Bollettino valanghe

Swan Range

Flathead Avalanche Center
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Pericolo valanghe

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

You could trigger a large avalanche on a weak layer buried 1-3 feet below the surface. Winds have drifted snow into thick, hard slabs in unusual locations. Both of these problems may not give you feedback before they fail. Continued cold conditions dictate a short, simple field plan. 

Problemi valanghivi

  • Persistent Slabs

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

    Weak layers are buried 1-3 feet below the surface. These layers are likely a facet/crust combination and have proven unstable. Natural and remotely triggered slides have occurred on these layers for over a 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. Otherwise, the only way to identify if a slope harbors these concerning layers is to dig into the pack, looking and feeling for density changes or getting propagation in your snowpack test.

     

    (1/11/2024)

    The avalanche crown averaged 2.5 feet thick and failed naturally or was remotely triggered. This is an out-of-bounds avalanche in a popular side-country location just east of the WMR ski area.

    Photo: Mark Dundas

  • Wind Slabs

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

    Arctic winds from the north through the east have been blowing since Thursday. These atypical winds drifted snow at all elevations. Given Arctic winds' consistent and strong nature, stiff drifts of dense snow will be in unique locations. Expect to find slabs downwind of saddles, ridgelines, and in cross-loaded gullies. 

    As you travel today, feel for wind-stiffened snow that may support you and your machine’s weight. Hard, dense dunes may crack or collapse, but they may not give you fair warning. With this in mind, look for visual clues, like pine needles, downed branches, or textured surfaces, to identify where the wind has been working the snow. Avoiding steep, wind-loaded slopes is a way to mitigate this hazard.

    Saturday’s shift to a more southerly flow may form thin drifts in more typical leeward locations.

    (1/12/2024) Crown in the upper reaches of Shed 9 on Snowslip Mtn Photo: Adam Clark

Avalanche Discussion

The current deep pool of cold air arrived in our advisory area at John F. Stevens Canyon on Wednesday, the 10th. Numerous weather highlights occurred during these five days, including air temperature readings of -49F in Polebridge and -48F at Marias Pass, along with impressive wind chills of -70F at Snowslip and Tunnel Ridge and -68F at the summit of the WMR ski area. To add insult to injury, Snowslip recorded air temperatures of -30F or greater for 45 consecutive hours, with Flattop below -40F for 16 consecutive hours.

Fun fact: It reached -40F in Ferndale Saturday morning, where a frequent contributor reminded us that  -40F = -40C

Yesterday (Saturday), winds shifted to the south or west at most mid and upper-elevation stations, ushering in a warmer air mass and increasing temperatures overnight. Readings this morning at mid and upper elevations are generally 15-25 degrees warmer than this time yesterday. Unfortunately, cold air remains entrenched in the valley floor, where warming will be slower to occur.

The cold temperatures have slowed the strengthening process with recently formed wind slabs and buried weak layers. Wind slabs encountered today should generally be of the hard variety and supportable to you or your machine weight. Hard slabs are unpredictable, and I would attempt to avoid them whenever possible. Our Persistent Slab problem remains a bit of a mystery due to a lack of observations. With rising temperatures, FAC staff hopes to view various avalanche terrain in the coming days to determine how the buried weak layers reacted to the recent loading event. We also plan on digging into the snow and seeing how these weak layers fare in snowpack tests.

Understandably, observations have been scarce these past few days. Saturday, the WMR ski patrol noted slabs in unusual places. A polar explorer visited southern Glacier Park just east of Marias Pass and found numerous signs of instability, including shooting cracks and substantial wind loading. On Friday, BNSF snow safety noted multiple wind slab avalanches and at least one larger Persistent Slab in John F. Stevens Canyon.

Unfortunately, an avalanche fatality occurred Thursday in the Idaho Panhandle National Forest. A preliminary report of the incident can be viewed here.

 

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