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Avalanche Forecast

Archived

Mar 17th, 2020–Mar 18th, 2020

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

Regions

Northwest Coastal.

Three things to note: 1) Wind slabs from outflow winds may linger. 2) Buried weak layers remain possible to trigger. 3) Sunny skies may weaken sun-exposed slopes and cornices during the heat of the day.

Confidence

High - Confidence is due to a stable weather pattern with little change expected.

Weather Forecast

TUESDAY NIGHT: Clear skies, light northeast wind, alpine temperature -3 C.

WEDNESDAY: Clear skies with afternoon clouds, light northwest wind, alpine temperature 0 C, freezing level 1400 m.

THURSDAY: Mix of sun and cloud, light east wind, apine temperature -3 C, freezing level 1100 m.

FRIDAY: Clear skies, light northwest wind, alpine temperature -2 C, freezing level 1300 m.

Avalanche Summary

On Monday, a large avalanche was observed near Shames. Check out this MIN for photos and a description. The avalanche may have been triggered by loose wet avalanche activity and it may have released on the surface hoar layer described in the Snowpack Summary.

Many loose wet avalanches and cornice falls occurred on sun-exposed slopes during daytime warming on Monday. This trend is expected to continue with generally sunny skies and relatively warm air temperature.

Snowpack Summary

Wind slabs and otherwise a wind-affected snow surface exist in exposed terrain at alpine and treeline elevations from predominantly northeast wind. In sheltered features, soft and faceted snow may be found. Clear skies and have melted the snow surface on sun-exposed slopes during the day and frozen it into a melt-freeze crust during nights and cloudy days.

A layer of surface hoar crystals is buried around 30 to 60 cm in sheltered areas at and below treeline. See this MIN from the Shames area that shows this layer when it was on the surface, prior to burial on March 9. There are several additional layers of surface hoar that are now buried 70 to 120 cm and 110 to 160 cm deep. 

An early-season layer of faceted grains and a melt-freeze crust may linger at the base of the snowpack. A large load, such as a cornice fall, also has the potential of triggering it.

Terrain and Travel

  • Carefully evaluate steep lines for wind slabs.
  • Avoid steep slopes when air temperatures are warm, or solar radiation is strong.
  • Minimize overhead exposure; avalanches triggered by warming or cornice fall may be large and destructive.
  • The more the snowpack warms-up and weakens, the more conservative you`ll want to be with your terrain selection.
  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.

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.