Boletín de aludes

Northwest Mountains

Crested Butte Avalanche Center
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Peligro de aludes

Tue
Alta montaña
2 · Moderado
Límite del bosque
3 · Notable
Bajo el límite
3 · Notable
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Desactualizado: este boletín caducó 4 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

The heatwave has turned our snowpack into a hot, wet, and dangerous mess.  A handful of large wet avalanches ran naturally yesterday, such as this destructive cement mixer that came close to Gothic Road.   Another poor overnight refreeze and warm temperatures again today will keep the threat of large natural and triggered wet avalanches going as the day progresses.  Start and finish your tours early and move to mellow terrain before thin surface crusts thaw into wet mush.  There is still a lingering chance of triggering a large, dry slab avalanche on northerly aspects as well.  

Problemas de aludes

  • Loose Wet

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Likely
    Tamaño
    1–2

    Wet loose activity has been increasing in size and expanding to more terrain during our warmup.   Warm temperatures and sunshine will cause the upper snowpack to lose cohesion throughout the day, making natural and triggered wet loose avalanches likely in steep terrain.  Rollerballs, pinwheels, or ankle-deep slushy snow are warning signs that steep slopes are unstable.  Move to a colder aspect or lower angle terrain to avoid this problem once it develops.  Wet loose avalanches are growing to dangerous sizes that are large enough to easily injure or bury you in some terrain.  With a poor overnight freeze, expect wet loose activity to begin early in the day on northeast and east aspects before moving south and then west as the day progresses.  

    3/26:  A large wet loose avalanche below Gibson Ridge.

  • Wet Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1.5–2.5

    Consecutive days of intense meltwater production and several nights with poor refreezes are causing natural and triggered wet slab concerns.  Several large natural wet slabs released yesterday, caused by meltwater weakening buried weak layers.   Our most concerning aspects are where our mid-winter weak layers are transitioning from dry to wet: east to northeast and west to northwest aspects.  There is also lingering potential for wet slabs on higher elevation southerly aspects.    Avoid travel on steep terrain and in runouts once the snowpack becomes unsupportive and trenchy.  Use extra caution below sunbaked cirques or rock bands, which tend to exacerbate meltwater production.

    3/27:  This destructive wet avalanche on Gothic Peak started as a wet loose and then propagated as a narrow wet slab as it ran further downslope.  

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Unlikely
    Tamaño
    1.5–2.5

    On the northerly aspects, there is a rare but dangerous chance you can trigger a hard slab avalanche that breaks up to several feet thick on weak faceted snow that was buried last month. Slabs can be triggered remotely from below or on adjacent slopes.  Given the thickness and hardness of slabs, you are unlikely to get direct feedback, such as collapsing or cracking, until it's too late.  In the Northwest Mountains, slopes holding a shallower snowpack (less than about 2.5 meters deep) are most suspect.  This could be in steep, rocky terrain or in terrain that has variable depths due to wind erosion.  Large cornice falls or wet loose avalanches caused by warming temperatures today could also act as natural triggers for this stubborn problem.   When in doubt, head for lower consequence terrain or slope angles in the lower 30-degree range and less. 

    This large persistent slab avalanche was remotely triggered on 3/18. 

Avalanche Discussion

This video that we made on Saturday helps illustrate why we have dangerous conditions in the backcountry.   Yesterday, we observed a handful of large natural wet avalanches and dozens of smaller ones.  The focus of natural activity, for both wet slabs and wet loose, was on the eastern quadrant of the compass.  In general, the snowpack in that terrain is transitioning from dry to wet for the first time, which makes it more sensitive to the effects of meltwater.  The same dry to wet transition is happening on west to northwest aspects, but activity has been a bit quieter, likely due to some combination of previous wind effects and observation biases.  South to southwest aspects have matured through more melt cycles this spring and are a bit more resilient to the ongoing flood of meltwater.    The snowpack is most saturated and has been producing more dangerous wet avalanches near and below treeline, where temperatures have been warmest and where the snowpack isn't as hardened by previous wind events.  These are somewhat minor details in the big picture, which is that our snowpack is a hot and wet mess capable of producing large and dangerous debris piles.  Another poor refreeze last night means that there is minimal recovery from meltwater continuing to saturate and weaken our snowpack.    Slightly cooler temperatures and some afternoon cloud cover may help reduce natural activity on the sunnier aspects, while the greenhouse effect might expand surface melt and spur more activity onto northerly aspects that have remained dry.  We're shifting the distribution of wet slabs to try and match the pace of the wetting front moving into dry weak layers, which is where wet slabs are most likely.  Yesterday's three wet slabs were on easterly terrain features between 10,500 and 12,000 ft (Redwell, Gothic 1, Gothic 2).  The largest ran an impressive distance and came within 1000 ft of crossing Gothic Road.  

With our primary concerns shifting towards wet avalanches, it's easy to forget that northerly aspects are still capable of producing large dry or mostly dry slab avalanches on a few slopes.  Just a few days ago, a skier had a close call with a deadly persistent slab just north of our forecast area in Express Creek, and yesterday, a skier in the same area triggered a large collapse and shooting cracks on a high northerly slope.  This destructive avalanche in the San Juan Mountains on Saturday appears to be a dry persistent slab that was triggered by a wet loose avalanche.  Similar concerns exist in our forecast area, especially where the snowpack is shallower.    

Tomorrow looks like a challenging day with storm instabilities developing over a saturated snowpack.  Cloud cover and arriving snowfall will insulate the snowpack from a refreeze.  We'll see a mix of newly developing dry avalanche concerns overlapping with lingering wet concerns, which isn't something that we commonly see here. Stay tuned.  

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