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Avalanche Forecast

Archived

Mar 5th, 2021–Mar 6th, 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

Purcells.

Human triggered avalanches are possible on steep slopes at higher elevations. Be extra cautious in wind-affected terrain and in the north tip of the region around Quartz Creek and the Dogtooth Range where there are buried weak layers.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

A weak front crosses the region on Friday night.

FRIDAY NIGHT: Cloudy with some scattered flurries and 5-10 cm of new snow, moderate southwest wind, treeline temperatures drop to -4 C.

SATURDAY: Mix of sun and cloud with some isolated flurries, light west wind, freezing level around 1600 m with treeline temperatures around -2 C.

SUNDAY: Mostly cloudy with some isolated flurries, moderate southwest wind, treeline temperatures around -4 C.

MONDAY: 5-10 cm of new snow by the morning then clear skies in the afternoon, light wind, treeline temperatures around -4 C.

Avalanche Summary

Warm sunny weather between Wednesday and Friday resulted in a widespread cycle of wet loose avalanches on sun-exposed slopes. Photos from this flight over the southwestern Purcells shows so some very large wet avalanches that ran to valley bottom. Clouds and cooling temperatures will heal this problem by the weekend.

Over the past week there have been several notable reports of large human triggered persistent slab avalanches in the Dogtooth Range and Quartz Creek area, including:

  • A remotely triggered size 2 avalanche on Tuesday on a northwest slope at 2100 m that failed on a 40 cm deep weak layer (MIN report)
  • A size 3 human-triggered avalanche on Feb 27 on a east-facing alpine slope (MIN report)
  • A size 2 avalanche on Feb 27 on a south-facing slope (MIN report)
  • A few remotely triggered size 2 avalanches on Feb 24 (MIN report)

While in most parts of the Purcells the main concern over the weekend is wind slabs (both old and new), backcountry travelers in the notrhern tip of the region should be extra cautious about the lingering persistent slab problem.

Snowpack Summary

5-10 cm of new snow will accumulate above moist and crusty interfaces that formed during the recent warm up. Some deeper accumulations can be expected in lee terrain features. High shaded terrain will have a mix of soft snow and some old buried wind slabs.  

Persistent weak layers have been most active in the northern end of the Purcells where a buried layer of surface hoar that formed in late January is 40-80 cm deep and an older surface hoar layer is 60-120 cm deep at treeline. These layers may exist as a combination of facets and crusts at other elevations. Recent observations suggest these layers have been less problematic as you move further south.

Steep rocky areas where the snowpack is thin likely has additional weak layers near the base of the snowpack.

Terrain and Travel

  • If you are increasing your exposure to avalanche terrain, do it gradually as you gather information.
  • Be carefull around freshly wind loaded features.
  • Caution around convexities or sharp changes in terrain.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

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.