textproduct: Portland

DISCUSSION

Currently a ridge of high pressure continues to sit overhead facilitating dry and fairly calm conditions. That said, a 5-8mb pressure gradient from PDX to DLS has been driving rather windy conditions through the Columbia River Gorge and other Cascade gaps with gusts at our typically "windiest" spots like Three Corner Rock and Corbett holding in the 35-60 mph range most of the day. This offshore gradient is expected to slowly ease over the 24-48 hrs but likely won't go away completely as cooler air is continually reinforced in the Gorge and locally adjacent Cascade valleys - a variable likely to come into play once precipitation arrives Thursday.

As we progress through tonight into Thursday morning the aforementioned ridge quickly shifts eastward thanks to an approaching closed upper-level low off the coast of southern Oregon and Northern California. This feature will help to swing a warm-frontal boundary into the Pacific Northwest by the afternoon hours while rapidly rising snow levels (sorry skiers/snowboarders). There remains about a ~6-8hr difference in the exact start time of light precipitation between the various modeling systems - an interesting point of forecast uncertainty. The EPS is generally the quickest to bring light precipitation northward (as early as 6-10am) while the GEFS and a number of high resolutions models push the start time back to the afternoon or even the early evening. Given how light precipitation appears initially, little impact is expected either way for the majority of the CWA.

It should be noted headed Thursday into Friday is that this overall set-up is a very favorable pattern for freezing rain to occur in the Central Columbia River Gorge as well as for Hood River County, especially as the offshore pressure gradient never completely goes away. Models are notoriously poor at scouring out cold air from these locations so with established cold/dry air in place and relatively warm precipitation expected to overrun said airmass, freezing rain with this system is likely. The current 12z HREF through 4am Friday depicts a 80-90% chance for at least 0.1" of ice accumulation or more for spots like Parkdale with a 50-60% chance to exceed 0.25" locally on the west side of the Upper-Hood River Valley. High confidence ice accumulation values fall at least into the 0.1"-0.25" range. Given the overall set-up and the latest forecast more bullish for a period of freeze rain Thursday night even down to portions of SR-14 and I-84, a Winter Weather Advisory was issued for the Central Gorge zones and Upper Hood River Valley from 10am Thursday through 10am Friday. While the moderation presented in the forecast may still be a bit too fast Friday morning, the rapid decrease in QPF by the midday hours will help to mitigate additional impacts the rest of Friday.

As we look towards the weekend into the start of next week, the specifics of the weather pattern become much more uncertain. Overall, ensemble guidance is pointing towards a fairly typical winter rain pattern for the PacNW continuing into Tuesday. Snow levels gradually lower back towards pass-level by Monday/Tuesday as well. Still, we'll need to keep an eye out of the placement of individual shortwaves rotating around the larger trough feature over the eastern Pacific for added impacts like breezy winds. This first of these potential set-ups appears on Saturday into Saturday night with the EPS showing a 20-45% chance for 58 mph gusts or higher along the coastline. Beyond Saturday/Sunday embedded shortwave placement with in the large trough pattern becomes extremely uncertain among ensemble modeling systems. By the middle of next week there is decent consensus for us to migrate into a progressive WNW upper-level flow pattern once the larger trough feature previously to our east kicks into the Rockies. However, large variations exist regarding the exact amplification and longitudinal placement of trough features within this flow regime. At the very least we can be confident in more pervasive Cascade snowfall with each passing system Wed/Thur onward and near normal temperatures for this time of year. -99

AVIATION

An upper level ridge continues over the region with southwest flow aloft. This will result in VFR flight conditions over the airspace through this evening. The main exception continues to be in the southern Willamette Valley where LIFR fog is favored to form again later this evening around 04-06z Thu. Otherwise, light offshore flow in persists in the lower levels, except much stronger east winds near the Columbia Gorge where gusts up to 30 kt continues at KTTD. A weak low pressure system lifting northward tonight will bring increasing chances for rain after 12Z Thu to southern portions of the forecast area, gradually spreading north through Thu morning. Of note, below freezing temperatures in the Hood River area will likely persist through Thu night, allowing for the potential of accumulating freezing rain. The probability for MVFR conditions increases across the area from south to north after 16-18z Thursday.

KPDX AND APPROACHES...VFR with mostly clear skies today, and increasing mid to high level clouds tonight. East winds expected through this evening around 4-8 kt, while stronger winds out of the Columbia Gorge continue at KTTD with gusts up to 30 kt. Light rain becomes likely by Thu afternoon, while chances for MVFR conditions increase to around 40-50% after 21z Thu. /DH

MARINE

High pressure over the waters will continue to weaken as more active weather returns late this week. Southeast winds around 10 kt or less continues into Thursday as a weak low pressure system off the northern California coast lifts northward. This will likely bring some breezier winds to the southern waters Thursday night. Could see southerly winds with gusts up to 20-25 kt into Friday morning. Winds turn back offshore later Friday as a stronger low pressure system develops farther off the northern California/southern Oregon coast. Southerly winds are expected to increase across the waters on Saturday. But, there still remains considerable uncertainty with the progression of this low through the weekend. Model guidance has pushed the timing a bit later, with Gale force wind gusts still appearing likely (60-70% chance) sometime Saturday into Saturday night. Though still a low probability (30% chance), a more compact surface low could lift across the coastal waters Saturday night with maximum gusts exceeding 48 kt.

Seas are expected to continue to hover around 6 to 7 ft through Friday. By Saturday morning, an incoming fresh southerly swell is expected to push seas to around 10 to 12 ft with a period of 10-12 seconds. Very steep and hazardous seas will likely come with the strongest winds later Saturday into Sunday. But, there remains quite a bit of uncertainty though guidance suggests there is up to a 10% chance of wave heights exceeding 20 ft.

Also of note, we are entering a perigean/king tide cycle. Therefore ebb currents are expected to be on the strong side, including near the Columbia River Bar, through January 4th. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory from 10 AM Thursday to 10 AM PST Friday for ORZ121-122. WA...Winter Weather Advisory from 10 AM Thursday to 10 AM PST Friday for WAZ210. PZ...Small Craft Advisory until 4 PM PST this afternoon for PZZ210. Small Craft Advisory from 1 PM to 5 PM PST Thursday for PZZ210.