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Avalanche Forecast

Archived

Feb 28th, 2020–Feb 29th, 2020

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

Regions

South Rockies.

The warm temperatures are a good thing for our snowpack long term, but warm temps Friday night into Saturday may allow for human triggerable avalanches Saturday, especially in more extreme terrain. Cornices are expected to remain fragile Saturday too.

Confidence

Moderate - Uncertainty is due to rapidly fluctuating freezing levels. Uncertainty is due to the limited number of field observations.

Weather Forecast

Warmer funky weather is with us for the forecast period.

FRIDAY NIGHT: Freezing level around 2200 m in the early evening lowering to about 1500 m by dawn, moderate to strong west/southwest wind, 2 to 6 mm of rain expected at lower elevations, wet heavy snow near ridge top,

SATURDAY: Broken cloud cover, freezing level holding around 1500 m, moderate to strong west/southwest wind, 1 to 6 cm of snow expected up high, rain down low.

SUNDAY: Broken cloud cover at dawn clearing to just a few clouds in the late afternoon, freezing level beginning at valley bottom rising to 1500 m, light west/southwest wind, no precipitation expected.

MONDAY: A few clouds in the morning building to broken cloud cover in the afternoon, freezing level beginning at valley bottom rising to about 1800 m, moderate to strong west/northwest wind, no significant precipitation expected.

Avalanche Summary

One of our field team members got out for a flight Friday, and she found a slab avalanche on a steep east/southeast facing alpine feature, but aside from that and a bit of loose wet activity on solar aspects, there has been little reported recent avalanche activity.

On Thursday a few small cornice falls were observed near Elkford, one of which triggered a size 2 slab on the slope below. A few small (size 1-1.5) slab avalanches were triggered by skiers on Tuesday and Wednesday, including a small storm slab that may have failed on surface hoar. 

The most recent deep persistent slab activity we know about is described in this MIN report from February 11 and this MIN report on February 9. 

Snowpack Summary

The 10 to 35 cm of snow from Sunday/Monday was absolutely hammered by wind Wednesday as evidenced in this MIN submission. Wind was strong enough to get into below treeline features even. Then it got quite warm Friday, the freezing level got to at least 2300 m. The beat up old snow is now quite warm. It sits on a variety of old surfaces including a firm crust on solar aspects, soft faceted snow, possibly surface hoar in sheltered areas, and old wind slabs near ridge crest, once it cools off it is unlikely to be much of a problem anywhere.

A thick rain crust sits 30-60 cm below the surface and can be found up to 2100 m. Avalanche activity was last observed at the faceting interface between this crust and overlying snow on February 17. 

The mid-pack is well settled and strong, but the base of the snowpack contains basal facets that are most prominent in shallow rocky start zones.

Terrain and Travel

  • Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.