Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Tue
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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In sintesi

Triggering an avalanche large enough to bury a person would require a dose of bad luck, but remains a possibility today. Weak layers of snow buried 3 feet deep have proven to be less reactive here than in our other zones. You'll be most likely to encounter this problem in terrain where the wind has stiffened the snow surface and added weight. With warming temperatures and rain today, wet snow avalanches will be possible, primarily at lower elevations and on slopes that face the sun, if the clouds clear earlier than forecast.

Problemi valanghivi

  • Persistent Slabs

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

    Weak layers of snow buried 2-3 feet deep have proven to be less reactive in this zone than in neighboring areas. On Friday, Chris went looking for this problem north of the highway. He found it in some of his snowpits but not in others. On an east-facing slope he found a thin layer of facets on top of a crust, all buried beneath a 3-foot thick slab. The block did not fail during standard loading tests in his snowpack tests, but he was able to get it to break cleanly by applying some extra force.

    Dealing with persistent weak layers is tricky business, particularly as they become more deeply buried. Snowpack test results become more difficult to interpret and obvious signs of instability are absent. The first clear sign of an unstable snowpack may occur when you trigger a large avalanche. These types of slides can be triggered remotely—from a distance above, below, or to the side of a steeper slope.

  • Loose Wet

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

    Above-freezing temperatures at lower elevations and rain falling below 7,000' are wetting the snowpack. If the sun comes out for longer than expected today it will also help to quickly melt the already warm snow. Keep an eye on the condition of the snow surface. If it is wet or if you are seeing wet balls of snow forming and rolling downhill it is time to think about looking for a slope where the snow surface is colder and drier. This problem is most likely on lower elevation slopes (regardless of the direction they face) and on middle and upper elevation slopes that directly face the sun.

Avalanche Discussion

Our snowpack is getting trickier to deal with by the day. Persistent weak layers that formed in March are buried in the upper few feet of the snowpack. These layers produced human-triggered avalanches six out of the past nine days. Many of the avalanches that failed during the widespread natural avalanche cycle (and earthquake) last week stepped down to deeper weak layers. For days we've been warning you about a snowpack that is behaving much more like a mid-winter snowpack than one that you typically find in April (read yesterday's Forecast Discussion for more details on this).  Today and in the coming days that picture will become even more complicated. 

Snowpacks do not respond well to rapid change. Last week we saw how strong winds and heavy snowfall can result in widespread natural avalanche activity (and that earthquake didn't hurt). Today and in the coming days, we will "get" to watch as our unique snowpack begins its annual springtime transition. Temperatures are expected for be 5-10 degrees F above seasonal averages, likely hitting the warmest sustained temperatures we've seen all season. Today, we will couple those warm temperatures with a dose of rain. Sound complex and a bit gross? Good.

What impact will this rapid warming and warm precipitation have on the snowpack? It depends on what elevation band you are traveling in. At lower elevations, where rain is falling on an unfrozen snowpack, we will see the increasing possibility of loose, wet snow avalanches. At middle and upper elevations the warm temperatures and heavy wet snowfall are quickly helping the snow that fell last week consolidate into a hefty slab. Slabs like this do a better job of "communicating" a fracture, which may help to keep the persistent slab problem on the menu for some time. At the transition, where a dry snowpack is becoming wet for the first time we may begin to see a wet slab problem developing as liquid water percolates down to and destabilizes weak layers in the snowpack. Yuck.

As backcountry travelers, some of the best tools we have in our toolkit are our powers of observation and pattern recognition. We can look at the snow surface and tell if the wind has been at work. We compare these observations to those of other wind slabs we have dealt with in the past and make a prediction about how a slab will behave based on that. However, if we are looking at a pattern that we don't recognize or don't have much experience with, this tool becomes useless. If you go into the backcountry today there is a pretty good chance that you will be dealing with a snowpack that you don't recognize. Bear this in mind as you travel through the mountains and build in an appropriate safety margin to help you manage that.  

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