Avalanche Forecast
Regions: Olympics.
Stormy weather with rising snow levels will increase the avalanche danger Wednesday creating dangerous avalanche conditions, especially at higher elevations receiving greater snow amounts and stronger winds. You will be able to trigger wind slabs Wednesday on steep slopes near or below ridgelines at higher elevations. Use visual clues such as snow drifts, uneven snow surfaces, and cornices to identify and avoid steep wind loaded slopes. Older weak snow exists in the snowpack and has caused at least two avalanches in the past week. You can avoid triggering a persistent slab by staying off of open slopes greater than 35 degrees on N-E aspects. At lower elevations receiving rain, expect wet avalanche conditions to develop.
Detailed Forecast
Dangerous avalanche conditions will develop Wednesday above treeline. A storm will bring renewed rain and snow, strong winds and rising freezing levels to the Olympics.Â
You will be able to trigger Wind Slabs on steep slopes near and below ridgelines, especially above treeline on slopes receiving wind transported snow. Watch for clues like variable snow height, drifts, cornices, and stiff snow that produces cracking. These are all indicators that you could trigger a Wind Slab. You can avoid triggering these avalanches by steering around steep roll-overs, unsupported features, and obvious start zones where you suspect Wind Slabs.
New snowfall amounts and wind speeds will increase with elevation, and this will increase the potential for larger avalanches at higher elevations. Â
Snow changing to rain below treeline will make Loose-Wet avalanches possible at lower elevations. Watch for new rollerballs, pinwheels, and small loose wet avalanches to occur during warming and periods of rain.Â
Older persistent weak layers exist in the snowpack around Hurricane Ridge. Some observations indicated this layer can fail and produce avalanches. It can be difficult to assess and manage persistent slab avalanches. You can use snowpack tests to confirm the presence of these weak layers, however, they are not a decision making tool. You can avoid triggering a persistent slab avalanche by staying off of open slopes greater than 35 degrees on N-E aspects near and above treeline.
Snowpack Discussion
Three to six inches (8-15cm) of snow fell at Hurricane Ridge Sunday through Monday morning. Light to moderate winds during the storm transported the new snow likely forming new shallow and reactive wind slabs at higher elevations. This new snow fell on melt-freeze crusts in most locations. These crusts are thickest on E-S-W aspects.
Older weak snow has been observed on N-E aspects near and above treeline. Weak sugar facets can be found just above a firm melt-freeze crust. This layer is generally 1.5 to 2 feet (45-60cm) below the snow surface. It resulted in two observed avalanches last Thursday.
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 over the weekend.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
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: Likely
Expected Size: 1 - 1
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
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