Avalanche Forecast

Issued: Mar 27th, 2017 3:56PM

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

mgrist, Avalanche Canada

Northwest Coastal.

Storm time again! With the new snow load, persistent slab problems are expected to worsen. Conservative terrain use is essential & avoid overhead hazards.

Summary

Confidence

Moderate - Forecast snowfall amounts are uncertain

Weather Forecast

A significant storm hits Monday into Tuesday, with lingering flurries thereafter. A repeat may be in the cards for Friday - the winter storm train keeps rolling into town! TUESDAY: 20-30 cm Monday overnight into Tuesday afternoon, STRONG southwest wind, freezing levels 1200m.WEDNESDAY: Lingering flurries (5-10 cm possible) moderate south wind, freezing levels 1100m, alpine temperature around -4 C. THURSDAY: Isolated flurries (5cm). Light-moderate south wind, freezing level around 1100 m with alpine temperature around -3 C.

Avalanche Summary

On Sunday, a machine-triggered Size 3 (with crown approximately 2m thick) was reported in the South Douglas area just north of Terrace on a northwest aspect near 1400m. The slab is suspected to have run on the late February interface and involves an incredible story of survival. Click here for details. On Saturday, several wind slabs to Size 1.5 were reported on (east) northeast aspects in the north of the region.On Friday, a spooky Size 3.5 persistent slab with impressive propagation was remote-triggered north of Terrace at 1500m elevation. See here for more details in this excellent post.A significant concern remains the potential for persistent slab avalanches triggered by people. See the forecaster blog for advice on managing a low probability - high consequence problem (here) and the accompanying photos (here).In addition to the daunting persistent slab problem, the incoming weather pattern is looking to continually build fresh storm slabs.

Snowpack Summary

Storm slabs (up to 40cm thick) developed at treeline and above, while cornices grew larger during another March storm: Weekend totals brought 10-20cm of new snow and moderate southwest / southeast winds. Snow may still be poorly bonded to a 40 cm deep crust and surface hoar interface from mid-March; however, the late-February interface has been the more troublesome persistent layer. It's composed of facets, crust, and surface hoar roughly 70-140 cm deep and is still reactive throughout the region, especially in shallower areas around Stewart and Bear Pass. See the Avalanche Summary above for surprising examples of this persistent problem re-awakening.

Problems

Storm Slabs

New snow may overload wind slabs sitting on the March 22nd surface hoar layer. Conservative terrain use is essential and manage your overhead exposure.

Be aware of the potential for wide propagations due to the presence of underlying windslabs.If triggered, the storm slabs may step down to deeper layers resulting in large avalanches.Minimize overhead exposure during periods of heavy loading from new snow, wind.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 3

Persistent Slabs

A weak facet/crust layer from late February has produced several large human triggered avalanches the past few days. Triggering this layer is most likely in shallower areas, but a small wind slab or cornice fall could act as a natural trigger.

Remote triggering is a concern, watch out for adjacent slopes.Use conservative route selection, choose moderate angled and supported terrain with low consequence.Avoid steep convexities or areas with a thin or variable snowpack.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

2 - 4

Valid until: Mar 28th, 2017 2:00PM