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Avalanche Forecast

Archived

Feb 27th, 2020–Feb 28th, 2020

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

Regions

Lizard-Flathead.

Warm temperatures and sun could destabilize the upper snowpack and potentially cause slab avalanches, loose avalanches, or cornice failures. Watch your exposure to overhead hazards in the heat of the day.

Confidence

Moderate - Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather. Uncertainty is due to the timing or intensity of solar radiation and its effect on the snowpack.

Weather Forecast

THURSDAY NIGHT: Cloudy, moderate southwest wind, freezing level drops to valley bottom overnight with alpine temperatures around -4 C.

FRIDAY: Mix of sun and cloud, freezing level climbing to 2000 m in the afternoon, moderate wind with strong gusts from the southwest. Increasing cloud and light snow starting in the evening.

SATURDAY: Cloudy with 5-10 cm of new snow, moderate wind from the southwest, freezing level drops from to 2000 to 1200 m.

SUNDAY: Mix of sun and cloud, light wind from the west, freezing level reaches 1200 m in the afternoon, alpine high temperatures around -2 C.

Avalanche Summary

Wind slabs will likely remain reactive, as there was a report of a size 2 skier triggered slab on Thursday on northeast aspect near ridgetop in the Lizard Range. No new avalanches were reported on Wednesday.

A few size 1.5 skier triggered avalanches were reported on Tuesday that appeared to fail on persistent weak layers roughly 30-40 cm below the surface. One was triggered by a skier on a east-facing slope at 1900 m and ran on the early February crust/facet combo. This MIN observation from Harvey Pass shows a skier triggered avalanche on an north-facing slope that looks like it could have run on surface hoar. Sun and warming temperatures could weaken the snow above these persistent weak layers on Friday.

Snowpack Summary

20 cm of settled snow rests on a variety of old surfaces including a firm crust on solar aspects, soft faceted snow, possibly a spotty surface hoar layer and old wind slabs near ridge crest. Recent snow has been blown into variable wind slabs in exposed terrain features, especially those near ridge crest.

A thick rain crust that has facets associated with it sits 30-60 cm below the surface and can be found up to ridgetop. The avalanche listed above is the first activity that we've seen on the crust since February 17.  

The mid-pack is well settled and strong, but the base of the snowpack contains basal facets that are most prominent in shallow rocky start zones.

Terrain and Travel

  • Avoid sun exposed slopes when the solar radiation is strong, especially if snow is moist or wet
  • Be careful as you transition into wind affected terrain.
  • Keep in mind the crust offers an excellent bed surface for avalanches.
  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.

Problems

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.

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.