Avalanche Forecast
Regions: Olympics.
By Sunday afternoon you may be able to trigger fresh and shallow Wind Slabs on lee slopes in the Hurricane Ridge area as well as Persistent Slabs on shaded slopes at mid to upper elevations. Persistent Slabs are difficult to manage and can break in surprising ways. Make sure to avoid avalanche terrain where new Wind Slabs and older Persistent Slabs likely overlap, such as N-E aspects near and above treeline.
Detailed Forecast
An incoming frontal system will bring light snowfall, increasing winds and much colder temperatures to the Olympics on Sunday.Â
By Sunday afternoon you may be able to trigger fresh and shallow Wind Slabs near and above treeline in the Hurricane Ridge area, especially on NW-N-E-SE aspects. Fresh Wind Slabs may form quickly on the leeward side of terrain features and ridges near and above treeline. Use visual clues such as snow drifts, cornices, wind textured snow, and firm snow surface conditions that produce cracks to identify and avoid wind loaded areas.
You may be able to trigger Persistent Slabs on shaded slopes at mid to upper elevations. Persistent Slabs are difficult to manage and can break in surprising ways. To stay safe, avoid where the problem exists. Make sure to avoid avalanche terrain where new Wind Slabs and older Persistent Slabs likely overlap, such as N-E aspects near and above treeline.Â
New snowfall will likely not be heavy enough to produce Storm Slabs in non-wind affected terrain. Anticipate a potential Storm Slab problem if more than 6-8 inches of snow accumulates in wind-sheltered terrain by Sunday afternoon. Cloudy and cool conditions will bring about an end to wet snow avalanche concerns. Â
Snowpack Discussion
Temperatures have warmed over the last several days with mild daytime temperatures. Very little snowfall has accumulated in the past week. This has allowed for snowpack settlement and the development of a strengthening melt freeze crust. Â
Slab avalanches likely released early last week, likely failing on a weak layer above a strong melt-freeze crust buried about 1.5 ft on lee slopes above Hurricane Ridge. This layer has been tracked over the past few weeks but has now produced at least two slab avalanches, each with crown depths about 1.5 ft and propagating over 100 ft wide on NE aspects between 5200-5400 ft.Â
There are no other significant layers of concern in the snowpack at this time.
Observations
No new avalanche activity was reported in the Hurricane Ridge area Saturday.Â
On Friday, NPS Rangers performed snowpack tests on a NNE aspect at 5,000 ft. They reported test results indicating propagation on a layer of weak old snow about 2 ft below the surface.
On Thursday, 3/29 NWAC professional observer Matt Schonwald saw evidence of two recent slab avalanches releasing on steep convexities on NE aspects between 5200-5400 ft. These avalanches released on a weak layer, poorly bonded to a hard crust. Several snowpack tests on this layer confirmed the propensity for avalanches to propagate.
Avalanche Problems
Persistent Slabs
Release of a cohesive layer of soft to hard 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 Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: North, North East, East.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
Wind Slabs
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1