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Avalanche Forecast

Archived

Dec 21st, 2020–Dec 22nd, 2020

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
Alpine
Natural and human triggered avalanches likely.
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 possible, human triggered probable.

Regions

Purcells.

A significant winter storm will lambaste the region Monday night further destabilizing the smorgasbord of weak layers in our upper snowpack. Avoid all avalanche terrain at this time, large to very large natural avalanches are likely as the snowpack adjusts to this new load.

Confidence

Low - We are confident the likelihood of avalanche will increase with the arrival of the forecast weather. Uncertainty is due to extremely variable snowpack conditions reported through the region. Uncertainty is due to the limited number of field observations.

Weather Forecast

One last pulse tonight, and then it looks like we’re moving into a bit of a clearing trend this week.

MONDAY NIGHT: Freezing level around 1000 m, strong southwest switching to northwest wind, 10 to 35 cm of snow expected.

TUESDAY: Broken cloud cover, freezing level near valley bottom, moderate northwest wind, 1 to 5 cm of snow possible.

WEDNESDAY: Clear skies, freezing level at valley bottom, light variable wind, no new snow expected.

THURSDAY: Clear skies, freezing level at valley bottom with potential for a light alpine temperature inversion, light southwest to west wind with strong gusts at ridgetop, no new snow expected.

Avalanche Summary

Saturday night's storm resulted in quite a bit of activity. Avalanches to size 2 failed naturally and were sensitive to avalanche control measures Sunday. Reported avalanche activity primarily came from northeast, north and northwest facing features at and above treeline. This MIN highlights that activity nicely.

On Friday and Saturday the buried surface hoar continued to show it's stuff making for touchy avalanche conditions which produced slabs to size 1.5, check out this great MIN report from the north of the region Saturday. Natural wind slabs and cornice failures were reported on alpine features in the central portion of the region to size 1.5. 

In the south of the region the snowpack is thin and there have been reports of whumphing at ridgetop. Check out all of the MIN reports for the region here

Snowpack Summary

30 to 75 cm of storm snow has fallen recently with the deepest amounts being found in the north around Golden. Southwest winds have built slabs in lee features at upper elevations, reactive cornices have grown too. 

Below the storm snow, the snowpack is a bit of a junk show. The early December surface hoar is down 30 to 80 cm below the surface now. On Saturday, before the big storm, it was sensitive to remote triggering which is well illustrated in this MIN post from the Golden area.

Below, or near the surface hoar you may encounter a crust. This crust can be bound as high as 2400 m in the south of the region and 1800 m in the north.  

The lower snowpack is characterized by more crusts, the most notable is from a rain event in early November that is 50 to 130 cm deep. This crust is sitting near the base of the snowpack, surrounded by a weak layer of sugary facets. It is most likely to be reactive to human triggers or in a step-down from a smaller avalanche in shallow, rocky areas.

Terrain and Travel

  • Avoid all avalanche terrain during periods of heavy loading from new snow and wind.
  • Only the most simple non-avalanche terrain free of overhead hazard is appropriate at this time.
  • Uncertainty is best managed through conservative terrain choices at this time.
  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.

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.