Bollettino valanghe

Elkhorns

Wallowa Avalanche Center
Vedi sulla mappa

Pericolo valanghe

Mon
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
Open full map
Non aggiornato — questo bollettino è scaduto 2 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Increasing winds and additional snow will make it possible to trigger a slab up to a foot deep along ridgelines and in cross-loaded mid-slope terrain features.  It remains possible to trigger a large destructive avalanche that could fail on weak layers in the upper 3-4 feet of the snowpack.  Avoid wind-loaded slopes and carefully evaluate snowpack structure on northerly aspects before committing to avalanche terrain. 

This forecast is sponsored by La Grand Sno-Drifters Snowmobile Club. 

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    As a few additonal of new snow falls in the mountains today, winds will increase this afternoon out of the Northwest.  This could build wind slabs near and above treeline in leeward terrain up to a foot deep.  These wind slabs could form along ridgelines, or cross-load mid-slope features such as convexities in the slope.  The wind-blown snow will be deposited on a firm melt-freeze crust and could be sensitive to being triggered by a rider.  Pay attention to where the wind is depositing the new snow, and avoid riding on slopes greater than 30 degrees where wind slabs may exist.  You may see signs of wind loading on the surface in the form of a smooth or rippled pillow-like texture.  You can find small inconsequential test slopes no taller than a two-story house to test the sensitivity of these slabs as you ride or ski across the top of them. 

    (2/19/2024)

    Example of a Wind Slab at 7400 on W aspect. Size D1.5

    Photo: Wallowa Avalanche Center

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2.5

    Several persistent weak layers exist in our upper snowpack.  Forecasters and observers have been tracking two layers of Buried Surface Hoar that have shown a concerning level of reactivity in snowpack tests.   These persistent weak layers have been found between 1-3' from the surface and are mostly on Northwest-North-Northeast aspects near and above the treeline in wind-sheltered terrain.   A slab of cohesive snow sits above these layers, and as this slab has stiffened, we have not been seeing obvious signs of stability without digging into the snowpack to look for the layer and test its reactivity with snowpack tests.  This means that the freebie red flag signs of instability such as cracking or collapsing (whumphing) will likely not be present as you travel in areas where these weak layers exist.   The zones where these layers have been noted the most are the Western and Southern Wallowas as well as the Elkhorn Mountains.   In my travels on Thursday in the Northern Elkhorns, I found the 2/26 Buried Surface Hoar in 1 out of 4 snowpits that I dug.  Most likely places to find this layer are on shady aspects near treeline that are protected from the wind.  Check out my observation here.  

    I traveled in the Norway Basin area on Friday in the Southern Wallowas.  I dug 3 snowpits on Northerly facing terrain around 7800' and did not identify the Buried Surface Hoar.  However, WAH guides last weekend found this layer 1 foot down from the surface and backed off consequential terrain due to its presence.  You can see my observation and snowpit structure here. 

    The Rain Crust that was buried on February 1st has weak snow around it and should also be on your radar.  This layer is widespread and has been found about 3 feet from the surface of the snow and sits below a hefty cohesive slab.  

    In shallower snowpack areas on Northerly aspects that hold snowpack depths of around 3-4 feet, there remains an off chance to trigger a deep persistent slab avalanche that could fail on weak faceted snow near the ground.  Snowpack tests from last week in the Northern Wallowas indicated propagation of the weak snow near the ground.  An avalanche of this magnitude would be catastrophic.  You can see an observation highlighting this here. 

    To decrease your chances of getting mixed up in this sort of fight, a thorough snowpack analysis is suggested before committing to any slope greater than 30 degrees on Northeast through Northwest aspects near and above the treeline.  When you have uncertainty around the presence of these persistent weak layers, choose simple terrain less than 30 degrees.  

    (3/23/2024)

    Tom Whipple found Buried Surface hoar (4-8mm) 100 cm/40" from the surface yesterday in a snowpit on a NNW aspect at 8,000' in the western wallowas.  It could be seen as a thin gray stripe and produced sudden planar results in his compression test.  

    Photo: Wallowa Avalanche Center

Avalanche Discussion

3-6" of new snow fell yesterday above 6500' in the Wallowas, with less in the Elkhorns.  This new snow fell on a firm melt-freeze crust above 7,000' and an unfrozen snow surface below that elevation.  Ski guides in the Western Wallowas did not observe slab development within the new snow and did not observe any new avalanches.  Wind-transported snow was noted in the alpine.  Additional snow and increasing wind out of the NW today will likely continue to build wind slabs in exposed terrain.  In more protected terrain, continue to evaluate how well the new snow has bonded to the old snow/crust surfaces through hand pits and riding across small test slopes.  Be on the lookout for dry loose avalanches in upper elevation steep terrain where the new snow sits atop a firm crust.  These could easily be triggered by a rider and could run fast and far on a firm crust surface. 

Stay vigilant in your mountain travels and snowpack assessment.  Many of us have built a mental winter road map that suggests stabilizing deeper snowpack conditions by this point in the winter.  The weather and snowpack this season demand that we recalibrate that operational memory and accept the current conditions.  When reactive weak snow exists deeper in the snowpack, the best tactic is to use wider terrain margins.  Make a conservative travel plan to avoid areas where you have a higher level of uncertainty around the presence of persistent weak layers. 

 The recent warm spell has destabilized cornices to the point that they could break naturally or as you step or ride near them.  Give them a wide berth for the rest of the riding season.  

Regioni coperte