Avalanche Forecast
Issued: Feb 23rd, 2020 5:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Deep Persistent Slabs.
ghelgeson,
Avalanche Canada
10 to 60 cm of storm snow rests on a layer of buried surface hoar. While the surface hoar is present at all elevations, it's been particularly problematic at lower elevations at and below treeline. Stick to simple, mellow, well-supported terrain and avoid convexities.
Summary
Confidence
Weather Forecast
The forecast period is marked by small amounts of snow and rising mid-week temperatures.
SUNDAY NIGHT: Freezing level at valley bottom, light variable wind, 1 to 3 cm of snow possible
MONDAY: Clear skies at dawn building to scattered cloud cover by sundown, freezing level at valley bottom, light variable wind, no significant precipitation expected.
TUESDAY: Broken cloud cover, freezing at valley bottom, moderate to strong southwest wind, trace of snow possible.
WEDNESDAY: Broken cloud cover, freezing level rising to about 1500 m, strong southwest wind, trace of precipitation possible.
Avalanche Summary
On Saturday storm slabs resting on surface hoar failed naturally and were sensitive to skier triggering producing slabs up to 60 cm in depth. These slabs seem to be more sensitive at and below treeline which is the reverse of how we normally think about avalanche hazard.
A few natural wind slab avalanches up to size 1.5 were observed on north to east aspects on Friday.
Some very large and destructive avalanches have been sporadically running on deeply buried weak layers near the bottom of the snowpack, predominantly on north through east aspects in the alpine. Last Tuesday, a natural size 2.5 was observed. It is suspected to have started as a wind slab in a thin rocky area before stepping down. Last Monday, two deep persistent slab avalanches size 2-3 were triggered from thin snowpack areas by very large loads pushed by snowcats.
Snowpack Summary
10-15 cm of recent snow has been redistributed by strong westerly winds in exposed alpine and treeline areas. In sheltered areas, the new snow rests on a layer of surface hoar, and on a sun crust on solar aspects.
An older, patchy layer of small surface hoar crystals and a sun crust both from early February may still exist under previous storm snow, 30-60 cm deep.
A couple of weak layers that formed in January are buried in close proximity to one another 80-140 cm below the surface and continue to give hard results in snowpack tests. Below, an early season crust lurks at the base of the snowpack. Some large avalanches have been triggered on these layers, usually during periods of rapid loading by new snow or wind and by large loads or from thin, rocky areas.
Terrain and Travel
- Watch for newly formed and reactive wind slabs 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.
- Storm slabs in motion may step down to deeply buried weak layers resulting in very large avalanches.
- The trees are not the safe-haven they normally are at this time. Terrain at treeline is primed for human triggered avalanches.
Problems
Storm Slabs
10 to as much as 60 cm of storm snow rests on a widespread layer of buried surface hoar. These slabs have been most sensitive to triggering at lower elevations (at and below treeline) which is a setup that can catch us off guard. Wind fueled storm slabs can be found in the alpine and may be most sensitive on solar aspects where they rest on a sun crust.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Deep Persistent Slabs
Some very large and destructive avalanches have been sporadically failing on deeply buried weak layers, predominantly on north through east aspects in the alpine. The bulk of events occur during periods of rapid loading by new snow or wind, or are triggered by very large loads (wind slab avalanche, cornice or snowcat).
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Feb 24th, 2020 5:00PM
