Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Thu
Alta quota
4 · Forte
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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Non aggiornato — questo bollettino è scaduto 6 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

A foot of dense, heavy snowfall, rain, and strong winds are stressing the snowpack. Large, destructive avalanches are likely, especially where wind is drifting snow. Stay off of and out from below all steep wind-loaded terrain. Avalanches may be triggered remotely—from below, above, and to the sides of steep slopes.

Problemi valanghivi

  • Deep Persistent Slabs

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

    We just added a large, rapid load (a foot of dense snow and about an inch of SWE) to the snowpack—a big flashing red flag. This will stress the unpredictable layer of weak, faceted snow that was buried about 3 feet deep before this storm.  The summary of Chris' video (right/below):

    • The good news: Because the weak layer is so deep, it's getting more difficult to trigger avalanches involving this layer. 
    • The bad news: Because the weak layer is so deep, avalanches you trigger on this layer may be large enough to destroy vehicles and break large trees. 

    In recent days, the snowpack was providing some feedback:

    • On Saturday, riders in this zone felt a huge collapse that they likened to an earthquake
    • Last week, Chris and Ben experienced a collapse that sounded like a gunshot. This occurred after they finished their tests (unstable results) as Chris stepped out of the pit.
    • Even though many slopes in this zone already avalanched during the Winter Solstice storm, a storm last week still produced some noteworthy slides (example, example).

    The added snowpack depth from this storm will make evaluating stability that much more difficult. Snowpack stability tests are less trustworthy, and snowpack collapses will be less frequent. In many cases, the first obvious sign of instability can be triggering a monster avalanche. Avalanches failing on deeply buried weak layers are as likely to be triggered by the 3rd, 5th, or 10th rider on a slope as the first rider. We urge backcountry skiers and riders to continue to choose low angled slopes and to pay extra attention to the terrain adjacent to them. Remotely triggering an avalanche from below, above, and to the sides of steep slopes remains a very real possibility.

    Build some extra safety margin into your travel plans today. The Lake Ck avalanche (click here for video) that released Sunday or Monday near Ketchum was over a quarter-mile wide and should probably be viewed as foreshadowing, not as an outlier event. 

  • Wind Slabs

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

    Today's strong winds will form 1-3 foot thick drifts and slabs in exposed terrain. Some slabs will be touchy (sensitive to triggering) and could release naturally. The fresh wind slabs will be large enough to pose a real threat on their own. Also, wind slab avalanches could "step down" as they move downhill, failing on layers deep in the snowpack and becoming much wider and thicker than the initial avalanche. 

    With over a foot of new snow and strong winds forecast for most of today, the travel advice is simple: stay off of and out from beneath all steep wind-loaded slopes. 

Avalanche Discussion

SCALES OF SPATIAL VARIABILITY

Much of today's forecast is "painted red", showing HIGH danger. Does that mean every slope, cirque, or drainage in our 3200 square mile forecast area is going to behave the same? Heck no. On the broad scale associated with backcountry avalanche danger ratings, spatial variability causes avalanche problems and danger to change as you move from one location to another. Let's look at a couple of ways conditions may differ within and between zones:   

  1. Within the Soldier & WRV Mountains zone: We view Avalanche Warnings and HIGH danger as "sledgehammers": blunt tools designed to perform a simple task. We avoid nuanced messaging when big, stomping natural or triggered avalanches are crashing down in the mountains.  A potential pitfall exists with the sledgehammer approach when a forecast zone has areas that are obviously meeting HIGH danger criteria (upper elevations in the Soldier Mtns and the Smoky Mtns crest/Dollarhide Summit area west of Ketchum and Hailey) versus areas that are still dangerous but lack the "in your face" natural avalanche activity (the mountains within a few miles either side of the highway between Ketchum and Bellevue, and the foothills in the Little Wood River drainage). If someone put a gun to my head and told me I had to ski or ride an avalanche path in this zone right now, I'd choose one as close to Hailey, Bellevue, or Carey as possible because 1) it snowed less  2) avalanches will be smaller  3) avalanches may not be as easy to trigger. A big caveat: I’d rather be forced to walk across Main Street in Ketchum or Hailey while blindfolded than ski or ride steep, consequential avalanche terrain near Ketchum.  
  2. Avalanche problems/behavior between and within zones: In the Banner Summit area and northern Sawtooths, the weak layer of faceted snow that has been causing problems all season is now over 3 feet deep. Read the Banner Summit Deep Persistent Slab Problem Discussion for the good, the bad, and the ugly repercussions. Now, contrast that with the widespread natural and remotely triggered Persistent Slab activity over the past week throughout the Smoky Mountains.

We try to provide as much information as possible each morning, but it’s important to recognize that the Forecast products cover large areas; they are designed to serve as a starting point, and you get to make the ultimate decisions about how and where to play.

Regioni coperte