Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

Tue
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto 4 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

You can trigger a dangerous avalanche today, similar to these large and very large avalanches that ran over the weekend (Augusta, Yule Pass).  Avalanches breaking near the ground are becoming harder to trigger and will give few obvious warning signs.   Over the past week, persistent slabs have been most active and most destructive on wind loaded alpine terrain.  Wind-sheltered terrain facing south and southwest offers the safest steep riding options.  

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2–3

    Storms this month have built thick and hard persistent slabs over a widespread weak layer of facets and faceted crusts. These slabs are generally 2 to 4-feet thick. The vast majority of avalanche activity this month has occurred on west to north to southeast aspects, with only a few avalanches from alpine south to southwest facing alpine terrain. Persistent slabs are notorious for erratic feedback and unusual behavior, such as remote triggering and wide propagation. To reliably avoid the problem, choose routes that keep you off and out from under terrain steeper than about 30 degrees that hold old, weak snow near the ground. In steeper terrain, you can reduce your chance of triggering an avalanche by avoiding trigger points where the snowpack is thinner than average.  Thin spots are most common in wind-affected terrain or near rock bands or vegetation.  

    This persistent slab avalanche wrapped from shady to sunny aspects and broke on weak layers near the ground. This a good example of the potential size and widely propagating behavior of our current persistent slab problem. 

Avalanche Discussion

The steady flow of natural avalanches all month has slowed to a trickle over the past couple of days due to dry weather and wind transport coming to a halt.  To be honest, Saturday's D3 natural near Yule Pass and the fresh D2 near Augusta observed yesterday both came as a surprise because wind transport has been minuscule during the past couple of days.  This just highlights how challenging and untrustworthy our weak layer is this winter. The gradual transition from Considerable to Moderate is occurring because slabs are getting increasingly stiffer and harder to trigger, while our basal weak layers are oh-so-slowly starting to adjust to the constant loading over the past three weeks.  Above treeline in the Northwest Mountains is the last to phase to Moderate simply because that elevation band has been most active over the last few days, and is capable of producing unsurvivable avalanches, up to D3 in size.

We're moving into a tricky period where the odds are in your favor that you can ride in a lot of avalanche terrain without triggering an avalanche. However, if you cross the wrong slope, or the wrong spot on the wrong slope, you can trigger a deadly avalanche.  And in many cases, those wrong slopes don't stand out uniquely from their peers.  However, unlike Russian Roullete, you do have the option to pull the trigger on a chamber that you know is empty.  We know that slopes that were bare dirt in late November (generally mid to low-elevation southerlies) don't have weak layer issues near the ground.  We know that the lower slope angles are safe.   Or you can swivel the chamber to terrain that is less likely.   High elevation south to southwest facing slopes held thick crusts in late November and have been generally quiet all month with only a few exceptions.  Slopes with a consistently deep and uniform snow cover (more than ~120 cm) are more difficult to impact the weak layer compared to slopes with variable snow depths.  And slopes that avalanched in the past couple of weeks have shed some combination of their slab or weak layer, making avalanches less likely.  

If you're stepping out into avalanche terrain, consider the potential size of an avalanche and terrain consequences.  I suspect the skier who was caught in an avalanche yesterday near Aspen came out unscathed because the slope was smaller and more forgiving than many of the slopes shown in the gallery below.  

Regioni coperte