Banner Summit

Problèmes avalancheux

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpin
  • Limite forestière
  • Sous la limite forestière

  Probabilité

  Possible

  Taille

  2–3

  We are still leery of persistent weak layers (facets and/or crusts, and occasionally surface hoar) buried in the top 3' of the snowpack. These layers have produced several avalanches over the past two weeks (Dollarhide, Galena Summit, Smiley Creek, Beaver Creek, Sawtooths, Banner Summit). They seem most reactive on sunny slopes where facets are associated with crusts (video, photo), but two fairly recent large avalanches occurred on shady aspects in the Sawtooths.

  Yesterday, experienced skiers backed off their objective in the Sawtooth Mtns due to concerns about how the past few weeks of snowfall has bonded to the hard crusts and surfaces beneath. Monday, experienced guides felt and heard a large collapse on a sunny ridgeline in the northern Boulder Mtns and switched to a more conservative plan B. Also Monday, I saw a natural persistent slab avalanche that failed in an area with a very thin snowpack in the Boulder Mtns; it released when hit from above by a small wet loose avalanche (see Additional Discussion). The warm temperatures and intense sunshine are adding uncertainty to the persistent slab stability equation.

  How can we manage or avoid this fairly complex, unpredictable problem?

  • Triggering a large avalanche is most likely on slopes steeper than about 35 degrees that are above about 8000' in elevation. These elevations received the most snowfall and wind-transported snow during last weekend's storm.
  • Rocky terrain where the snowpack is thinner gets your skis or sled track closer to the weak layers in question and is more suspect. Avoid craggy slopes that are peppered with exposed rock. 
  • Picture the consequences of being caught in an avalanche before committing to riding or skiing that steep slope. If a 2-3 foot thick slide released, would you be carried through trees or rocks? Would you be buried in a creek bottom or gully where snow can pile up deep?

  ADDITIONAL DISCUSSION:  Today's warm temperatures and sunshine could cause small, loose snow avalanches to release naturally. These slides can be dangerous on their own in rocky, consequential terrain, and they can trigger larger slab avalanches as they travel downhill. We saw evidence of this earlier this week in the Boulder Mountains (observation here, photo to right and here). Wet loose slides are most likely to release in very steep, rocky terrain that directly faces the sun and is sheltered from the wind. This type of instability follows the sun around the compass, beginning on E-SE aspects in the morning, moving to SE-S facing terrain near noon, and finishing up on SW-W aspects later in the afternoon. Fortunately, this hazard is easily managed:

  • Watch for rollerballs and pinwheel moving down slopes facing the sun. This is a sign the snow surface is heating up, and wet loose activity may not be far away.
  • Minimize your time spent basking in the sun below large avalanche paths that fit the description above. While the T-shirt conditions make these great places to take a break, be sure you're out of the firing zone in case slides release on their own above you. 

  The heat is also affecting the large cornices that have formed throughout the season. These beasts sag and can break during the heat of the day. Yesterday, I saw chunks from a recent natural cornice failure in alpine terrain in the Smoky Mtns. Cornice fall can be a great trigger for the persistent slabs described above. Give cornices a wide berth if you're traveling on ridgelines, and minimize the time you spend on slopes directly beneath large cornices.