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Avalanche Forecast

Archived

Mar 4th, 2014–Mar 5th, 2014

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

Regions

South Rockies.

In localized areas with deeper storm snow accumulations or wind loading, the danger rating may be HIGH on Wednesday.  Watch for local signs of instability.

Confidence

Poor - Timing, track, or intensity of incoming weather system is uncertain

Weather Forecast

Snow or rain is expected for Wednesday and Thursday but amounts are uncertain. Freezing levels should rise to around 2000m. Dry conditions are expected for Friday with some clearing. Wednesday: Rain or snow 10-20mm, freezing level around 2000m, ridgetop winds 30-50 km/h SWWed. Night/Thursday: Rain or snow 10-20mm, freezing level 1000-1500m, ridgetop winds 40-60 km/h SWFriday: A mix of sun and cloud, mainly dry conditions, freezing level around 2000m, ridgetop winds 20-40 km/h SW-NW

Avalanche Summary

On Tuesday, we received a report of a natural storm slab avalanche and multiple human-triggered storm slab avalanches up to size 2 releasing down as deep as 35cm.

Snowpack Summary

20-40cm of new snowfall overlies surface hoar, facets, and/or a sun crust on south aspects. Recent moderate to strong SW winds have created wind slabs in leeward and cross-loaded features. The primary concern remains a persistent slab that sits on a stubborn persistent weak layer consisting of a crust, facets, and/or surface hoar. The layer continues to show a high degree of sensitivity to human triggers but is becoming more isolated. In the thinner snowpack areas such as the Crowsnest and northern Elk Valley, the slab is typically 30-60cm thick (below new snow). In thicker areas such as the Flathead and around Fernie, the slab typically varies in thickness from 70-150cm. Check out the South Rockies Blog for a video discussing the persistent weak layer and the current avalanche problem. A weakness at the base of the snowpack may still exist in isolated areas of the region but triggering has become unlikely.

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.