Avalanche Forecast

Issued: Mar 19th, 2013 9:59AM

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

Peter, Avalanche Canada

Northwest Coastal.

Summary

Confidence

Fair - Forecast snowfall amounts are uncertain on Wednesday

Weather Forecast

Synopsis: A frontal system is spreading moderate snowfall to the region today and through Wednesday. Conditions remain unsettled in the wake of the system on Thursday before high pressure develops on Friday resulting in sunshine. Tonight and Wednesday: 10-15 cm (maybe more near Bear Pass). The freezing level is around 400-500 m. Winds ease to moderate from the southwest, shifting to northwest. Thursday: Mainly cloudy with a chance of flurries. The freezing level drops to valley bottom and winds ease to moderate from the west. Friday: Mainly sunny. Temperatures remain cool with the freezing level at valley bottom. Overnight lows near treeline could be in the -15 range. Watch for outflow winds to develop.

Avalanche Summary

Natural, human triggered, and remotely triggered avalanches up to size 2 continued on Monday with most avalanches releasing on the March 9th layer. A very large (size 3.5) avalanche was triggered in Cherry Bowl in the Shames backcountry on Sunday. Click here for a full report. This slide illustrates clearly the potential for deep and destructive releases on the March 9th surface hoar layer. On Saturday, natural slab avalanches were reported up to size 3 on all aspects in the alpine.

Snowpack Summary

30-60 cm of recent storm snow rests on a variety of old snow surfaces, including crusts, previous wind slabs and surface hoar. Warmer temperatures and recent strong southwest winds have now set this new snow into a reactive slab, with wind slabs building in exposed lee areas. The surface hoar (buried March 9th) is reported to be well-developed and fairly widespread, at treeline and alpine elevations. Recent snowpack tests have produced sudden failures with moderate loading force at this interface. The mid snowpack is generally well settled and strong. Cornices are large and untrustworthy, especially when the sun is out.

Problems

Storm Slabs

Recent new snow has consolidated into a slab and is reacting on steep terrain. Watch for deep and weak wind slabs to form in North through East facing terrain well below ridge crests.

Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.>Be cautious as you transition into wind affected terrain.>Stay off recent wind loaded areas until the slope has had a chance to stabilize.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely - Very Likely

Expected Size

2 - 6

Persistent Slabs

A weak layer comprising surface hoar crystals is buried around 50 cm below the surface and is reacting readily to natural and human triggers. This layer appears to have the ability to propagate over large areas of terrain.

Be aware of the potential for large, deep avalanches due to the presence of buried surface hoar.>Avoid exposure to overhead avalanche terrain, as large avalanches may reach the end of runout zones.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

3 - 7

Cornices

Cornices are reported to be large and unstable, especially when temperatures become warm or the sun is out. Watch for reverse loading from outflow winds.

Give cornices a wide berth when travelling on or below ridges.>Do not travel on slopes that are exposed to cornices overhead.>

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

Elevations: Alpine.

Likelihood

Possible

Expected Size

2 - 6

Valid until: Mar 20th, 2013 2:00PM