Bulletin d’avalanche

Tetons

Bridger-Teton Avalanche Center
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Danger d’avalanche

Thu
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 3 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Dangerous avalanche conditions exist. A widespread weak layer lies below a stiff slab that is becoming more difficult to trigger in most areas.  Human triggered avalanches are most  likely in steep terrain and thin spots such as rocky slopes and wind affected areas. Use cautious route finding around steep terrain and watch out for terrain traps.

Yesterday, a rider was fully buried and extricated with no injuries after unintentionally triggering a slope from below. We are glad to hear they are OK. This occurred in the far north part of Grand Teton National Park where we had another report of widespread collapsing on Monday. The snowpack remains variable throughout the zone.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1.5–3

    Weak snow from December is widespread and located 2-4 feet down at mid and upper elevations and 1-2 feet down at lower elevations. You are most likely to trigger an avalanche at a thin spot in the snowpack near rocky areas or wind affected slopes. The slab of January snow sitting on top of this weak layer has settled and stiffened over the last few days as a result of very warm temperatures.  It is becoming more difficult to impact the underlying weak layer through this stiffening slab and therefore less likely to trigger an avalanche. The decreasing trend of avalanches and red flags is probably a result of this stiffening slab more than it is a result of the weak layers adjusting. More time is needed than just a few days for the weak and widespread December drought layer to adjust. If you find the thin spot and collapse the weak layer, the resulting avalanche could still propagate long distances and be very destructive. The difference between now and a few days ago is that warning signs might not be present until you trigger the avalanche from on the slab.  With a 1-5 inches of snow overnight (continued loading) and warming temperatures, conditions can be more sensitive during the heat of the day. Continue to maintain wide safety margins in the current conditions by choosing to ride lower angle terrain, using cautious route finding and safe travel techniques.

    The Moderate hazard rating at low elevation reflects the stiffening slab that we are seeing in many places. Likelihood may be decreasing, but the consequence of getting strained through trees or lured into a terrain trap remains high.  Continue to give steep low elevation slopes room, particularly deep shady aspects that hold older, weak snow. Terrain traps, convexities and gullies remain suspect and keep an eye on pockets of avalanche terrain that could carry you into consequential terrain.

    (1/20/2024)

    A large, wide propagating persistent slab avalanche that occurred south of JHMR.

  • Deep Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    2–3.5

    This weak layer is located 3-6 feet below the surface at the base of the snowpack and is more likely to be found on northerly aspects at mid and upper elevations where old November snow lingered before being buried in December. It remains on the problem list because it is still in the snowpack even though we have not seen recent activity on this weak layer. Avalanches that step down to this layer could be very large and destructive.

    (1/22/2024)

Avalanche Discussion

Today might be on the low end of Considerable, particularly at mid elevations. There is some uncertainty involved, including the spatial variability of the snowpack around the forecast zone. Snow depths vary across the zone. In deeper areas like the commonly skied zones out of the Bradley Taggart portal we are seeing a thick and stiff slab that is in essence protecting the weak layer from impact more so than in thinner areas where the weak layer underneath is more easily impacted. Snowpack variability and uncertainty go hand in hand. If you are uncertain, dig down and see what’s below the surface

Over the last few days there has been a significant decrease in reported avalanches and red flag clues like cracking and collapsing in most areas. Clearly it is becoming less likely to trigger an avalanche. However, it has only been a few days, and we know that it will take much more time for the December drought layer to adjust and a trickle of precipitation overnight doesn't help. The more probable explanation for decreased activity is a stiffening slab. The danger, of course, is deciding to step out into more consequential terrain prematurely while the current weak layer is still likely to propagate long distances. The current balance of decreasing likelihood but similar consequence (including size and distribution) lands us in that gray zone between Considerable and Moderate.

As mentioned above, there was a full burial in the north end of the forecast zone. While this is the only reported avalanche in the last few days, this event is significant and highlights the variability and uncertainty discussed above, and indicates a higher liklihood in that area compared to others.  Once again, variability contributes to uncertainty.

The decision to go Considerable today reflects some of this uncertainty. But if we dig a little deeper, we are certain to find a widespread and well-preserved persistent weak layer in most locations that shows propagation in pit tests. This weak layer can produce very large and destructive avalanches. As a result, our risk management hasn't changed much, and conditions still warrant conservative decision-making and cautious route finding as we remain patient and allow the snowpack to adjust.

Régions couvertes