Avalanche Forecast

Issued: Dec 14th, 2019 5:00PM

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

Avalanche Canada ghelgeson, Avalanche Canada

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Improving visibility beckons us into the alpine, but we need to watch for fresh slab formation as northwest wind picks up Sunday. At and below treeline, 20 to 30 cm of storm snow rests on surface hoar which teeters on the brink of becoming a touchy storm slab problem.

Summary

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

We’re entering a rather quiet period with no significant snow expected until Monday afternoon. The next 48 hours are marked by seasonally normal temperatures, potentially sunny periods and a bit of northwest wind at ridgetop.

SATURDAY NIGHT: Freezing level around 800 m, light northwest wind largely confined to the high peaks, no significant precipitation expected.

SUNDAY: Scattered cloud cover at dawn building to broken by sundown, freezing level at valley bottom, moderate northwest wind at ridgetop, no significant precipitation expected.

MONDAY: Scattered cloud cover in the morning building to overcast by sundown, freezing level at valley bottom, moderate west/southwest wind, light snow beginning late in the day, potential for 1 to 5 cm by Tuesday morning.

TUESDAY: Broken cloud cover, freezing level around 700 m, moderate to strong southerly wind, 1 to 5 cm expected during the day, 1 to 5 more cm possible Tuesday night.

Avalanche Summary

Reports from Thursday and Friday mostly describe sloughing and small loose dry avalanches up to size 1. Soft storm slabs propagating at ridgetops but breaking up quickly as they ran, were also limited to size 1.

Prior to the new snow, natural and skier triggered wind slab and persistent slab avalanches averaging size 2 were widely reported in the north of the region and in neighboring Glacier National Park. Natural events have been reported on a range of aspects and are suspected to have been triggered by wind loading or solar radiation. Keep in mind that the new snow, or small storm slab avalanches could also act as triggers to these deeper avalanches.

Snowpack Summary

20-30 cm of new snow has covered a widespread layer of large, feathery surface hoar crystals. The new snow can be expected to gain cohesion and settle into a slab in the mild temperatures, especially at treeline and below.

The new snow also buried soft wind slab found in the lee of features such as ridge tops, ribs and gulleys.

A weak layer formed in late November is now buried around 1 m below the surface. This is the layer of concern relating to the persistent slab avalanche problem. The weak layer may present as surface hoar, a crust, facets or a combination, depending on elevation and aspect. It can most likely be found in areas sheltered from the wind, but open to a view of the sky, such as near treeline. Below this, variety of crusts from late October are buried deeper in the snowpack.

Problems

Storm Slabs

An icon showing Storm Slabs

20 to 30 cm of recent storm snow rests on a widespread layer of large surface hoar. The storm snow is teetering on the edge of becoming a potentially touchy slab problem, but it hasn't happened yet. Northwest wind Sunday is likely to form fresh sensitive wind fueled storm slabs in wind exposed features, especially steep pieces of terrain near ridge crest.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

1 - 2

Persistent Slabs

An icon showing Persistent Slabs

A couple of weak layers formed in late November and early December are now sitting about 1 m below the surface. This layer may present as surface hoar, a crust, facets or a combination of those, depending on elevation and aspect. They can most likely be found in areas sheltered from the wind, but open to a view of the sky, such as near treeline.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Unlikely - Possible

Expected Size

1 - 2

Valid until: Dec 15th, 2019 3:00PM