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Archived

Avalanche Forecast

Feb 20th, 2012–Feb 21st, 2012
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Northwest Inland.

Please read the new Forecaster's Blog post on the "Tipping Point"

Confidence

Fair - Intensity of incoming weather is uncertain on Tuesday

Weather Forecast

Tuesday: New snow accumulations up to 20cm overnight Monday throughout the day on Tuesday. Ridgetop winds 50km/hr from the West. Freezing levels near 600m. Wednesday: Trace amounts. Ridgetop winds 45-55km/hr from the NW. Treeline temperatures near -6. Thursday: Mainly dry. Freezing levels will lower to valley bottom.

Avalanche Summary

No new avalanches have been reported.

Snowpack Summary

About 15cm of snow that fell over the past week sits over a variety of old surfaces. These surfaces include: melt-freeze crusts that exist on all aspects at lower elevations and on south-facing slopes higher up and well-settled powder on shaded aspects in the alpine. Weak surface hoar crystals (size 3-4mm) are sandwiched between the old surfaces and the newer snow and have recently shown easy compression test results. The distribution of the surface hoar seems to be up to treeline on all aspects and on sheltered features in the alpine. This upper snow pack structure will be the big thing to watch as the overlying slab develops. I expect to see gradually increasing reactivity in the surface hoar interface with incremental loading and higher winds forecast for this week. Check out our Forecaster's Blog for some insight on incremental loading. The mid and lower snowpack are generally well settled and strong.

Avalanche Problems

Wind Slabs

Increasing winds and a possible 20cm of new snow through the forecast period will form wind slabs on lee slopes. As wind slabs build, it is likely the buried layers could reach their threshold and produce avalanches up to size 2 or larger.

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

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 4

Persistent Slabs

Forecast snow and wind will increase the soft slab load on the variable buried weak layers. I suspect this to be a concern as these layers reach their "Tipping Point" where natural and human triggered avalanche activity will increase.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Likely

Expected Size: 1 - 4