Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

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

The wet snow avalanche hazard will remain elevated until cooling temperatures arrive this afternoon. This hazard exists on all aspects, including shady slopes where dry snow is warming for the first time. Mushy snow is a sign of instability. It remains possible to trigger a large dangerous avalanche on weak layers buried 3-6 feet deep.  Stick with slopes less than 30 degrees in steepness that are not below steeper terrain or cornices.  

Problemi valanghivi

  • Cornices

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

    This week's warm temperatures have weakened the large wind features found on the lee sides of ridgelines. Many of these whales are enormous and are capable of triggering a large destructive avalanche on buried weak layers. Give slopes harboring these beasts a wide berth. While traveling on ridgelines, remain well back from the edge, which in many cases is further than you think.

    (3/5/2021) The large Rubideau cornice in the Apgar Range in GNP. This cornice develops from westerly winds.

  • Loose Wet

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

    Above-average temperatures and plenty of sunshine spurred a wet loose cycle on sunny aspects at low and middle elevations Wednesday and Thursday.  These slopes have been roughed up, and the natural cycle has ended in these areas. Cloud cover overnight has kept temperatures above freezing at many mid and upper elevation stations. We expect the wet loose problem to migrate to the north side of the compass at mid and upper elevations, where dry snow will be getting wet for the first time. This will also be the case on those upper elevation sunny slopes, which did not warm earlier this week. Look for fresh rollerballs that are a precursor to wet loose slides. If you can make a snowball from the surface snow, triggering a wet sluff is possible.  If this is the case, head to slopes less than 30 degrees that are not connected to steeper terrain above. Limit your exposure on slopes directly above a terrain trap where debris can pile up surprisingly deep.

    (3/4/2021) A wet loose slide triggered by warming and sunshine. This occurred during the day on Mt Vaught in GNP.

  • Persistent Slabs

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

    Recent warm weather contributed to a natural Persistent Slab cycle, with most of the slides reported on sunny aspects. Last night, many mid and upper-elevation weather stations remained above freezing which may result in natural slides on slopes warming for the first time. It remains possible to trigger a large dangerous slide on weak layers buried 3-6 feet below the surface. These layers formed in late-January and have been responsible for numerous triggered slides for over a month. Terrain selection remains your go-to for dealing with this challenging problem. Choose slopes less than 30 degrees in steepness free of rollovers and not connected to steeper terrain above. Steer around areas where the slope dramatically steepens, even for just a few feet. These are the areas where you are most likely to trigger a slide.

    (3/4/2021) A very large Persistent Slab avalanche on an SW aspect in southern Glacier Park. This was triggered by warming and sunshine.

Avalanche Discussion

This week’s spring teaser produced several days of above-average temperatures and sunshine. Temperatures spiked Thursday and Friday, with Marias Pass and Flattop reaching 50 degrees Fahrenheit. Unfortunately, last week’s dry powder became this week’s mank. Warming of the surface snowpack resulted in both wet and dry avalanche activity. Loose Wet occurred in dry near-surface snow on sunny aspects. Last night's cloud cover results in possible greenhouse conditions today, which will allow dry snow on shaded aspects to warm. Terrain that was a safe refuge during the past few days of warming is now poised to develop a wet loose cycle.

Persistent Slab avalanches this week started as dry avalanches but picked up wet debris as they moved downhill. Observations have been sparse, but Persistent Slab’s hotspots seem to be the John F. Stevens corridor and the Whitefish Range. With continued warm conditions, natural and triggered slides on buried weak layers remains a concern. The Persistent Slab problem related to weak layers formed in late January has been listed daily since January 31 and responsible for numerous triggered slides and close calls.

We removed glide avalanches from today’s list of problems. Glide failure is notoriously difficult to predict, but they often occur during periods of extended warm temperatures and sunshine or heavy rain events. We have not received any reports of these occurring during our recent warm-up. Due to their unpredictable nature, we recommend not recreating on or below slopes harboring the brown frown.

Our weekly summary and weekend outlook video is found below in our media gallery or here.

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