Bollettino valanghe

Swan Range

Flathead Avalanche Center
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Pericolo valanghe

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In sintesi

Shifting winds have expanded the areas where you can trigger an avalanche in wind-affected snow. Choose terrain sheltered from the wind or lower-angle slopes for a safer, higher-quality experience.

Problemi valanghivi

  • Wind Slabs

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

    North and east winds arrived Saturday afternoon. Many shady aspects have abundant dry snow available for transport onto atypical aspects where fresh slabs and cornices will develop. New wind features may form on a sun crust formed yesterday or wind-board from recent southwest winds. Thursday, blustery southwest winds transported substantial snow at all elevations, creating fresh slabs and cornice growth reactive to the weight of riders. Winds and transport tapered a bit Friday, leaving behind a patchwork of soft and stiff features. Today, new and older wind-affected snow can be found around the compass. Look for these near ridgelines, saddles, and prominent terrain features at mid and upper elevations. At lower elevations, north and east winds are notorious for wreaking havoc and affecting the snow surface. 

    In terrain exposed to the strongest winds, slabs may be hard and supportable to you and the weight of your machine. On these slopes, the snowpack may not offer signs of instability until it is too late. These conditions are particularly concerning due to the ability of the slab to fracture above you and propagate wider than expected.

    Stiff, dense snow indicates the wind has been at work. A scalloped surface and dune-like formations are clues you are in wind-affected terrain. Shooting cracks and collapsing means instability directing you to a more sheltered slope or lower-angle terrain.  

     

    (3/2/2023)

    A shooting crack in a wind slab immediately below a saddle.

    Photo: md

Avalanche Discussion

Yesterday afternoon, an arctic air mass moved west over the Continental Divide, resulting in cooling temperatures and north-through-east winds. Dry snow is abundant in sheltered locations and available for transport. Winds from the northeast affect vast areas, such as the front side of WMR and the Swan front, while leaving many narrow drainages in the Middle Fork corridor and elsewhere virtually unscathed. They often intensely affect the valley floor as the cold, dense air sinks and spills out to the west. 

Yesterday's observations noted up to 12 inches of glorious dry, loose snow in areas sheltered from recent southwest winds. Blue sky and champagne powder made one skier think they were in Colorado. The intense March sun warmed southerly aspects throughout the day and affected the snow quality. We expect to find a thin sun crust this morning on these slopes. I traveled to the Cascadilla drainage in the Middle Fork to look closely at a very large hard slab avalanche from Thursday's intense wind event. A wind slab just below the ridgeline triggered this D3 slide. As the debris moved downhill, it initiated an avalanche on a deeply buried weak layer. The crown width is approximately 1500 feet with a depth of 5+ feet. This is the first slide reported on a buried weak layer since the February 20 and 21 Atmospheric River event. The debris came to rest in a significant depression, where it initiated another deep avalanche. 

We have dropped the Persistent Slab problem from the Swan Range, where we have not observed slides on the buried mid-January weak layers for the past 11 or 12 days. In most locations, the weak layer is buried well over 3 feet. This is at a depth where the weight of a rider or their machine is less likely to affect. Also, a strengthening process occurs as the overlying weight compresses this weak layer. This problem remains in the Flathead Range, where the likelihood of triggering remains POSSIBLE, and in the Whitefish Range, where the possibility is UNLIKELY.

FAC would like to thank Ted Steiner and Adam Clark from the BNSF snow safety team for help in replacing the temperature and relative humidity sensor at the Aeneas Ridge weather station. This sensor has been out of service all season, and FAC staff are delighted to have this data to help us forecast this spring's freeze/thaw and wet snow problem in the Swan Range. They also fixed issues with the wind anemometer. 

Avalanche Encyclopedia: The National Avalanche Center has issued a new and updated encyclopedia containing 257 avalanche-related terms, a new graphic design, a new hierarchical term structure and navigation, and a new user interface. The US Forest Service funded the project. The writing and content were produced by Zach Guy, former Director of the FAC. You can find it here.

State of the Snowpack and Fireside Chats. Join us at 6 pm on March 9 for a virtual evening combining two popular community ed workshops. New this season, State of the Snowpack is a presentation from FAC on recent weather and avalanches to help you digest the forecast and plan your weekend. Fireside Chats are an intimate, conversational forum for open dialogue about human and avalanche factors. With skier/psychologist Dr. Sara Boilen as our facilitator, we dig into the tough decisions, tricky snow, and lessons learned from local near misses. No registration is needed; click here for a Zoom invite.

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