Avalanche Forecast

Issued: Mar 12th, 2013 10:55AM

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.

slemieux, Avalanche Canada

Purcells.

Warm temperatures and heavy precipitation are increasing the avalanche danger in the days to come. Good time to avoid avalanche terrain or to make very conservative terrain choices out there.

Summary

Confidence

Fair - Forecast snowfall amounts are uncertain

Weather Forecast

Tuesday night and Wednesday: The strong Westerly flow bringing significant precipitation amounts into the interior should taper off a little tonight but should keep delivering moisture until Friday. Amounts vary in the region from around 10-20 mm. in water equivalent till Wednesday evening with winds blowing strong from the W. Freezing levels are expected to rise to close to 2000 m. during the day and stay high until Friday.Thursday: Similar amount of precipitation and freezing level is expected for Thursday with slightly lighter winds from the SW.Friday: The Westerly flow is forecasted to weaken and precipitation to taper off. Freezing levels and wind speeds should also start to drop.

Avalanche Summary

A couple slab avalanches size 1 have been triggered by skiers out of steep alpine terrain on a S and NE aspect. These would have initiated in ~30 cm deep wind loaded pockets. There was also a couple natural slab avalanches up to size 2 one of which was triggered by a cornice fall on a NE alpine slope. More and bigger avalanches are expected in the days to come.

Snowpack Summary

Currently, 5-20 cm of new snow (more in the W part of the region) is covering a variety of surfaces; a 2 cm thick suncrust on SE-S-SW facing slopes at all elevation, a melt-freeze crust all the way up to 1600 m. and some well developed surface hoar on shaded slopes above that elevation. The forecasted precipitation and warm temperatures will add a good load to the snowpack (could be another 30 mm in water equivalent till tomorrow night). The variety of surfaces described earlier are already showing reactivity to this new load and creating a good failure plane for avalanches to slide on. With freezing levels rising again tomorrow, the new dryer snow layer at the upper below treeline and treeline elevation band (between ~1200 m. and ~1800 m.). could get soaked and create wetslabs and loose wet avalanches. The surface hoar persistent weak layer buried down up to 100 cm is still producing some sudden planar snowpack test results. This layer could be triggered by the heavy load in the coming days.

Problems

Storm Slabs

Warm and wet storm is expected to create new storm slabs, new wind slabs lee of strong SW winds in the alpine and treeline, loose dry avalanches above the freezing level (rising possibly to 2000 m.) and wet slab and loose avalanches below that level.

Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.>Avoid exposure to overhead avalanche terrain, as large avalanches may reach runout zones.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Very Likely

Expected Size

1 - 6

Persistent Slabs

A significant load on the surface hoar and a sun crust buried down 60-90 cm could trigger this layer.

Be aware of the potential for large, deep avalanches due to the presence of buried surface hoar.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

2 - 7

Valid until: Mar 13th, 2013 2:00PM