Avalanche Forecast

Issued: Dec 4th, 2017 3:44PM

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

cgarritty, Avalanche Canada

Northwest Coastal.

Avalanche danger is on an increasing trend. More new snow is the first factor, followed by a bout of sustained warm temperatures that will truly test the snowpack.

Summary

Confidence

Moderate - Forecast snowfall amounts are uncertain

Weather Forecast

Tuesday: Flurries bringing 10-20 cm of new snow, mainly in the morning, rain below about 1000 metres. Moderate to strong southwest winds. Freezing level to 1500 metres with alpine temperatures around -1.Wednesday: A mix of sun and cloud with no new snow. Likely alpine temperature inversion. Light south winds. Freezing level climbing to 3000 metres with alpine temperatures around +5. Cooler temperatures at valley bottom.Thursday: Mainly sunny with valley cloud due to a lingering alpine temperature inversion. Light south winds. Freezing level to 3400 metres with alpine temperatures around +5. Cooler temperatures at valley bottom.

Avalanche Summary

On Sunday, dry loose avalanches were reported up to size 1. On Saturday, reports indicated several natural wind slab avalanches up to size 2 on northerly aspects in the alpine and dry loose avalanches from steeper terrain up to size 1 treeline and above. Beyond surface instabilities, the strength of the bond between our recent storm snow and deeper buried crusts remains in question (especially at the basal crust). With this in mind, use a conservative approach to route selection by traveling through low consequence terrain, especially as we head into forecast warmer weatherCheck out the Mountain Information Network for recently posted observations around the Shames area. Give info, get info.

Snowpack Summary

Average snowpack depths in the region are roughly 100-150 cm at treeline elevations. This means that our recent accumulated storm snow (100-130 cm) forms the vast majority of our existing snowpack. Strong winds (mainly southwest) over the course of the storm developed deep deposits of wind slab on leeward slopes. The recent snow seems to be settling quickly and sits above a 5-10 cm thick crust that was buried a week ago. The crust/facet interface that was buried at the end of October can now be found 90-160 cm deep. Recent snowpack test results are showing hard compression tests with sudden collapse results on this interface. The strength of these two deep weak layers will be an increasing concern as the snowpack is forced to adjust under continued loading and forecast rapid warming.

Problems

Storm Slabs

Storm slabs may be reactive to rider triggers, especially at upper elevations where the storm snow has been affected by wind. Wet flurries and rain may promote wet loose avalanches at lower elevations on Tuesday.

Use conservative route selection, choose moderate angled and supported terrain with low consequence.Be cautious as you transition into wind affected terrain.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely - Very Likely

Expected Size

1 - 2

Deep Persistent Slabs

A lot of uncertainty exists around the reactivity of layers sitting near the base of the snowpack. A conservative approach to terrain is warranted while the snowpack adjusts to recent loading and forecast warming.

If triggered the storm/wind slabs may step down to deeper layers resulting in large avalanches.Use conservative route selection, choose moderate angled and supported terrain with low consequence.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

2 - 4

Valid until: Dec 5th, 2017 2:00PM