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Avalanche Forecast

Archived

Feb 19th, 2020–Feb 21st, 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 possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

North Rockies.

Conditions have really improved & it's largely about two things now: avoiding wind slabs at upper elevations and staying away from cornices which continue to fail naturally in somewhat spectacular form.

Confidence

Moderate - Uncertainty is due to the extreme variability of wind effect on the snowpack.

Weather Forecast

The later half of the week features plenty of periods of sun along with potential for some pretty intense wind.

WEDNESDAY NIGHT: Strong southwest wind, freezing level at valley bottom, no significant precipitation expected.

THURSDAY: Scattered cloud cover, strong west/southwest wind, freezing level rising to about 1200 m during the day, no significant precipitation expected.

FRIDAY: Broken cloud cover, strong southwest wind, freezing level rising to about 1400 m, no significant precipitation expected. Potential for 5 to 10 cm Friday night.

SATURDAY: Broken cloud cover at dawn, clearing throughout the day, moderate to strong west/northwest wind, freezing level near valley bottom, trace of snow possible.

Avalanche Summary

Given that there has been 50 to 100 cm of storm snow in the last week with quite a bit of wind, surprisingly little slab avalanche activity has been reported. Cornices are a different story, there have been quite a few reports of size 2.5 cornice releases from the last few days, a trend that is expected to continue through the forecast period with the sun making a rather prominent appearance.

Snowpack Summary

Dribs and drabs of snow from the last few days add to last week's phenomenal storm that produced 50 (Kakwa) to 100 (Torpy) cm of storm snow under mild temperatures and moderate wind from the south, southwest and southeast. Most of the observations we're seeing suggest that the copious amount of storm snow arrived warm and it continues to settle out nicely. The storm snow still has has very little slab property, but we're short on observations from more wind effected terrain.  

The presence of buried surface hoar about 40 to 60 cm below the surface has been confirmed in the Pine Pass, Torpy & Renshaw zones, it may be more widespread throughout the region too. If you're seeing it in your travels please let us know about it!

Below all the recent storm snow there is a thin rain crust from the warm, wet storm on Feb 01. This rain crust has been reported to exist up to treeline elevations around 1700 m. No recent activity involving this crust has been reported.

Deep in the mid-pack there may still be a layer of surface hoar buried in late December. It's gaining strength, but should remain a concern since snowpack tests still demonstrate this layer has the potential to slide, albeit in isolated areas and/or with very large triggers.

Terrain and Travel

  • Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.
  • Cornices become weak with daytime heating or solar exposure.
  • Avoid exposure to steep, sun exposed slopes, especially when the solar radiation is strong.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

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.