Avalanche Forecast

Issued: Feb 13th, 2017 4:52PM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Storm Slabs, Deep Persistent Slabs and Wind Slabs.

Avalanche Canada cgarritty, Avalanche Canada

Warm weather will continue to undermine stability over the coming days. Our deep persistent slab problem is being tested. Choose low-consequence terrain and maintain constant awareness of overhead hazards.

Summary

Confidence

Moderate -

Weather Forecast

Tuesday: A mix of sun and cloud. Winds light to moderate from the southwest. Freezing level to 3400 metres with alpine temperatures around +5.Wednesday: Cloudy with sunny periods. Winds strong gusting to extreme from the southwest. Freezing level dropping from 3500 to 2000 metres with alpine temperatures around +4.Thursday: Cloudy with scattered wet flurries bringing up to 5 cm of new snow. Winds strong gusting to extreme from the southwest. Freezing level around 2000 metres with alpine temperatures around +1.

Avalanche Summary

Explosives control in the south of the region on Saturday yielded a few Size 1 results, limited to wind slabs in the upper snowpack. Friday's post-storm observations included reports of storm slab avalanches having run generally to Size 2 in our region and in the adjacent Waterton National Park over the course of the recent natural avalanche cycle. As for persistent slab activity during that cycle, one Size 3 was suspected to have run on a persistent weak layer while one Size 2 was reported to have run to ground. On Thursday there was a report of a Size 2.5 natural avalanche 50-120cm deep on a northwest aspect in Waterton Park. Nearby on the east slope of the Rockies, a Size 2.5 wind slab was brought down with explosives, leaving behind a 40-100cm crown fracture. For Tuesday, warm temperatures can be expected to promote instability within wind slabs and storm slabs. As the warm temperatures persist, and especially if we don't experience refreezing overnight, the potential for deep persistent slab releases will rise steadily each day. Any avalanches releasing on a basal weakness will likely be large and very destructive.

Snowpack Summary

Roughly 10 cm of new snow received late last week has been redistributed into wind slabs by strong southerly winds at upper elevations. Where it hasn't blown away, this new snow now lies above the 15-30 cm of moist snow that fell over the region on Thursday, which was mixed with periods of rain at lower elevations. Together, these more recent accumulations overlie the 90cm of low density storm snow that we received last weekend. Strong shifting winds redistributed this previous storm snow in exposed terrain, forming wind slabs over a variable old surface which includes wind affected surfaces, facets, and/or a rain crust below around 1500 m. The bond at the interface between this older storm snow and the previous surface is suspected to be improving slowly.In deeper areas, the snowpack appears to be well settled with only isolated concerns about the mid-December facet layer which was buried 50-100 cm deep prior to the weekend storm. In shallow snowpack areas and at lower elevations, the snowpack is weak, faceted, and has no structure. In these areas the snowpack's weakness may reach a tipping point as warm temperatures persist over the coming days.

Problems

Storm Slabs

An icon showing Storm Slabs
Roughly half of our current snowpack is essentially a storm slab. Its bond to the underlying snow is improving, but warm temperatures will put this bond to the test on each day of the current forecast period.
Use conservative route selection, choose low angled and supported terrain with low consequence.Minimize exposure to sun exposed slopes when the solar radiation is strong.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

2 - 3

Deep Persistent Slabs

An icon showing Deep Persistent Slabs
A weak layer at the bottom of the snowpack is creating the potential for full depth avalanche releases. Triggering is most likely in areas where storm snow piled onto a previously thin snowpack and becomes more likely as warm temperatures persist.
The likelihood of deep persistent slab avalanches will increase with each day of warm weather.Be aware of the potential for full depth avalanches due to weak layers at the base of the snowpack.Avoid exposure to overhead avalanche terrain, large avalanches may reach the end of run out zones.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

3 - 4

Wind Slabs

An icon showing Wind Slabs
Strong winds have been blowing our recent snow into reactive wind slabs in lee areas. Solar radiation and warming temperatures will make fresh wind slabs especially touchy.
Be careful with wind loaded pockets, especially near ridge crests and roll-overs.Be cautious as you transition into wind affected terrain.

Aspects: North, North East, East, South East, South.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 2

Valid until: Feb 14th, 2017 2:00PM