Avalanche Forecast

Issued: Feb 27th, 2019 3:28PM

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

dsaly, Avalanche Canada

South Rockies.

Recently formed wind slabs and wind-affected snow may quickly be hidden under a thin cover of flurries falling through Thursday. Expect a more reactive snowpack in areas where winds pick up and 10 cm snowfall accumulates.

Summary

Confidence

Moderate - Wind effect is extremely variable

Weather Forecast

WEDNESDAY NIGHT: Clear with cloudy periods. Alpine temperatures near -12C. Ridgetop winds 10-15 km/h from the east.THURSDAY: Cloudy with scattered flurries, 5-10 cm. Alpine temperatures near -12C. Ridgetop winds 10-20 km/h from the southeast.FRIDAY: Mainly cloudy with isolated flurries, trace to 8 cm. Alpine temperatures near -19C. Ridgetop winds 15-25 km/h from the east.SATURDAY: Sunny with cloudy periods. Alpine temperatures near -22C. Ridgetop winds 10-15 km/h from the east.

Avalanche Summary

On Monday, small (size 1) wind slabs were reactive to skier traffic, and an isolated natural wind slab avalanche event up to size 2.5 likely occurred overnight Monday.Several large (size 2) wind slabs were triggered with explosives control in the Castle area on Friday after a bout of strong to extreme southwest winds. Slab depths ranged from 20-60 cm.A MIN report from Grizzly Peak in the Elkford area last Thursday describes a persistent slab failing on the mid-January persistent weak layer as a ski touring group ascended a steeper slope near treeline. See the report here.Also last Thursday in adjacent Waterton National Park, a recent large (size 2) deep persistent slab avalanche was observed. It occurred on a steep feature below treeline that was noted for its thin snowpack and for being reverse loaded by winds.

Snowpack Summary

Widespread wind affect has smoothed alpine terrain and loaded lee features. Harder wind slabs are found in more exposed terrain and higher elevations, softer wind slabs have formed in more sheltered areas.Below this stack of wind-affected snow, a weak layer of surface hoar or faceted grains buried mid-January is down 40 to 80 cm. It is found on shaded and sheltered slopes and is most prominent between 1600 m and 1900 m but has been found up to 2200 m. It may overly a melt-freeze crust on south aspects. Recent avalanche reports and snowpack test results suggest that this layer remains reactive to human triggers.The bottom half of the snowpack is composed of weak and sugary faceted snow crystals.

Problems

Wind Slabs

5-10 cm flurries falling with wind through Thursday will build new wind slabs with deeper and more sensitive deposits in wind-loaded terrain. Harder, recently formed, wind slabs may be hidden under a thin cover of new snow.

Keep an eye out for reverse loading created by variable winds.If triggered, wind slabs may step down to deeper layers resulting in large avalanches.Use caution in freshly wind-loaded features, especially near ridge crests and in steep terrain.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 2

Persistent Slabs

A weak layer of surface hoar is buried around 40 to 80 cm, which is best preserved in shaded and sheltered areas between 1600 m and 1900 m.

Be aware of the potential for wide propagation.Use added caution in open terrain features such as cutblocks, gullies and cutbanks.

Aspects: All aspects.

Elevations: Treeline, Below Treeline.

Likelihood

Unlikely - Possible

Expected Size

2 - 2.5

Valid until: Feb 28th, 2019 2:00PM