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Avalanche Forecast

Archived

Feb 26th, 2021–Feb 27th, 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.

Don't let the post storm pockets of sunshine lure you into complex avalanche terrain. A reactive persistent slab exists and may be easily triggered by a skier or rider. A conservative mindset and patience is required. 

Confidence

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

Weather Forecast

Saturday: A mix of sun and cloud. Ridgetop wind moderate from the southwest. Alpine temperatures near -8 and freezing levels valley bottom.

Sunday: Cloudy with light snow 5 cm. Ridgetop wind moderate to strong from the southwest. Alpine temperatures near -7 and freezing levels valley bottom.

Monday: Light snow 5-10 cm with gusty strong ridgetop wind. Alpine temperatures near -3 and freezing levels 1300m. 

Avalanche Summary

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.

Earlier this week (Monday-Wednesday) Many large size 2 to 3 slab avalanches were triggered naturally, by skiers, and using explosives. The slabs were generally 30 to 70 cm thick, most often at treeline and alpine elevations, and occurred on all aspects but many of them on north to east aspects. 

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

Up to 30 cm of new snow fell Thursday accompanied by strong ridgetop wind forming new reactive storm and wind slabs. This brings up to 50 cm of snow over 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 almost daily. Avoidance of consequential slopes is the best way to ensure a safe day right now.

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.