Intermountain

Problemi valanghivi

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  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

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

  Probabilità

  Likely

  Dimensione

  1.5–2.5

  There is a layer of weak snow (buried surface hoar) that remains the primary avalanche concern in the Intermountain and Continental zones. In the Maritime zone, recent warm temperatures have destroyed this buried surface hoar, and it is no longer expected to be an active problem.

  Last week, the setup below 3000’ was a classic persistent slab structure: a stout rain crust, buried surface hoar immediately above it, and 4–10 inches of dry snow forming the slab. At that time, the slab was generally too thin and soft to create a widespread, dangerous avalanche package. This week, the same crust + surface hoar combo likely remains, but the slab is becoming deeper and more cohesive as we add snow and wind effect. Thompson Pass picked up about 8 inches overnight, with Nick’s SNOTEL reporting 10 inches, and moderate winds from variable directions have likely increased slab thickness and stiffness in specific start zones and lee features.

  It is still uncertain whether the slab has reached the point where human-triggered persistent slabs are likely, but we have applied a CONSIDERABLE hazard rating to account for the possibility that we are now at (or very near) that threshold. Even if we aren’t quite there yet, conditions are trending that direction, with moderate to heavy snowfall forecast to return Thursday, adding additional load to a known weak layer.

  One advantage of this problem is that it is near the surface, making it easier to assess than our second concern (deep persistent slabs). Identifying where it exists can be straightforward: if you can probe or feel a firm rain crust beneath the new snow, assume buried surface hoar may be sitting on top of it. Use quick hand pits, hasty tests, and small test slopes to look for clean shears or easy failures at this interface. If you get red flags like shooting cracks or whumphing/collapsing, treat that as a strong signal that triggering an avalanche is likely on steep enough terrain. Avoid steep or complex terrain below 3000’ anywhere you find a cohesive slab over the crust, especially in the Intermountain zone. The Continental zone is still expected to have a thinner, less developed slab overall, so this problem may be less prominent there—but watch for pockets where wind has stiffened the surface.

  Storm Slab (above 3000’): Buried surface hoar is not expected above 3000’, so the focus shifts to new snow bonding and wind effect. Pay attention to how reactive the storm snow feels, and look for signs of drifting (pillows, textured snow, cracking). The same quick assessment tools—hand pits and small test slopes—work well for determining how sensitive storm slabs are in the terrain you’re traveling through.

   

  

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  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

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

  Probabilità

  Unlikely

  Dimensione

  2–3

  Weak snow near the ground exists at all elevations and is now deeply buried in most locations. In many areas, basal facets are bridged by stronger, stiffer layers in the middle of the snowpack. As this problem gets deeper it becomes harder to trigger and is unlikely to produce obvious red flags like cracking or collapsing. Expect a stubborn, unreactive deep slab in most places—this is a low-likelihood / high-consequence setup.

  The most likely trigger points are thin spots, especially steep, rocky terrain where the weak layers are closer to the surface. A rider may be able to stress the basal weak snow in these shallow areas, and a small trigger could step down or propagate into deeper, thicker parts of the snowpack, producing a large and destructive avalanche. The best way to manage this problem is to avoid steep, rocky features and other shallow-snow areas where the snowpack is thin and variable.

  In the Continental zone, the hazard will remain CONSIDERABLE where the snowpack is thinner, weaker, and more easily affected by human triggers. Human-triggered deep slab avalanches are possible in the Continental zone. They are unlikely in other zones, but if triggered could produce very large avalanches with a very low chance of survival.