Avalanche Forecast

Issued: Mar 19th, 2013 10:01AM

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.

pmarshall, Avalanche Canada

South Columbia.

There is much uncertainty surrounding the March 10 Surface Hoar and how it will respond to Wednesdays storm. Hedge your bet and stick to simple terrain free of overhead hazard.

Summary

Confidence

Fair - Timing, track, or intensity of incoming weather is uncertain for the entire period

Weather Forecast

Synopsis: A low pressure system approaches from the SW which should deliver strong wind and significant precip. A trailing cold front in it’s wake will bring the freezing levels down to valley bottom on Thursday. Wednesday: Freezing Level: 1500m. Precipitation: 15 /20mm – 30/40cm Wind: Strong SWWednesday night: Precipitation: 10/15mm – 20 – 30cm.Thursday: Freezing Level: Valley Bottom. Precipitation: Isolated convective flurries. Wind: Moderate to strong NW.Friday: Freezing Level: Valley Bottom. Precipitation: Isolated convective flurries. Wind: Moderate NW.

Avalanche Summary

Skier triggering to size 1.5 continued to be a problem Monday on steep unsupported features. A widespread slab avalanche avalanche cycle to size 2 took place on Sunday. The activity occurred in response to new snow and variable winds at all elevations. One notable occurrence was a size 3 slab avalanche which took place in the southeast corner of the region. The avalanche occurred on an east-northeast aspect at 2500m and was likely triggered by rapid loading from strong northwest winds. Observations were limited, but its size may suggest that a persistent weakness was the failure plane.

Snowpack Summary

Light to locally heavy amounts of new snow fell over the weekend and have have been distributed by northerly winds into deeper windslabs in lee terrain. These recent accumulations overlie a thick rain crust which is reported to extend up to 2000m. About 1m below the surface is a layer surface hoar or a sun crust buried on March 10th. Natural avalanche activity dropped off in the wake of the last storm but isolated natural activity and remote triggering is still occurring in some areas. I expect another round of natural activity on this layer as the coming storm adds a significant new load on Wednesday. At lower elevations where last week's heavy rain saturated the snowpack, subsequent cooling has formed crusts and has dramatically strengthened the snow.Weaknesses deeper in the snowpack have become unlikely to trigger.Widespread cornice development has also taken place, and cornices are reported to be very large and weak.

Problems

Storm Slabs

Significant new snow combined with strong SW winds on Wednesday will begin to stack up on a variety of old surfaces including old storm/wind slabs and variable crusts below treeline. This will likely set the stage for a natural avalanche cycle.

Avoid exposure to terrain traps where the consequences of a small avalanche could be serious.>Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Very Likely

Expected Size

1 - 4

Persistent Slabs

Around 1m of snow rests on the March 10 surface hoar/sun crust combo. This layer may roar back to life Wednesday as storm totals stack up which would result in a surprisingly large and destructive avalanche cycle.

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.>Stick to simple avalanche terrain with zero overhead hazard.>Avoid open slopes and convex rolls where buried surface hoard may be preserved.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 7

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