - something to keep in mind.

DISCUSSION...Saturday morning through Friday

Satellite imagery early around 2 AM PST Saturday morning indicates mostly clear skies over NW Oregon and SW Washington except for pockets of fog and low stratus in the Lane County lowlands, SW Washington lowlands (I-5 corridor), and the Upper Hood River Valley. Not expecting much more of an expansion of fog/stratus over the morning hours as current dew point depressions across much of the Willamette Valley are too wide due to drying northerly winds. Strong high pressure continues over the region through at least Tuesday, promoting a strong subsidence inversion and radiational cooling in the lowlands. Daytime temperature forecasts remain above normal for January, peaking in the upper 40s to low 50s across the Willamette Valley and upper 50s to 60s along the coast and western foothills of the Coast Range and Cascades. Additionally, morning low temperatures will be near or below freezing across low lying areas that are away from easterly winds from the Gorge, leading to the promotion of widespread frost in these areas. However, elevations around ~1000-5000 ft will see much warmer than normal low temperatures due to the strong subsidence inversion.

The main hazard of concerns remains the east winds through the western Columbia River Gorge. A thermally induced trough at the surface remains situated just over the Oregon and Washington coast, leading to a strong surface pressure gradient over the Cascades. Current observations show a -9 to -10 mb pressure gradient between KPDX and KDLS, which is expected to continue through much of today. This is resulting in peak wind gusts up to 40-55 mph into the Troutdale area, 25-45 mph in the eastern Portland metro, and 55-70 mph for our climatologically windiest spots like Corbett, Crown Point, Three Corner Rock, etc. A Wind Advisory remains in effect through this afternoon for the eastern Portland metro area. Breezier east winds with peak gusts up to 35-45 mph are also expected through some Coast Range gaps and peaks of the Coast Range and Cascades. Most ensemble guidance indicates the pressure gradient begins to slowly weaken tonight into Sunday as the ridge weakens slightly causing the thermal trough near the surface to also weaken. Breezy winds are still expected into Sunday and early next week, just not as strong as yesterday and today. Elevated east winds are likely to continue in the Gorge into at least Wednesday, but not expecting much more than peak gusts up to 25-30 mph making it farther west than the Troutdale area beyond Sunday.

As the thermal trough weakens and strong easterly winds diminish outside of the Gorge, fog chances increase each night and morning, becoming more widespread across the interior lowlands. Overnight lows near or below freezing in combination with any fog may lead to locally slick road conditions. 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 15-30% chance of precipitation returning sometime Thursday, mainly over the coast and terrain. Precipitation chances increase Friday to 20-40% chance for the interior lowlands and 30-55% for the coast and terrain. One thing to note is overnight temperatures will remain on the cold side across most of the area, and a colder airmass will remain in place near the surface around Hood River valley and surrounding areas. This along the lingering easterly flow through the Cascade gaps could lead to a period of wintry precipitation as precipitation begins for lower elevations, mainly in the Columbia River Gorge, Hood River Valley, and surrounding locations. Specific details will become more clear as we get closer to precipitation returning. -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 will continue in sheltered portions of the Cowlitz and southern Willamette Valleys through 20-21z Sat, although major terminals including KEUG have already cleared out this morning. 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 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.

KPDX AND APPROACHES...VFR flying conditions with largely clear skies through the period. East-southeast winds will increase to 10-15 kt with gusts up to 25 kt by 20-21z Sat, then easing again to around 10 kt or less after 00-03z Sun. -36

MARINE

Strong upper level high pressure continues into the weekend with a thermally-induced surface trough along the coast 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 20-25 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, where a Small Craft Advisory has been extended through 10 AM this morning. East winds otherwise persist through the weekend, generally less than 15-20 kt. Seas remain below 10 ft around 10-13 seconds through early 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.

Wind Advisory until 3 PM PST this afternoon for ORZ112.

WA...None.

PZ...Small Craft Advisory until 10 AM PST this morning for PZZ271.