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

A snowmobiler triggered a large avalanche yesterday (Thursday) on a weak layer buried 2-4 feet deep. Warming temperatures can increase the potential for natural and human-triggered avalanches. Give steep slopes a wide margin, particularly during the heat of the day. 

Problemi valanghivi

  • Persistent Slabs

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

    Yesterday (Thursday), a snowmobiler triggered a large avalanche on a buried weak layer at 5500' in the southern Whitefish Range. Otherwise, most avalanche activity has occurred in the Flathead Range on slopes near and above 6500 feet that face NW-NE-E-SE. Carefully assess terrain on slopes that fit this description. Slabs can break 2 to 4 feet deep on facets formed in early February. The resulting avalanche will be difficult to escape. The most conservative option is to stick to lower-angled slopes in areas where the problem exists. Avoid convex rollovers and rocky, wind-effected slopes if you enter steeper terrain.

    Today's warming temperatures and solar radiation may stress weak layers buried in the snowpack.  Give steep terrain a wider margin during the heat of the day.

    (3/7/2024)

    A snowmobile trigged slab stepped down to a deeper buried weak layer and produced a large avalanche in the southern Whitefish Range.

  • Loose Wet

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

    A Wet Loose problem may develop today due to intense solar radiation and warming temperatures. Steep, rocky areas hit by the sun are typically the first place to slide. Expect this problem to develop most quickly where warm temperatures meet dry snow. Middle elevations may see the most activity, but upper elevations can't be ruled out. Move to lower-angle terrain or shaded aspects, IF you see signs of warming, like rollerballs and pinwheels scuttling down the hill. 

    (2/6/2024)

    Wet loose avalanches may run naturally in steep and rocky terrain. 

    Photo: Sarah for Beth Steele

Avalanche Discussion

This week I traveled to the Northern and Southern ends of the advisory area. In both outings, my party found seemingly benign conditions; limited effects from wind and sun, and unremarkable findings in our test pits. After wrapping up in one pit my partner commented, “If it wasn’t for all the action, we’d be riding everything.”

The action they were referring to consists of:

  1. A handful of near-misses and accidents in the Marion Lake area during the President’s Day weekend
  2. A skier-triggered hard slab on Slippery Bill Mountain in the Flathead Range on 2/26
  3. A cycle of very large avalanches in the Flathead Range during the wind-loading event 2/28-2/29
  4. Fatalities north and west of our advisory area

You can add Thursday's snowmobile-triggered avalanche in the Whitefish Range to this list. These slides all involved buried weak layers from early February. In some cases, the weak structure has been subtle and has fooled several parties including professionals. Stability tests have been a pretty mixed bag, although BNSF Avalanche Safety was able to reproduce propagating results in John F Stevens Canyon.  In this situation I'm placing more emphasis on behavior than appearance.  These buried weak layers may not look like much, but the above rap sheet has me wanting to steer clear.   

We removed the windslab problem as observations indicate tapering wind-loading and slabs trending toward unreactive. Continue to exercise caution where you find signs of wind-loading, like a scalloped surface or dune-shaped features. Recognize that previous wind-loading occurred on slopes harboring buried weak layers. Three human-triggered avalanches have occurred in the southern Whitefish Range since Monday. One of these (the previously mentioned snowmobile-triggered slide) stepped down to a buried weak layer and resulted in a larger avalanche. 

Regioni coperte