Avalanche Forecast
Issued: Jan 3rd, 2019 6:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
Josh Hirshberg,
The Bottom Line: Dangerous avalanche conditions exist due to strong wind, new snow, and old weak layers well below the surface. Be very cautious on slopes where you find one foot or more of fresh snow with underlying weak layers. If you see obvious signs of danger like recent avalanches, cracks that shoot through the snow, or a dramatic collapse of the snowpack, stay off of slopes steeper than 30 degrees.
Summary
Regional Synopsis
Happy New Year!
Thanks to all of you who volunteer, send observations, and support NWAC in various ways - we appreciate it.
December of 2018 was fun (from a forecasting perspective) with three pronounced avalanche cycles, a couple different persistent weak layers, some rain events, and a flurry of human triggered avalanches to ring in the New Year. Most importantly, it seems that we made it through the last days of 2018 without anyone getting seriously hurt by an avalanche.
The deep (Dec 9) layer responsible for many of the avalanches early in the month no longer seems to be a problem in the western zones. That said, it is still possible to trigger an avalanche on its counterpart (or basal facets) in the eastern areas.
A widespread layer of surface hoar formed around Christmas. Late December storms preserved this layer in areas above the rain line and we have numerous (more than a dozen) reports of people triggering avalanches on it in the last three days. At least 4 people were caught and carried during this period, but so far we have no reports of serious injury. Most of these avalanches were soft slabs, D1-D2+, but there were several harder wind slabs in the mix.
It appears that the layer is most reactive and/or prevalent in the Crystal Mountain backcountry and in the mountains around Leavenworth and west of Mazama.
Surface Hoar can be an especially tricky and persistent weak layer. Read more about it here.
Weather Forecast
Fri 4th Jan 14:57 - Robert Hahn
Weather Synopsis for Friday night through Sunday
Very light post-frontal showers decrease and generally end as SW flow continues behind a departed short-wave trough. Friday night with relatively mild with snow levels around 3000-4000 feet. A trough will approach from the SW as it digs into northern California, spreading some light precipitation into the Olympics in the early morning hours.
The main portion of the trough will move into the near-shore waters with light rain and snow brushing the coastline by the end of the day on Saturday.
Saturday night, the surface trough crosses the region, bringing a brief shot of strong westerly winds and moderate to locally heavy precipitation behind it, impacting Mt. Hood in the late evening and the Washington Cascades after midnight. At Snoqualmie Pass, precipitation will be a mix of rain and snow with east winds initially keeping temperatures cool, changing back to rain as the trough pushes westerly winds through and then rapid cooling changes precipitation back to snow behind the trough by Sunday morning. Snow shower activity appears initially poorly organized under the upper-level trough on Sunday with light to moderate winds.
Problems
Wind Slabs
Use extra caution at upper elevations on leeward, wind-loaded slopes steeper than 35 degrees. Watch for fresh drifts and areas of variable height storm snow as indicators that you could trigger a wind slab avalanche. Steer around thickly pillowed areas and convex rolls where the terrain gets steeper. Use small, inconsequential test slopes to clue you in to how well the new snow is bonded. In some areas of the zone, wind slabs will be your main concern. In areas where you find underlying weak layers, triggering an avalanche in the new snow could result in a much bigger and more dangerous persistent slab avalanche.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Persistent Slabs
Snow, rain, and wind are putting a new load on an already weak snowpack. While you can trigger avalanches at all elevations, the most dangerous slopes will be near and above treeline where the wind is loading leeward slopes. You can trigger dangerously large and surprising avalanches on slopes over 30 degrees. Avalanches may be more reactive and break wider than you can expect. You can trigger persistent slabs from a significant distance. Put plenty of space between you and any suspect avalanche terrain. Make sure you are well out from under any areas where avalanches could start. Two weak layers of concern are a thin layer of surface hoar buried 1-2 feet below the surface and sugar-like facets near the bottom of the snowpack.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Jan 4th, 2019 5:00PM
