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Avalanche Forecast

Archived

Mar 13th, 2023–Mar 14th, 2023

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
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, Dogtooth, East Purcell, West Purcell.

Storm snow blankets the region and has created reactive storm slabs at all elevations. If triggered storm slab avalanches could step down to weak layers deep in the snowpack creating large destructive avalanches. Careful snowpack evaluation, cautious route-finding and conservative decision-making are essential for a safe day out in the mountains.

Confidence

Moderate

Avalanche Summary

On Monday, poor visibility made for limited observations in the alpine. Several natural loose dry and storm slab avalanches were reported on steep terrain. Ski cutting produced numerous size 1.5 storm slab avalanches in open terrain.

On Saturday, a natural size 2.5 was observed at 2400 m on a northwest aspect. The avalanche was +100 cm deep and occurred on one of the persistent buried layers within the snowpack.

If you head out in the backcountry, let us know what you are seeing by submitting a report to the Mountain Information Network.

Snowpack Summary

10-25 cm of storm snow overlies a plethora of surfaces including hard, wind-affected surfaces in open areas, surface hoar and facetted snow in sheltered areas and a sun crust on steep solar slopes.

Buried surface hoar sits 60-80 cm deep in sheltered terrain features, and a thin sun crust exists at the same depth on steep south-facing terrain. Several other layers from January can also be found in the top 100cm of the snowpack.

The lower snowpack is made up of a widespread layer of large, weak basal facets and depth hoar in some areas. this weakness has been responsible for a number of recent very large, destructive avalanches and will continue to be a concern.

Weather Summary

Monday Night

Cloudy with mixed precipitation, 2-6 cm of new snow at higher elevations. The Western Purcells will see an additional 15 cm. Alpine temperatures reach a low of -6 °C. Ridge wind southwest 20 km/h gusting 40 km/h. Freezing levels lower from 1700 meters to valley bottom through the night.

Tuesday

Mainly cloudy with sunny periods. Alpine temperatures reach a high of -4 °C. Ridge wind southwest 15 km/h gusting 30 km/h. Freezing levels rise to 1200 meters.

Wednesday

A mix of sun and cloud with isolated flurries. Alpine temperatures reach a high of -3 °C. Ridge wind northwest 20 km/h. Freezing levels rise to 1300 meters.

Thursday

Mainly sunny. Alpine temperatures reach a high of -2 °C. Ridge wind northwest 20 km/h. Freezing levels rise to 1400 meters.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Be especially cautious as you transition into wind affected terrain.
  • Be aware of the potential for larger than expected storm slabs due to the presence of buried surface hoar.
  • Avoid areas with a thin or variable snowpack.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep 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.

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.

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.