Bollettino valanghe

Swan Range

Flathead Avalanche Center
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Pericolo valanghe

Sat
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

You're most likely to trigger a large avalanche on a wind-stiffened snow surface. The chance you could trigger a slab avalanche in sheltered terrain is increasing. Default to conservative terrain if there are obvious signs of instability, such as cracking or collapsing.

 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1.5–2.5

    Triggering a large 1-3 foot thick avalanche on a buried weak layer is becoming more likely as the surface snow consolidates into a slab. Suspect slopes include those stiffened by the wind or exposed to recent sun and warming temperatures. Buried weak layers (facets, facets + crusts, and surface hoar) are found on many slopes at all elevations and rest beneath February's snowfall. 

    Initiating an avalanche is possible if the slope you are on or connected to is steeper than 35 degrees. The odds go up if this terrain has a convexity.

    The shallower the weak layer is buried, the more sensitive it will be to the weight of a rider. In wind-drifted areas, avalanches will be easier to trigger in thin spots at the slab's edge or where rocks stick out of the snow.

    Signs of instability, such as collapsing (whumpfing), shooting cracks, and a remotely triggered avalanche, have been concentrated in the Whitefish Range. Still, a weak snowpack structure is found in all zones. Signs of instability may not always be present, and digging is the best way to identify if the slope you are on harbors a slab and a weak layer.

    A rider triggered this slab on a southerly mid-elevation slope in the southern Whitefish Range on February 17. Note the convex shape of the slope near the fracture line.

  • Loose Dry

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

    Unconsolidated surface snow remains on slopes unaffected by Tuesday’s sun and recent wind. On very steep slopes, you could trigger a sluff in this soft snow that can potentially entrain significant amounts of snow and travel long distances. These can also occur naturally from clumps of snow falling from trees or rocks. Today is expected to be mostly cloudy, but natural slides may develop on slopes directly facing the sun if sustained sunshine occurs.

    This problem is generally easy to manage unless you get surprised and swept off your feet or machine. These slides are most dangerous in long-running gullies where escape can be difficult and above cliff bands or surface obstacles where you do not want to end up. Limit your time above terrain traps, where debris can pile up surprisingly deep. Utilize sluff management to mitigate this problem, or stick to lower-angle terrain where sluffing will be limited.

    These natural Dry Loose sluffs were observed in the Flathead Range on 2/19/2025. Photo: Michael Reavis

Avalanche Discussion

Air temperatures during the first two-plus weeks of February were well below seasonal readings, which preserved the snow surface as low-density blower powder in locations sheltered from the wind.  A change began this week with moderating temperatures and mid-elevation readings reaching freezing (Graph).  Area-wide, snow stations show a gradual decrease in the height of snow (HS), indicating the surface snow is beginning to settle into a slab. 

A developing surface slab increases the chance that a rider could trigger an avalanche on weak layers buried 1-3 feet deep. This remains our primary concern today (Friday) and Saturday. Increasing winds yesterday and this morning may have formed thin slabs of wind-drifted snow near summits, ridgelines, and cross-loaded features. An etched snow surface, dune-shaped, or a pillow-like appearance can identify these. 

The big news is that an Atmospheric River targets NW Montana Saturday night into Monday. The rain/snow line may rise above 5000 feet, with the potential for 1+ inches of precipitation. The avalanche danger will increase as rain on snow and heavy, wet snow above the rainline impact the relatively cold, dry surface snow and the weak layers formed during the January drought. If the forecast is verified, we expect to issue an Avalanche Warning as the danger may reach HIGH. Traveling through the mountains on Sunday and Monday will be challenging and dangerous. 

Upcoming Events: Check out our calendar and sign up now!

  • Feb 26th - State of our Snowpack Presentation at 7 pm for Pint Night at Blackstar with donations going to Flathead Avalanche! 

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