Avalanche Forecast
Jan 20th, 2019–Jan 21st, 2019
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Regions
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The Bottom Line: Dangerous conditions have developed in the East Central Cascades. Incremental loading has stressed multiple persistent weak layers to their breaking point. This is a good time to step back, and avoid recreating in avalanche terrain.
Snow and Avalanche Discussion
On Saturday, Mission Ridge Ski Patrol triggered multiple avalanches up to 3 feet deep on northwest through northeast aspects between 6-6500ft. To the north, a skier was caught and carried in an avalanche near Washington Pass.
Toward the end of the cold and dry period, shaded aspects harbored surface hoar and facets east of the crest. One observer reports skiing on a ‘glass carpet’ of surface hoar between 4,000 and 5,500ft. Another noted ‘dramatic faceting’ near and below treeline on shaded aspects. The new snow doesn't appear to be bonding very well, especially where it sits over facets and surface hoar. A few natural avalanches occurred overnight on the 18th at Mission Ridge, these ran on a crust from early January. Observers have noted loud whumphs and collapses near Blewett Pass the past few days. As the snow accumulates with warming temperatures and the winds blow into Saturday, we will create a cohesive slab and further stress these layers. More on the recently buried snow surface in this observation here.
Problems
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.