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Avalanche Forecast

Archived

Feb 29th, 2020–Mar 1st, 2020

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

Regions

Purcells.

Be on guard for signs of instability in the snowpack, especially in immediate lee of ridge features and where snowfall amounts exceeded 15cm. A buried surface hoar layer lurks and is showing signs of reactivity. Check out this Fx blog on Surface Hoar.   

Confidence

Moderate - Recent weather patterns have resulted in a high degree of snowpack variability within the region.

Weather Forecast

SATURDAY NIGHT: Cloudy with clear periods and isolated flurries, Trace accumulation. Winds light to moderate Northwest. Alpine low temperatures -12 C with freezing levels at valley bottom.

SUNDAY: A mix of sun and cloud. Winds light to moderate West. Alpine high temperatures near -9 C and lows of -12 C, with freezing levels 1200m.

MONDAY: Cloudy with sunny periods and scattered flurries, accumulations Trace to 5cm. Winds moderate West /Northwest gusting strong. Alpine temperatures high of -8 and lows -13 C with freezing levels near 1300m.

TUESDAY: Snow with accumulations of 5-15cm. Winds moderate to Strong Southwest gusting extreme. Alpine high temperatures -8 and low temperatures -10 C, freezing levels 1600m.

Avalanche Summary

There were a few early reports of human triggered avalanches on Saturday, including a snowmobile triggered small avalanche (size 1) at a wind affected NE treeline feature. Additionally, cornices have grown large with recent snow and wind.  

Reports from Friday of several small (size 1-1.5) skier triggered avalanches, one large (size 2) Skier remotely triggered avalanche and another natural avalanche on the widespread layer of Surface hoar. Depths ranged from 30-45cm and were reported from 1600 - 2100m predominantly on NW to Eastern aspects. 

A few small (size 1 to 1.5) wind and storm slab avalanches were triggered by humans each day Monday thru Wednesday with activity tapering on Thursday. They occurred on all aspects, were from 2100 to 2700m and were 15 to 40cm thick. In the Western (& deeper) portion of the region a few of the avalanches released on the surface hoar layer described in the Snowpack Summary.

Snowpack Summary

5-20cm of snow fell Friday into Saturday, with highest amounts reported in the North and Western mountainous areas of the forecast region. Winds were moderate to strong from the Southwest as new snow fell. This new snow likely formed widespread wind affect and wind slab in Alpine and exposed treeline locations, particularly near ridges.  

30-45 cm of recent snow now overlays the widespread Surface Hoar (size 5 to 15 mm) that was found at all elevations and on all aspects. Recent warm temperatures have built slab like properties in this recent snow, particularly at mid and low elevations.  

The middle of the snowpack is strong. The base of the snowpack contains basal facets, which have been responsible for occasional and hard-to-predict Deep Persistent Slab avalanches in shallow, rocky start zones.  

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Shooting cracks, whumphs and recent avalanches are strong indicators of an unstable snowpack.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.