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Avalanche Forecast

Archived

Mar 20th, 2025–Mar 21st, 2025

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Purcells, East Purcell, West Purcell.

Forecast snow and wind are expected to form fresh storm slabs reactive to human triggers.

Watch for rapidly changing conditions during periods of heavy loading from new snow and wind.

Confidence

Moderate

Avalanche Summary

On Wednesday, numerous explosive triggered persistent slab avalanches up to size 3 on primarily northerly and easterly aspects at treeline and above.

On Tuesday, several explosive triggered deep persistent slab avalanches up to size 3.5 were reported on north and west aspects in the alpine. The weak layer was around 150 cm down.

Snowpack Summary

Forecast snow and wind are expected to form fresh storm slabs reactive to human triggers.

30 - 50 cm of snow overlies a crust on sunny slopes and lower elevations, and surface hoar or facets on sheltered and shady upper elevations.

Several persistent weak layers consisting of a crust, facets, and/or surface hoar from February and January remain a concern. They are down 50 to 100 cm in most areas.

The bottom of the snowpack is composed of large facets in most areas. There is evidence that this layer is becoming reactive again, and some very large avalanches have failed on this layer recently.

Weather Summary

Thursday night

Cloudy with 5 to 15 cm of snow. 10 to 20 km/h south ridgetop winds. Treeline temperature -7 °C. Freezing level valley bottom.

Friday

Cloudy with 10 to 25 cm of snow. 10 to 20 km/h southwest ridgetop winds. Treeline temperature -4 °C. Freezing level 1600 m.

Saturday

Mix of sun and cloud. 10 to 20 km/h west winds. Treeline temperature -5 °C. Freezing level 1500 m.

Sunday

Mostly cloudy. 10 to 20 km/h southwest ridgetop winds. Treeline temperature -1 °C. Freezing level 1900 m.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Watch for signs of instability like whumpfing, hollow sounds, shooting cracks, or recent avalanches.
  • Watch for rapidly changing conditions during periods of heavy loading from new snow, wind, or rain.
  • Use small, low consequence slopes to test the bond of the new snow.
  • Avoid steep, rocky, and wind-affected areas where triggering slabs is more likely.
  • Be aware of the potential for large, destructive avalanches due to deeply buried weak layers.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.