Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Sun
Alpin
3 · Marqué
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 5 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

While the major storm event has ended, conditions remain dangerous. Avalanches are most likely on slopes with wind drifting but could occur in wind-sheltered terrain. Triggering a large, catastrophic avalanche breaking very deep in the snowpack is unlikely but can't be ruled out. Safer conditions—and the best skiing and riding—can be found on gentler slopes sheltered from the wind.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    The past 5 days dropped 30-40" (!!) of new snow in this zone. Strong winds accompanied much of this snowfall. Yesterday saw another bout of strong, gusty W winds with the passage of a cold front that marked the end of this extended storm cycle. On Titus Ridge yesterday, I observed large quantities of snow being transported by the wind (watch the video above/right).

    Avalanche conditions are most dangerous in wind-exposed terrain. Wind slabs will be most common (and dangerous) at upper elevations, but this week's wind also drifted some open middle and lower elevation terrain (example). Triggering a wind slab is most likely near ridgelines, beneath cornices, and on the sides of cross-loaded gullies. Avoid slopes steep enough to slide where you see evidence of wind loading such as drifted, pillowed areas,  or feel denser/stiffer snow beneath your board(s) or sled.

    While wind-loaded slopes are most dangerous, there has been enough storm snow—as well as warming temps Thursday into Friday—that it may be possible to trigger an avalanche on very steep (>35 degrees), wind-sheltered or semi-sheltered slopes (avalanche photo). The new loading is also adding weight to deeply-buried persistent weak layers (see Problem #2). This weekend will be a good time to keep it mellow and let the dust settle from this recent storm cycle.

  • Deep Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    2–3

    A mix of facets and crusts leftover from October snowfall make up the base of the snowpack on most shady middle and upper elevation slopes. Some sunnier, upper elevation aspects have this layer too. The recent storm event just added a huge load to this weak layer, especially on wind-loaded slopes. This layer is now buried 6' deep or more throughout much of this zone. Once things calm down after this storm and visibility allows us to see the pattern of avalanche activity, we may begin to feel better about putting this layer to bed. But if this layer still has some life left in it, it's going to be the most reactive following this major loading event.

    Most savvy backcountry skiers and riders have chosen to avoid steep slopes where this old snow exists, and that strategy will remain a prudent one in the wake of this major loading event. This is not a tough strategy to swallow given the excellent riding and skiing conditions on slopes lacking the old October faceted snow.  

Avalanche Discussion

Pardon my enthusiasm, but the last month has been mind-blowing. Since December 11th, we've had three separate multi-day storm events that each dropped 2-4+" of SWE (snow water equivalent). Not to mention additional "dribs and drabs" between these storms. Back on December 10th, things were grim and our snowpack was hovering around 60-80% of average. Fast forward...in less than a month, we're now at 140-160% of normal. 

Accurately measuring snowfall is tough, especially during multi-day storm events. The only way to do it with any reliability is to have a live human checking a snow stake daily. Any volunteers? Remote weather station snow sensors measure both snow height gain (snowfall) and snow height loss (settlement). Both of these processes happen simultaneously during big storms—hence the inaccuracy. One thing we can do is look at the amount of SWE that's accumulated over a given timeframe, and using an average snow density, we can come up with an estimate of snowfall. For the calculations, I use an 8% average density which seems reasonable (and likely conservative) given the colder temperatures we've had over the past month. Here is the estimated snowfall since December 11th at a handful of stations:

  • Banner Summit: 150”
  • Vienna Mine: 180”
  • Galena Summit: 130”
  • Dollarhide Summit: 145”
  • Baldy: 100”
  • Hyndman: 85”
  • Soldier Ranger Station: 100”

I suspect that for a number of our stations, this was record-setting snowfall!

These storms have produced rapidly-changing conditions that can be difficult to track. As we head into the first period of extended high pressure since these storms began, it'll be beneficial to see what avalanches occurred where, fit this information into our theories of snowpack stability, and see where that leaves us. Stay tuned!

Be sure to catch up on the week's conditions in Chris' latest Weekend Update.

Régions couvertes