Blues

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

  1–3

  The persistent slab avalanche problem demands respect and patience.  The past week's impressive storm totals have overloaded the weak snowpack structure and many slopes near and above treeline are just waiting for a trigger to tip the balance.  During my travels in the Northern Wallowas yesterday, my group noticed multiple large collapses (whumphs) while traveling along ridgelines.  If we were on steeper terrain and this occurred, I'm confident we would have triggered a large avalanche.  We also viewed a large natural avalanche on a North aspect around 7,000' (near treeline) that looked to have failed on old, weak, faceted sugary snow underneath the 2 feet of recent storm snow.   Wallowa Alpine Huts Guides in the Wing Ridge area got a look into the Big Sheep Creek drainage and noted multiple natural avalanches of the same character.   An extended column test showed easy propagation (ECTP7 50 cm from the surface).   

  All of this information is the snowpack screaming at us that dangerous conditions persist.  

  In the Blues, Elkhorns, and Southern Wallowas, snowpack structure is similar, but last week's storm has deposited even more snow and wind-blown snow on top of this house of cards.  No reports of natural avalanches have surfaced from these zones, but it's hard to believe that they aren't out there.  An AIARE 2 motorized course yesterday in the Southern Wallowas noted large collapses of the buried weak layers in flat terrain near treeline.  They conducted a propagation saw test on a layer of weak faceted snow that sat on a crust and was buried 2.5 feet below the surface.  Their results (PST 40/100 END down 73cm) shows that these conditions are primed and ready for a trigger.  Don't let it be you or your riding buddies! 

  The scenarios that I think would most likely trigger a large to very large deadly persistent slab avalanche today would be:

  1.) Traveling on slopes greater than 30 degrees and finding a thin spot in the most recent storm snow where the weight of a rider and/or machine will stress the weak layers that are buried 2-4 feet deep.  These avalanches could be triggered from a distance (remotely) and run further than you expect.  

  2.) Triggering a wind slab avalanche that will bring additional stress to the weak snowpack and could "step down" to deeper buried weak layers in the snowpack.  Again, these avalanches could run further and be larger than you expect.  Within the last week, there have been two large avalanches triggered by this scenario.  One in the Southern Wallowas that was remotely triggered, and one in the Elkhorns.  See more on Wind Slab avalanches below. 

  

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  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

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

  Probabilità

  Likely

  Dimensione

  1–2

  The strong wind event during and following the storm has done its damage.  Observers noted wind-stiffened slabs in most all exposed terrain near and above treeline in the Northern Wallowas, Southern Wallowas, Blues, and Elkhorns in the past few days.  Wind slabs could be up to several feet thick.  They will start to stabilize in the next couple of days, but it is recommended to continue to stear clear of wind-loaded terrain. 

  Some good clues to look for with wind slabs are a stiffened snow surface, rippled/wavy texture, and cornices along ridgelines.  A wind slab avalanche would be enough of a trigger to step down into deeper persistent weak layers under the latest storm snow.  This would create a much larger and more destructive avalanche.