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Avalanche Forecast

Archived

Dec 27th, 2019–Dec 30th, 2019

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

Regions

North Rockies.

Watch for fresh slab formation this weekend, especially in wind exposed terrain. If there's more than 30 cm of new snow in your location the danger may well be high. Central & northern areas have weak layers 40 to 70 cm below the surface which remain a concern for triggering. 

Confidence

Low - Uncertainty is due to the complexity of the snowpack’s structure. Uncertainty is due to the timing, track, & intensity of the incoming weather system. Uncertainty is due to field data and reports showing a wide variation in conditions throughout the region.

Weather Forecast

Saturday looks to be a storm day and then the weather is a bit quieter Sunday & Monday. The latest model runs are showing a pretty juicy storm system for the region Monday night into Tuesday, stay tuned for more details.

FRIDAY NIGHT: Freezing level at valley bottom, strong west/northwest wind switching to strong southwest by dawn, trace of snow possible.

SATURDAY: Overcast, freezing level between 500 and 900 m, strong southwest wind, 1 to 10 cm of snow expected.

SUNDAY: A few clouds, freezing level at valley bottom, light southwest wind, no significant precipitation expected.

MONDAY: A few clouds at dawn building to overcast by sundown, freezing level at valley bottom, strong southwest wind, no significant precipitation expected during the day, potential for 5 to 20 cm Monday night.

Avalanche Summary

Last week's storm resulted in a natural avalanche cycle in Pine Pass and northern areas that released either at the bottom of the storm snow or in a deeper weak layer. In the south and west areas there were fewer reports of avalanches and they were small.

Looking forward, the weather forecast suggests strong wind events with some new snow; even without additional snow there's enough on the ground available for transport so I expect natural wind slab activity on lee slopes with it primed for human triggering.

Snowpack Summary

On Friday 15 to 30 cm of new snow fell across the region, this snow likely comes to rest on surface hoar in many locations.

Over the last week, northern and western areas of the region received 30 to 50 cm of new snow, eastern and southern regions received much less with anywhere from a trace to around 25 cm. That means deeper snowpack areas have around 250 cm on the ground near treeline, thinner areas have around 100 cm.

Layers of concern vary through the region too. Areas around Pine Pass and Tumbler have, within the top 40 to 80 cm, surface hoar and facet/crust layers formed in late November that are reactive in snowpack tests. We think they exist around treeline elevations throughout the region, and can likely be found in alpine terrain as well.

The lower snowpack contains several crust layers, which could be a concern in shallow areas along the eastern slopes of the region. Having said that, we have little to no information from places like Kakwa.

In the west (e.g. Torpy) and south (McBride) where the snowpack is deep it is also stronger and missing some of the weak layers.

Throughout the region a new layer of surface hoar is being buried -- here's a pic from our NoRo field team.

Terrain and Travel

  • Sheltered slopes at lower elevations will offer the best riding.
  • Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
  • Back off if you encounter whumpfing, hollow sounds, or shooting cracks.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Caution around slopes that are exposed to cornices overhead.

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.