textproduct: Pendleton

KEY MESSAGES

- Areas of dense fog this morning.

- Active winter weather today into Wednesday.

- Potential for additional winter weather headlines through the week.

UPDATE

Dense fog has lifted in central Oregon, so have cancelled the advisory early. Farther north, fog is showing signs of dissipating in portions of north-central Oregon, but remains thick enough in trouble spots along US 97 and US 197 to keep the advisory in effect. Elsewhere, fog is still lingering along the foothills of the Blue Mountains.

Confidence has increased (to 80 percent from 70 percent) in a windy day in Wallowa Valley, so have opted to issue a Wind Advisory, valid from 9 AM until 4 PM PST. 86

DISCUSSION

Water vapor imagery reveals a closed low spinning off the coast of California. Farther north, an elongated shortwave trough is diving south-southeast along the Pacific Northwest coast from southeast Alaska to the OR/WA coast. A secondary vorticity maximum and low is getting wrapped up in strengthening southwesterly steering flow. Synoptic-scale forcing associated with this vort max and the mid- to upper-level jet coupled with a developing surface low will drive precipitation chances through the day. Indeed, radar shows a band of precipitation blossoming over the Pacific Northwest, stretching from southwest Oregon all the way north and east to the northern Blue Mountains. As precipitation becomes more widespread, and surface winds increase in magnitude later this morning, existing areas of dense fog should (80 percent confidence) be scoured. For now, have left the Dense Fog Advisories in place for central and north-central Oregon and the foothills of the Blue Mountains of Oregon. Anticipating fog will lift by 10 AM PST, if not sooner.

Advisory-worthy snowfall is forecast for the Oregon Cascades and northern Blue Mountains of OR/WA today with showers extending through Tuesday and into Wednesday. However, there is also some potential for lowland snowfall, especially for the Kittitas Valley and higher portions of the Yakima Valley today as the band pivots across south-central Washington. Confidence in reaching advisory amounts (2 inches) is too low (<50 percent) to issue any highlights, but it is worth keeping an eye on as the morning progresses. Elsewhere, as the aforementioned cold front swings east this afternoon and evening, there is also low (20-50 percent) confidence in reaching advisory-level snowfall for the Grande Ronde and Wallowa valleys.

Southwesterly winds aloft, around 40-55 kts) are advertised by latest HREF and REFS NWP guidance. Coupled with the potential for mountain waves, this may induce windy conditions for Wallowa Valley by mid-morning (70 percent confidence). High-end solutions from CAMs are placing gusts up to 55 mph through the valley, strongest along Alder Slope and areas just to the northeast such as Joseph. Ultimately, did not quite have the 80 percent confidence needed to issue a Wind Advisory, but did significantly increase forecast winds through the valley and along the crest of the Wallowas.

Ensemble NWP indicates the elongated shortwave will transform into a closed low as it tracks just offshore later today and ultimately onshore over Oregon through Tuesday. Cold air associated with this low will keep snow levels low enough to facilitate low-elevation snowfall, albeit the showery nature means it will be difficult to forecast precise totals; some areas will receive snow and others will not. By Wednesday, ensemble clusters show this low decaying into an open wave as it treks inland, though another shortwave of some flavor from the northwest is expected (95 percent confidence) to be hot on its heels. Ensemble cluster analysis shows a range of scenarios for this second wave, with the most notable solution showing advisory-level snow potential for central Oregon, perhaps extending north and east across much of the forecast area if the alternate scenario materializes (roughly a 50/50 chance based on current 00Z ensemble clusters).

Lastly, will mention there is potential for an atmospheric river, heavy snow, and windy conditions by next weekend. Ensemble clusters all show some form of an anomalously deep closed low offshore by Sunday, but details are still unclear. 86

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Dense fog has lifted at BDN/RDM and is not expected to return this morning. Elsewhere, MVFR or lower conditions are forecast as low stratus and/or fog are still lingering in the Columbia Basin/Gorge. Meanwhile, a band of precipitation will continue spreading north through morning. While some form of precipitation is forecast at all sites, the best chances of persistent precipitation will be at DLS/YKM. A cold frontal passage this afternoon and evening may facilitate brief heavy showers at PDT/ALW/PSC, and winds will become gusty today at all sites. In the wake of the front, CIGs and VSBYs are forecast to improve to predominantly VFR for all sites. 86

Preliminary Point Temps/PoPs

PDT 27 45 27 41 / 70 30 40 30 ALW 30 46 29 41 / 80 40 50 50 PSC 28 48 28 45 / 50 10 20 20 YKM 24 41 23 41 / 10 10 10 10 HRI 27 46 27 44 / 50 10 30 20 ELN 22 38 21 36 / 20 20 10 20 RDM 22 38 18 36 / 20 30 10 20 LGD 24 41 24 37 / 90 70 60 70 GCD 24 38 22 35 / 90 70 40 60 DLS 29 43 28 43 / 30 50 20 40

PDT WATCHES/WARNINGS/ADVISORIES

WA...Winter Weather Advisory until 3 PM PST Wednesday for WAZ030. OR...Wind Advisory until 4 PM PST this afternoon for ORZ050. Winter Weather Advisory until 3 PM PST Wednesday for ORZ502. Dense Fog Advisory until 10 AM PST this morning for ORZ507-508- 510. Winter Weather Advisory until 10 PM PST Tuesday for ORZ509.