Lawinenlagebericht

Flathead Range & Glacier NP

Flathead Avalanche Center
Auf der Karte ansehen

Lawinengefahr

Wed
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
Open full map
Veraltet — dieser Lagebericht ist 2 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Today will be a great day for mountain travel. Move to shadier aspects or lower-angled terrain if you feel the warm sun affect the snow surface. If you choose to enter steep or consequential terrain, only expose one rider at a time. Avoid riding over common trigger points, such as steep breakovers or locations where the snowpack appears thin and rocky. 

Lawinenprobleme

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2–3

    Although weak layers buried 1 to 4 feet deep continue to show signs of improvement, sustained solar radiation today may increase the likelihood of triggering on sun-exposed slopes. Since the new year, we have received no reports of instability on buried weak layers, and the last natural persistent slab avalanches likely failed on December 30th during the post-Christmas storm. 

    Travel advice:

    • As the day progresses, move to shadier aspects. 
    • Only expose one rider at a time in steep or consequential terrain.
    • Avoid common trigger points (i.e. steep rollovers or thin and rocky terrain)
    • Choose slopes with long, clean runouts free of surface obstacles. 
    • If you want to continue to avoid the problem, you’re in luck. Little to no wind has left soft snow on many aspects. Enjoy the fluffy snow (minus the zipper crust) on low-angled terrain on most aspects. 

    (1/6/2025) Photo: Michael Reavis

    This photo shows distant crowns of Persistent Slab avalanches that were likely triggered by loose dry avalanches from above. These likely failed sometime around December 30th. 

Avalanche Discussion

Today, uncertainty about solar-induced triggering of slabs on persistent weak layers led us to bump to the danger back up to Moderate. We know there are shallowly buried weak layers on sun-exposed slopes. There are simply too many unknowns to remain at Low danger during the first full day of solar input. Solar input can change slab properties and increase creep rates, which can lead to the fracture of a weak layer (research paper here). The takeaway from this is to move to shadier aspects after sustained sun. 

Apart from this uncertainty, evidence for instability on persistent weak layers in the snowpack are scarce. Here is a summary of observations since New Year's Eve: 

  • Signs of instability: 4 of 35 observations found unstable snow. None of which involved whumpfing collapses and all appeared to have occurred in new and/or wind-drifted snow.
  • No human-triggered avalanches on persistent weak layers. The last reported human-triggered avalanche failing on a persistent weak layer occurred on December 29th (observation). The last reported natural avalanches failing on persistent weak layers seem to have occurred after a loading event around December 30th (observation, observation). 
  • Snowpit tests show stable results. One exception was a propagating snowpit test outside the forecast area at the new/old snow interface (observation). The last propagating snowpit test on persistent weak layers occurred on December 31st (observation).

Old slabs of wind-drifted snow may still linger on exposed ridgelines in the alpine. Look for subtle cues of wind-textured snow and dune-shaped drifts. Slabs should be easy to identify and ride around. 

Sunshine may intiate a round of small, wet loose slides. Rollerballs are a precursor to this easy to manage problem. Move to shadier slopes if you see signs of warming. 

Thank you to everyone for taking the time to submit observations!

Abgedeckte Regionen