Central Sierra Nevada

Problemi valanghivi

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta quota
  • Limite del bosco
  • Sotto il limite del bosco

  Probabilità

  Possible

  Dimensione

  1

  A new round of wind slab formation will add to existing avalanche problems today. Prior to last night's increase in winds, 4 to 12 inches of unconsolidated snow was on the ground on all aspects and available for wind transport. Redistribution and subsequent wind loading is expected to have occurred last night and continue today. New wind slabs will be most prevalent in near and above treeline terrain on NW-N-NE-E-SE aspects. New wind slabs may form in more isolated areas on S aspects as well. In some locations these new wind slabs will form of top of older wind slabs that remained unstable yesterday.

  Identify areas of recent wind loading and avoid steep or consequential slopes that likely hold wind slabs. Many clues exist to identify the likely location of wind slabs. Looking for areas of blowing snow. Look for rough or scoured snow surfaces on windward aspects. Look for cornices, wind pillows, and areas of smooth snow in leeward areas to identify where wind slab development has occurred.

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta quota
  • Limite del bosco
  • Sotto il limite del bosco

  Probabilità

  Possible

  Dimensione

  1

  Much of the instability observed yesterday was associated with old storm slabs and in some cases wind slabs that have now become persistent slabs. In many but not all areas, faceted snow sitting above the Dec 15 rain crust and below recent storm snow is a problematic weak layer in near treeline and below treeline areas on W-NW-N-NE-E aspects. Locations were avalanches have occurred on this weak layer are frequently low on the slope and away from the traditional ridgetop wind slab start zones. The icy Dec 15 rain crust is generally easily identifiable at the bed surface.

  Managing persistent slab problems is more challenging than managing wind slab or within new snow avalanche problems. Time will tell, but as recent storm snow above the faceted weak layer becomes increasingly cohesive and slab like, the potential exists for instability to become more widespread and avalanches to possibly become more easily triggered. Take home point: Anticipate avalanche activity lower on the slope than usual. Adjust travel tactics to account for avalanches in less than typical places. Think slope angle, not historical start zone. Don't accept previous tracks or the passage of time as the sole argument for the absence of instability.