Avalanche Forecast

Issued: Jan 2nd, 2018 6:17PM

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

mgrist, Avalanche Canada

South Rockies.

We're into a tricky period as a critical amount of storm snow settles (into a slab) and sits on a buried weak layer. Keep your terrain choices on the conservative side until conditions stabilize.

Summary

Confidence

Moderate - Due to the number of field observations

Weather Forecast

Dry with gradually warming alpine temperatures during the week. Cold in the valley bottoms. WEDNESDAY: Cloudy with sunny breaks. Ridge wind light west. Alpine temperature -5. Freezing level surface.THURSDAY: Mix of sun and cloud. Ridge wind light southwest. Alpine temperature -5. Freezing level surface.FRIDAY: Cloudy with isolated flurries. Ridge wind light west. Alpine temperature -3. Freezing level surface.

Avalanche Summary

On Monday, a machine triggered Size 1.5 persistent slab avalanche was reported running on the mid December surface hoar layer, down 35 cm at that location (1700m). Even mountain goats looked like they were triggering avalanches! See the details here in the MIN post. On Sunday a Size 2 persistent slab (stepping down to late November crust/facets) was intentionally triggered on an east aspect near 2000m in the south of the region.On Saturday several natural and human triggered storm slab avalanches to Size 2 were reported on a wide variety of aspects and elevations. Furthermore, several persistent slab avalanches to Size 1.5 were reported (with some remote-triggered by skiers). A week ago, a larger persistent slab (Size 2) was triggered low on a slope that propagated up and across the slope on a 50 cm deep crust. This highlights the potential for persistent slab avalanches to have wide propagations.

Snowpack Summary

Up to 45 cm of snow fell on at the end of last week. Since then, strong winds from a wide range of directions have blown snow around: In the south of the region winds were from the south west; whereas in the north the winds were from the north east. The main question in the snowpack surrounds the mid-December layer sitting beneath the recent snow. This layer includes crusts on solar aspects and feathery surface hoar in sheltered terrain at and below treeline, and has consistently shown its reactivity, even on small features below treeline. The lower snowpack is composed of mostly soft sugary snow and a few early season crusts that have not produced significant test results recently.

Problems

Persistent Slabs

A weak layer composed of surface hoar and/or crusts is buried 35-70 cm below the surface. This layer continues to be very reactive to human triggers and may not improve anytime soon.

Watch for signs of instability such as whumpfing, or cracking. Use caution on open slopes and convex rolls at treeline where buried surface hoar may be preserved.Choose well supported terrain without convexities.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

1 - 3

Wind Slabs

Moderate winds have created fresh wind slabs at higher elevations. In the south of the region, the winds were from the south-west. In the north of the region the winds were from the north east.

Be careful with wind loaded pockets, especially near ridge crests and roll-overs.If triggered, the wind slabs may step down to deeper layers resulting in large avalanches.Avoid freshly wind loaded features.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 2

Valid until: Jan 3rd, 2018 2:00PM