Bollettino valanghe

Swan Range

Flathead Avalanche Center
Vedi sulla mappa

Pericolo valanghe

Mon
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
Open full map
Non aggiornato — questo bollettino è scaduto 4 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Natural and human-triggered avalanches in wind-drifted snow occurred over the past several days. Today, you could initiate an avalanche in wind-affected surface snow or on a buried weak layer. Choose terrain sheltered from the wind or a lower-angle slope to minimize your chances of getting into trouble.

Problemi valanghivi

  • Wind Slabs

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

    Yesterday, observers noted wind transport, shooting cracks, and recent large wind slab avalanches. Friday, a rider accidentally triggered a Wind Slab avalanche in the Swan Range. Southwest winds have left several generations of slabs on leeward aspects. These features are most widespread in the Flathead Range and southern Glacier Park. Today, you may encounter slabs of wind-drifted snow reactive to a rider's weight near ridgelines, below saddles, or on the sidewalls of cross-loaded gullies. You will likely find these near any prominent terrain feature where the wind decelerates and deposits snow.

    Slabs will increase in thickness and become more dangerous as you gain elevation. Look for blowing snow as you travel. A scalloped surface or dune shape formation may identify these wind features. Older, harder slabs may feel and sound hollow like a drum. Slabs are generally denser than the surrounding snow and may keep you or your machine from sinking into the snow.

    Cracking in the snow surface beneath you is a sign directing you to seek out sheltered terrain for a safer, higher-quality experience.

    Windward terrain or areas sheltered from recent wind offer slopes without a wind slab problem. If you plan on riding steep leeward terrain, choose runouts free of surface obstacles which can magnify the consequences of a slide or terrain traps where the snow can pile up deep.  

    (2/11/2023)

    A shooting crack in a freshly formed wind slab. The blue arrow denotes the end of the crack.

    Photo: Dundas

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–3

    Friday, a motorized party triggered what we believed to be a Persistent Slab in the Swan Range. This slide fractured above a rider and was around 40 inches deep. Weak snow above and/or below a crust formed mid-January continues to be a concern. Depending on your location, the weak layer is either facets or surface hoar. A Level 1 motorized class had propagating results in 4 of 4 snowpits in the Swan Range Thursday. This layer is 1-2 feet below the surface in the Flathead Range and Southern Glacier Park and 2-4 feet below the surface in the Swan Range. This layer has been found in most aspects but is most developed on NW-NE-SE slopes.

    Unfortunately, the only way to identify this hazard is to dig down and see if this weak layer is present. Where you find it, choose lower-angle terrain free of overhead hazards and convexities. Small avalanches have the potential to step down to buried weak layers.   Slopes with clean runouts are an excellent way to build a margin of safety into your day.

    (2/11/2023)

    In a snowpit at 6900' on an east aspect, the mid-January crust is 1 cm thick and caps a thin layer of facets. The slab has been inverted to expose the underlying facets for this image.

    Photo: Dundas

Avalanche Discussion

And the beat goes on. Saturday was another blustery day in wind-exposed terrain. Our area has experienced windy conditions for most of the past couple of weeks, resulting in a wind slab problem that won't go away. Fortunately, Thursday saw quiet air allowing Two Bear Air to fly in replacement parts for the Tunnel Ridge weather station. More on that adventure below. 

Yesterday, observers in the "early" Middle Fork and Essex areas noted moderate to high winds drifting snow on exposed ridgelines and the higher peaks. Two large wind slab avalanches were spotted in steep, rocky terrain. Both propagated wide and ran far. Shooting cracks were experienced on wind-affected slopes. In the same neighborhood, I found up to 12 inches of loose, dry snow in sheltered locations that is faceting due to the dry weather. However, anything open and exposed to the wind was a different story. Windward slopes were scoured, and cross-loaded pitches had a wind crust that was occasionally tough to navigate. Cornices were stiff and difficult to trigger. Some areas held hard slabs that were hollow sounding and a bit unnerving. We also noted shooting cracks in freshly formed wind slabs and blowing snow.

Sunny slopes sheltered from the wind received solar warming and softened as the day progressed. Rollerballs and moist surface snow developed. Cool temperatures overnight should result in a crust at low and mid elevations this morning. We are unsure how high the crust extends, but the WMR (6730') station reached 32F for a couple of hours yesterday afternoon. 

 A Pacific cold front moves into the Flathead Monday and Tuesday with a decent shot of snow and wind. The snow will fall on various surfaces, including weak faceted surface snow, melt-freeze crusts, and stiff wind-affected snow. These surfaces may inhibit bonding with the new snow. There may not be enough snow to form a storm slab problem, but fresh wind slabs are a guarantee. We anticipate rising danger if the storm delivers as advertised.

Tunnel Ridge Weather Station: A huge thank you to Two Bear Air and the BNSF Avalanche team for their work to get this station up and running. The wind tower was destroyed in the fall during a strong wind event. The associated snow station survived in a lower, more sheltered spot but needed the wind station to transmit data. Two Bear Air flew in the replacement parts as a community service. We are so fortunate to have this resource in the Flathead Valley. Hooray for Two Bear! Both the wind and snow stations are online transmitting data. We hope to have this data available on the FAC weather table in the near future.

 

Regioni coperte