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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

Lizard-Flathead.

Persistent slabs 60-100 cm thick continue to be reactive to skier and rider triggers. These are producing large and consequential avalanches. A conservative mindset and patience are required. 

Wind and more wind! Fresh wind slabs may be reactive on northeast- east slopes on Sunday.

Confidence

High -

Weather Forecast

The dominating feature associated with this next frontal system will be WIND. Ridgetop wind is forecast 25-70 km/hr from the West through the forecast period. Light snowfall amounts expected.

Sunday: Cloudy with some flurries. Ridgetop wind moderate from the southwest and alpine temperatures near -5. Freezing levels valley bottom and rising to 1500m by the afternoon.

Monday: Cloudy with light snow 5-10 cm. Ridgetop wind moderate to strong from the southwest and alpine temperatures near -3. Freezing levels 1200 m.

Tuesday: Mix of sun and cloud. Ridgetop wind light to moderate from the southwest and alpine temperatures near -5. Freezing levels 1000 m.

Avalanche Summary

On Saturday, a size 2 skier triggered persistent slab avalanche was reported. This was on an East aspect at 2000 m and ran on the late January interface. The person was able to ski out with no injuries. 

On Friday, numerous loose-dry avalanches up to size 1.5 were triggered with the use of explosives as well as a storm and wind slab up to size 1.5.  

On Thursday, numerous skier-controlled and natural slab avalanches were reported up to size 2. A significant natural explosive triggered persistent slab size 2.5 was initiated from a NE slope at 1700 m. It was 80-100 cm deep and ran 200 m in length. Loose-dry avalanches were reported up to size 1.5 and running fast and far. 

On Wednesday in the South Rockies region, 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 wind, and persistent slabs will remain likely. Conditions are not easily managed right now, warranting a conservative mindset!

Snowpack Summary

Strong west-southwest winds have formed deep and reactive wind slabs on leeward slopes by redistributing some of the recent storm snow that fell Thursday. Up to 50 cm of snow sits above a variety of old interfaces that formed from mid-February's cold drought. These mostly include sugary facets, possible surface hoar in wind-sheltered locations and sun crust on steep solar aspects. Recently, some reports have indicated that this weak interface has failed and caught people by surprise. This MIN report is a good example. A persistent weak layer found around 60 to 100 cm deep that was buried late-January has been prevalent in the region during February. Persistent slab avalanches remain reactive and continue to surprise skiers and riders. 

There are presently no deeper concerns.

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.
  • Make conservative terrain choices and avoid overhead hazard.
  • 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.