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Avalanche Forecast

Archived

Dec 30th, 2021–Dec 31st, 2021

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

Lizard-Flathead.

Lingering wind slabs may remain reactive to human triggers on lee features at treeline and above.

Check out the latest Forecasters' Blog for tips on managing the ongoing Persistent Slab problem in this region.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

THURSDAY NIGHT: Partly cloudy / Light west wind / Low of -29

FRIDAY: Sunny / Light west wind / High of -20

SATURDAY: Increasing cloudiness in afternoon / Moderate northwest wind / High of -20

SUNDAY: Mostly cloudy with flurries; 3-5 cm / Strong west wind / High of -11

Avalanche Summary

No new avalanches were reported in this region on Wednesday.

An explosive triggered size 3 persistent slab was reported in the alpine on December 27th. The avalanche failed on the crust formed in early December which is down 50-150 cm.

Snowpack Summary

Recent winds have created wind slabs on a variety of aspects and created a lot of variability in the upper snowpack.

The most notable layer of concern in the snowpack is a crust that was formed in early December and is now down 80-150 cm. The most recent reported avalanche on this layer was on December 27th. Activity on this layer has been sporadic. It has generally been triggered on steep, rocky slopes, with a shallow or thin to thick snowpack.

This layer has created a low likelihood, high consequence scenario which is best managed through conservative terrain choices and disciplined backcountry travel techniques.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
  • Seek out wind sheltered terrain below treeline where you can avoid wind slabs and find great riding.

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.