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Avalanche Forecast

Archived

Feb 27th, 2021–Feb 28th, 2021

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

Regions

South Rockies.

Wind and more wind! Fresh wind slabs may be reactive on Easterly slopes Sunday.

A reactive persistent slab exists and can be easily triggered by a skier or rider. A conservative mindset and patience are required. 

Confidence

Moderate - Uncertainty is due to the extreme variability of wind effect on the snowpack.

Weather Forecast

The dominating feature associated with this next frontal system will be WIND. Ridgetop wind is forecast 50-90 km/hr from the West through the forecast period. Light snowfall amounts expected.

Sunday: Mix of sun and cloud. Ridgetop wind strong from the West and alpine temperatures near -6. Freezing levels valley bottom.

Monday: Cloudy with light snow 5-10 cm. Ridgetop wind strong to extreme from the West and alpine temperatures near -3. Freezing levels 1600 m.

Tuesday: Mix of sun and cloud. Ridgetop wind strong to extreme from the southwest and alpine temperatures near -5. Freezing levels valley bottom.

Avalanche Summary

On Saturday, a natural size 2 slab avalanche was spotted from outside the Castle Ski Hill Boundary as well as a few natural loose-dry size 1. 

On Friday, isolated storm slab and loose-dry avalanches were reported up to size 1.

On Thursday, several skier controlled avalanches were reported up to size 1. 

On Wednesday, this MIN report highlights a critical incident of a rider-triggered persistent slab size 3. Also on Wednesday, this MIN report indicates another remotely-triggered size 2 slab avalanche. Whether it's a wind slab or a persistent slab problem, they're catching folks by surprise and will not heal quickly. 

This weekend, human triggering of storm, wind, and persistent slabs will remain likely. Conditions are not easily managed right now, warranting a conservative mindset!

Snowpack Summary

Recent snowfall amounts vary across the region with 10-20 cm accompanied by strong to extreme wind from the West. Reactive wind slabs are likely in exposed terrain at treeline and alpine elevations. 30 to 60 cm of snow now sits above sugary faceted snow that formed mid-February. This layer may be easily reactive to skier and rider triggers on slopes that have a stiffer and more cohesive slab. This recent MIN report is a prime example. 

Deeper in the snowpack a persistent slab (50 to 90 cm deep) lingers above a weak layer of surface hoar and may be found in sheltered terrain or otherwise faceted grains and/or a hard melt-freeze crust. This weak layer has also been easily reactive to human triggers around the region. Check out this MIN report from yesterday in the Ruault Lake area in the western flank of the region. 

Weak faceted grains and a decomposing melt-freeze crust can be found near the base of the snowpack. There have not been recent avalanche observations on this layer, but steep and rocky slopes, as well as shallow snowpack areas, should be approached with caution.

Terrain and Travel

  • The trees are not the safe-haven they normally are at this time. Terrain at treeline is primed for human triggered avalanches.
  • Fresh snow rests on a problematic persistent slab, don't let good riding lure you into complacency.
  • Back off if you encounter whumpfing, hollow sounds, or shooting cracks.
  • Choose simple, low-angle, well supported terrain without convexities.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.

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.