Avalanche Forecast

Issued: Mar 19th, 2013 11:24AM

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

pmarshall, Avalanche Canada

Northwest Inland.

Wednesdays storm is expected to be quite mild, but there is much uncertainty surrounding the March 09 Surface Hoar and how it will respond to additional loading The danger rating will quickly rise to high at tree line if it snows more than 15cm.

Summary

Confidence

Poor - Due to limited field observations

Weather Forecast

Synopsis:Â A low pressure system approaches from the SW which will likely deliver meager amounts of snow and strong winds to the NW Inland. A trailing cold front will keep freezing levels near valley bottom for the duration of the forecast period. Wednesday: Freezing Level: 500m.Â Precipitation: 5 /10mm â€“ 5/15cm Wind: moderate gusting strong SWWednesday night: Isolated flurries.Â 0 â€“ 5 cm expected.Thursday: Clear skies.Â Freezing Level: Valley Bottom.Â No precipitation. Wind: Moderate NWFriday:Â Â Clear skies.Â Freezing Level: Valley Bottom.Â No precipitation. Wind: Moderate NW

Avalanche Summary

Numerous skier controlled avalanches up to size 1 failing on the March 09 surface hoar were reported from the greater region on Monday. On Saturday, a size 2 avalanche was triggered remotely on a south aspect at 1800 m. This avalanche ran on the new snow/crust interface. On Friday, a size 2 remote-triggered avalanche was reported from a west aspect at 1450 m, likely failing on the March 9th surface hoar layer down around 50 cm. Another human-triggered avalanche was reported on Friday from a north aspect slope.

Snowpack Summary

20-40 cm of recent storm snow rests on a variety of old snow surfaces, including crusts, previous wind slabs and surface hoar (buried March 9th). Warmer temperatures and recent winds have now set this new snow into a reactive slab. The March 9th surface hoar layer is reported to be very touchy and appears to exist at all elevations and on a variety of aspects. I suspect cornices have become well-developed and could easily become unstable with daytime heating. The mid snowpack is generally well settled and strong.

Problems

Storm Slabs

New snow will add to the previous storm snow which has consolidated into a slab and is reacting to human triggers. The primary weak layer comprises surface hoar and appears to be touchy.

Avoid large alpine features.>Choose conservative lines and watch for clues of instability.>Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.>Avoid open slopes and convex rolls where buried surface hoar may be preserved.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 5

Cornices

Cornices are well developed and may become more sensitive with increased snow and wind Wednesday.

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

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

Elevations: Alpine.

Likelihood

Possible - Likely

Expected Size

2 - 6

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