Lawinenlagebericht

Banner Summit

Sawtooth Avalanche Center
Auf der Karte ansehen

Lawinengefahr

Fri
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
2 · Mässig
Open full map
Veraltet — dieser Lagebericht ist 5 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Triggering a large, 1-4 foot thick avalanche is possible. These slides are most likely in very steep, rocky areas where the snowpack is thinner—less than about 4 feet thick. Intense sunshine and warm temperatures may cause small, loose snow avalanches on slopes that directly face the sun. These slides could "step down", triggering larger avalanches as they move downhill. 

Lawinenprobleme

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2–3

    Persistent slab avalanches remain at the top of our problem list. Weak layers of facets and/or crusts, and occasionally surface hoar exist in the top 4' of the snowpack. These layers produced several avalanches over the past two weeks (Dollarhide, Galena Summit, Smiley Creek, Beaver Creek, Sawtooths, Banner Summit). They seem most reactive on sunny slopes where facets are associated with crusts (video, photo), but large avalanches have also occurred on shady aspects in the Sawtooths.

    Although obvious signs of instability are generally decreasing, Monday's reports of large snowpack collapses and poor snowpack test scores are a reminder that triggering a large and dangerous avalanche remains possible. This has motivated SAC forecasters and local guides to continue to make conservative terrain choices. 

    Triggering a persistent slab avalanche is most likely:

    • On steep (>35 degrees) middle and upper elevation slopes that received the most snowfall and wind-transported snow during last weekend's storm.
    • In shallow areas where the snowpack is thinner than about 4' thick. These spots can be found near windswept ridges and on the edges of wind-loaded slopes. They are also common in craggy terrain where shallowly buried rocks bring weak layers closer to the snow surface. 

    Don't let the spring-like weather lull you into complacency. These weak layers don't simply heal overnight. It's a gradual process, and we're not out of the woods yet. 

    ADDITIONAL DISCUSSION:More wet loose avalanches were observed in the Boulders, Smokys, and southern White Clouds yesterday and similar slides are possible again today. So far the majority of the natural wet loose activity has been relatively small. However, small avalanches can spell bad news if they sweep you off your feet or push you into a terrain trap like a stand of trees or bury you in a gully. Fortunately, this avalanche problem is fairly predictable. 

    You're most likely to find this problem on very steep, rocky slopes that directly face the sun and are sheltered from the wind. This type of instability follows the sun around the compass, beginning on E-SE aspects in the morning, moving to SE-S facing terrain near noon, and finishing up on SW-W aspects later in the afternoon.

    Direct sun and warm temperatures will also weaken the large cornices that have formed throughout the season, which may sag and break during the heat of the day. Cornice breaks, as well as wet loose avalanches, can trigger larger slab avalanches as they travel downhill. We saw evidence of this earlier this week in the Boulder Mountains (observation here, photo). 

    (2/26/2021) Small natural avalanche observed on Banner Summit on Friday. This slide likely ran on a weak layer at the old snow surface.

Avalanche Discussion

SPRING WEATHER, WINTER SNOWPACK

Warm weather and climbing sun angles have many of us thinking about spring. But, while the weather is "spring-like" and the snow surface on steep, sunny slopes is softening, it's still very much winter deeper in the snowpack. 

Daytime high temperatures have increased steadily in the last few days and will creep into the low-40s F again this afternoon. Couple that with direct sun and light wind and you could be easily fooled into thinking it was late March. The snow surface has reacted as expected, releasing wet loose avalanches on very steep, rocky, sun-drenched slopes. This activity will continue today, but so far these slides have been confined to the upper several inches of the snowpack, a sign that the melting process is still fairly shallow. 

This warmup is just the first in a series of warming events that we will see this spring. When people imagine a "spring snowpack", they're generally thinking of a snowpack that's seen several melting and re-freezing events that help to transform the dry, weak, faceted layers that plague much of our area into a denser, stiffer, stronger and more uniform snowpack.

This transition happens in different mountain ranges, aspects, and elevations at different rates. Generally, we will see this transition sooner in the low elevation terrain of our southern mountains, and it will take a bit longer at cooler, middle, and upper elevations as you head north. Because this process takes time, and the transition varies by elevation and mountain range it can be hard to track. What we can say with certainty, is that we're nowhere near that point yet. The weak layers in our snowpack remain mostly dry and are far from out of the picture.

It is good, however, to think ahead to the inevitable warmups and what that may mean for stability. As spring marches on, we need to anticipate melt-water moving deeper into the snowpack. Stability can change rapidly when this water hits buried weak layers. It doesn't seem likely that we'll get there in any widespread fashion before cooler weather returns next week, but the timing of these events is notoriously hard to predict.

Abgedeckte Regionen