Avalanche Forecast

Issued: Mar 15th, 2014 11:23AM

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

jlammers, Avalanche Canada

Purcells.

Incoming weather combined with a complex snowpack means this is no time to be pushing into steep terrain. Conservative slope selection is critical to safe mountain travel.

Summary

Confidence

Poor - Forecast snowfall amounts are uncertain on Sunday

Weather Forecast

Overview: A pacific frontal system will push inland on Saturday bringing moisture to the interior until early Monday morning. Expect a ridge of high pressure to develop for Monday and Tuesday. Due to significant weather model disagreement for Saturday night and Sunday, I have limited confidence in forecast snowfall amounts and freezing levels.Overnight Saturday: Light to moderate snowfall / Moderate westerly winds with strong gusts / Freezing levels between 1500m-2000mSunday: Moderate to locally heavy (western slopes) snowfall / Moderate westerly winds with extreme gusts) / Freezing level at about 1600mMonday: Light flurries with a mix of sun and cloud / Moderate westerly winds with strong gusts / Freezing level at 1500mTuesday: Mainly sunny skies / Light northwest winds / Freezing level at about 1200m

Avalanche Summary

Expect fresh wind slab and loose dry snow avalanche activity to increase throughout the forecast period as snow flurries load exposed alpine and treeline feature. At treeline elevations and below, significant warming has made large persistent slab avalanches are highly sensitive to human triggers. In parts of the forecast area, very large destructive avalanches running to valley bottoms from major start zones remain a concern. Neighboring forecast regions have recently reported large avalanches initiating in the new storm snow, then stepping down to deeper layers, some running full path to the ground. Cornice failures triggering large deep persistent avalanches have also been reported.

Snowpack Summary

Convective flurry activity has already accumulated 10-15cm of fresh snow in some drainages, which has likely blown into touchy wind slabs on leeward slopes. These recent accumulations overlie crusts on previously sun-exposed slopes and surface hoar in shaded terrain. More snow and wind forecast for Saturday night will add to this developing storm slab. Below the new snow 50-70 cm of settled snow has been bonding poorly to buried surface hoar and crusts. The deeper facet/crust persistent weakness buried at the beginning of February, now down 60 to 160cm, has been highly variable in terms of reactivity and is very difficult to trust. Needless to say, any avalanche at these deeper, persistent interfaces would be large and destructive.Weak basal facets exist in many areas, but without a large load, triggering now is unlikely.

Problems

Storm Slabs

Forecast wind and snow will add to an existing storm slab which overlies hard crusts and weak surface hoar. Expect increased reactivity in wind exposed terrain.

Choose conservative lines and watch for clues of instability.>Be cautious as you transition into wind affected terrain.>Be alert to conditions that change with elevation.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely - Very Likely

Expected Size

1 - 4

Deep Persistent Slabs

In many areas the deeply buried weak layers that formed in early February are still showing significant reactivity in snowpack tests. Avalanches at this interface would be large and destructive and are possible at all elevation bands.

Be aware of the potential for large, deep avalanches.>Good day to make conservative terrain choices.>Avoid convexities or areas with a thin or variable snowpack.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

2 - 5

Wet Slabs

Forecast precipitation is expected to fall as rain at lower elevations with the potential to trigger deep and pushy wet slab avalanches. Be cautious of slopes that did not avalanche during the last big cycle.

Be alert to conditions that change with elevation.>Avoid steep unsupported terrain below treeline especially if the snow is moist or wet >

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

2 - 4

Valid until: Mar 16th, 2014 2:00PM