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Avalanche Forecast

Archived

Dec 24th, 2019–Dec 25th, 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 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

North Columbia.

Considerable avalanche hazard means that human triggered avalanches are likely, and this is exactly the situation right now. Very large recent avalanches have had crowns up to 200 cm in depth. Anything besides non avalanche terrain, or simple terrain is out of the question.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

We’re entering a rather quiet weather period with coolish temps and no significant snowfall expected, at least through boxing day. The region should see winds pick up later in the week and there is potential for a small shot of snow Thursday night, and then another one Saturday Night.

CHRISTMAS EVE: Freezing level at valley bottom, light variable wind, no significant precipitation expected.

CHRISTMAS: Scattered cloud cover with clearing expected after lunch, freezing level at valley bottom, light northwest wind, no significant precipitation expected.

BOXING DAY: A few clouds in the morning, cloud cover building to broken by sundown, freezing level at valley bottom, moderate to strong south/southwest wind, trace of snow possible during the day, potential for 5 to 10 cm by Friday morning.

FRIDAY: Broken cloud cover, freezing level at valley bottom, moderate to strong west/southwest wind, trace of snow possible during the day.

Avalanche Summary

The recent storm was a significant one, and there is a great deal of uncertainty at this time as to how quickly the snowpack will gain strength. Avalanche occurrences are tapering, but recent avalanches have been very large. Patience and conservative terrain selection is the best way to manage this uncertainty.

On Monday natural avalanche activity from size 2 to 3 was observed on a variety of aspects above 2100 m. Control work produced avalanches from size 2 to 4 that involved all elevation bands, some mature timber was taken out by very large avalanches.

On Sunday, there were dozens of reports of natural, human and explosives triggered avalanches up to size 4. These were a mix of storm slab and persistent slab avalanches.

On Thursday, Friday and Saturday, there were numerous reports of natural, human and explosives triggered avalanches up to size 3.5. A number of these were triggered remotely.

Snowpack Summary

The North Columbias received upwards of 80 cm between Thursday night and Saturday night. Storm slabs are likely widespread and may still be reactive to human triggers.

There is anywhere from 100-160 cm of snow on top of a widespread layer of large, feathery surface hoar crystals. This layer has reportedly been responsible for numerous recent avalanches and may be reactive to human triggers.

A weak layer that formed in late November is now over 170 cm deep. This is the layer of concern relating to the persistent slab avalanche problem. The weak layer may present as surface hoar, a crust, facets or a combination, depending on elevation and aspect

Terrain and Travel

  • Make conservative terrain choices and avoid overhead hazard.
  • Don't be too cavalier with decision making, storm slabs may remain sensitive to human triggering.
  • 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.