Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

Sun
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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In sintesi

Keep your plan simple to stay safe during extreme and dangerous cold. You might find sensitive slabs of wind-drifted snow in unusual locations. A slide failing on a buried weak layer may result in a larger, more destructive avalanche.

Problemi valanghivi

  • Wind Slabs

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

    During the last two days, winds blowing from the north through east have drifted recent low-density snow at all elevations. Slabs reactive to the weight of a rider can now be found on slopes that are usually windward. Look for slabs downwind of saddles, ridgelines, and in cross-loaded gullies. Unusual winds can form slabs in unique places at low and mid elevations, including on or just above the valley floor. The Tunnel Ridge station continued to record southerly winds, and encountering fresh slabs on typical leeward aspects remains a possibility in the Flathead Range. 

    Blowing and drifting snow, slopes littered with pine needles, and scoured or scalloped surfaces are all signs of recent wind.  Stiff slabs may support your weight but can break well above you, wider than expected, and are extremely difficult to escape.  You've found a hard slab if your boards or machine start to ride on top of, instead of through, recent snow. 

    (1/12/2024) This crown is located in the upper reaches of Shed 9 on Snowslip Mtn.  This is on the leeward side of a ridgeline on a southeast-facing slope, cross-loaded by prevailing westerly winds. Photo: Adam Clark

  • Persistent Slabs

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

    Several feet of recent snow have buried weak layers 1-3+ deep. Natural and remotely triggered slides occurred on these layers during the last week. We do not have the distribution of these layers pinpointed, and all steep slopes at mid and upper elevations should be considered suspect until proven otherwise. Collapsing or shooting cracks may give you a warning of their location. However, the only sure way to identify if a slope harbors these concerning layers is to dig into the pack and perform snowpack tests. Look for a layer of facets and/or surface hoar resting on a crust that formed 12/22. 

    (1/12/2024)

    A large natural avalanche that failed on a buried weak layer in Shield's Creek. This is referred to as a Persistent Slab avalanche. In the sun, in the upper right corner, is a crown of a wind slab avalanche.

    Photo: Adam Clark

Avalanche Discussion

Things break when it’s this cold. Many weather stations stopped reporting overnight. The ones still online are recording temperatures of -40 to -20 degrees Fahrenheit. Winds seem to have tapered overnight, which will technically decrease the wind chill hazard. Regardless, I still plan to limit my time outside warm buildings and vehicles today.

A break in the wind and snowfall will also start a trend of decreasing avalanche hazard.  However, we’ll begin this trend gradually. Cold temperatures can slow the healing of instabilities in the snowpack. Limited observations and weather data increase uncertainty.

We added Persistent Slab to our list of problems yesterday (Friday). With the precipitation ending, our attention focuses on weak layers below the surface. We currently have limited information on the exact distribution of this problem. Here’s what we’ve seen so far:

  • Sunday, 1/7: An experienced rider remotely triggered slides on Six Mile's 12/22 crust/facet combination. I returned to the same area on Wednesday and had propagating test results in my snowpit.
  • Monday, 1/8: In Lost Johnny, Cam found facets resting on a crust that initiated a collapse and triggered a small 2-foot-thick, soft slab.
  • Monday, 1/8: Blase found widely propagating natural avalanches and unstable snowpit test results on a layer of facets in the southern Whitefish Range.
  • Thursday, 1/11: Cam and Mark visited the site of a Persistent Slab avalanche in a popular side-country location east of the WMR ski area. This slide was near a well-used skin trail and occurred either naturally or remotely triggered. The crown was up to 3 feet thick and failed on a thin layer of facets just above the 12/22 crust.
  • 1/10-1/12: BNSF Avalanche Safety reported avalanches (D1-D2) failing on faceted snow near the ground (1/10, 1/11, 1/12)

Regioni coperte