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Avalanche Forecast

Archived

Apr 4th, 2018–Apr 5th, 2018

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Olympics.

Another stormy day Thursday with further warming will maintain dangerous avalanche conditions, especially at higher elevations receiving greater snow amounts and stronger winds. You will be able to trigger Wind Slabs 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 dangerous wind loaded terrain. 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 open slopes greater than 35 degrees on N-E aspects. At elevations receiving rain, expect wet avalanche conditions.

Detailed Forecast

Dangerous avalanche conditions will develop again Thursday above treeline. A storm will bring renewed rain and snow, strong winds and rising freezing levels to the Olympics. 

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.  

Rain near and 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. 

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

Warming precipitation Wednesday caused rain to above Hurricane Ridge with small amounts of wet snow accumulating above about 5500 ft. 

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. 

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.

Problems

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.

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.