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Avalanche Forecast

Archived

Jan 7th, 2021–Jan 8th, 2021

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

Regions

Lizard-Flathead.

  

Sporadic large natural persistent slab avalanches have recently been reported in this region. This low likelihood, high consequence scenario is best avoided by choosing conservative terrain away from overhead hazard.

Confidence

Moderate - Uncertainty is due to the limited number of field observations. Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

  

THURSDAY NIGHT: Mostly clear / Light, southwest ridgetop wind / alpine low temperature -13 / Freezing level valley bottom.

FRIDAY: Mix of sun and cloud / Light, southwest ridgetop wind / alpine high temperature -5 / Freezing level valley bottom.

SATURDAY: Mix of sun and cloud / Light, southwest ridgetop wind / alpine high temperature -8 / Freezing level valley bottom.

SUNDAY: Cloudy with sunny periods / Moderate, northwest ridgetop wind / alpine high temperature -7 / Freezing level valley bottom.

Avalanche Summary

A couple large (size three) naturally triggered persistent slab avalanches were reported on large alpine features on Thursday. These avalanches were triggered by either smaller wind slabs in motion or cornice falls. Another reminder of the "low probability; high consequence" scenario that persistent slab problems often create.

Several size 1.5 human triggered avalanches were reported on Wednesday in the top 30 cm of storm snow. Over the past few days there have been several natural and explosive triggered storm slab avalanches in the size 1.5-2.5 range, as moderate amounts of snow and wind each day have kept the avalanche danger elevated.

On Sunday there was remotely triggered persistent slab avalanche in a north facing gully at 1350 m (see the photo here). The avalanche failed on a 50 cm deep surface hoar layer. This layer has been reported in sheltered terrain below treeline.

Snowpack Summary

Moderate snowfalls and strong southwest wind over the past few days has likely left hard surfaces and wind slabs in exposed terrain. Roughly 30-50 cm of recent snow is settling above a crust that extends up to 1900 m. The main feature we are monitoring in the snowpack is a layer of feathery surface hoar and/or faceted grains over a hard melt-freeze crust found 50 to 100 cm deep. Reports suggest this layer is strong and bonded in most terrain, but remains suspect in the isolated areas where it is preserved. Unfortunately, we do not yet have a clear picture of where these isolated areas exist.

Terrain and Travel

  • Be carefull around freshly wind loaded features.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.
  • If triggered loose wet avalanches may step down to deeper layers resulting in larger avalanches.
  • Avoid exposure to slopes that have cornices overhead.

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.