Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
Vedi sulla mappa

Pericolo valanghe

Sat
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
Open full map
Non aggiornato — questo bollettino è scaduto 6 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

The danger of triggering large, dangerous avalanches lingers on steep, shaded slopes. These can break 1 to 4 feet deep on weak snow above the distinct mid-January crust. Avoid being on or under slopes steeper than about 35 degrees likely to harbor this structure. These are unusual conditions for our region; riding safe requires more attention to slope angles than usual. Look for and avoid stiff, dense slabs of drifted snow near ridges and in cross-loaded gullies. The extreme weather will magnify consequences of mishaps; keep your trips short and simple. 

Problemi valanghivi

  • Persistent Slabs

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

    Dig down a few feet on northerly and easterly slopes, and you'll find thin layers of weak snow at or just above a distinct crust. That structure remains dangerous, though it's growing less susceptible to the weight of a person or snowmachine. It's very difficult, however, to determine whether a particular slope will fail under your weight. Given that uncertainty, avoid slopes that are steeper than about 35 at any point. Triggering slides from below remains possible, so stay out from under steep start zones. Use an inclinometer so you aren't relying on guesswork to keep yourself and your partners safe. The riding is great on sheltered, low-angled slopes.

    (2/9/2021) The results of collapsing buried surface hoar

  • Wind Slabs

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

    A large natural avalanche on a westerly slope shows that northeasterly winds drifted snow onto usually windward slopes earlier this week. While winds have eased, the slabs remain. They'll feel denser and firmer than snow on sheltered slopes. They'll be most common near ridges and exposed summits, though you may find cross-loaded gullies at mid-elevations. An unambiguous clue that winds have been at work is a ridge with drifts and cornices on both sides. Avoid being under steep start zones if you see blowing snow near ridges above you. Triggered slabs may step down and release slides that break on weak snow above the mid-January crust, resulting in much larger, more destructive avalanches. 

    (2/11/2021) Outline of a recent large natural avalanche on the west face of Running Rabbit Mtn in the Goat Lick path.

Avalanche Discussion

Mark summarized Chapter 2 of our winter - the mid-January storm to the Arctic intrusion - on our blog. It's a great review. The plot summary: "Slabs build slowly above a hard crust and surface hoar/facets, culminating in a rash of avalanches, near-misses, and accidents the first week in February." Chapter 3 started when Arctic air pushed over the Divide, ending the snowfall, switching winds to the east and northeast, and plunging the region into a week, so far, of bitterly cold temperatures.

The break in precipitation is allowing the slabs that formed in early February to gradually stabilize. The cold temperatures are slowing the process. For now, many northerly and easterly slopes harbor slabs 1 to 4 feet thick that are sitting on collapsible layers of surface hoar or facets. A person's weight can still trigger some of these slabs, resulting in large or very large avalanches that would have serious consequences. The slabs that remain sensitive to the weight of a person won't be obvious; you may not see shooting cracks or feel whumpfing collapses on nearby terrain. And slope-specific stability tests are not particularly predictive, because likely trigger points are often mid-slope, on convexities, or in glades or meadows where the surface hoar was best developed. The most reliable way to survive Chapter 3 is to avoid steep slopes likely to harbor the slab-surface hoar/ facets- crust structure. "Steep" translates as more than 35 degrees, though if you want to build in a bigger margin of safety, use 30 degrees as your threshold. Keep in mind that many of the slides reported the 1st week in February were breaking very wide, with fracture lines running between trees. The key is slope angle, not perceived anchoring. Measure slope angles, because getting it wrong will have serious consequences.

A second character in Chapter 3 are northerly and easterly winds. These have drifted low-density snow onto usually windward slopes. That is, southerly and westerly slopes. That drifted snow formed unstable slabs, as evidenced by a large natural avalanche that likely ran Tuesday near Essex. The winds in the easterly half of the region peaked Tuesday, but look to be spiking today in the Swan Range. If you brave the cold, expect this hazard to be more widespread as you climb higher, and to be more reactive in the Swan Range. Triggered wind slabs can step down to the surface hoar and facets above the 1/13 crust, making them more dangerous. 

Keep in mind that the weather is exaggerating the consequences of even small missteps and accidents. A minor medical or mechanical problem can spiral into an emergency in subzero temps and dangerous wind chills in exposed terrain. More serious issues can be unsurvivable. Keep your trips short, with straightforward access back to warmth and shelter.

Regioni coperte