Avalanche Forecast

South Rockies

Date issued

Nov 25, 2017 - 4:39 PM

Valid until

Nov 26, 2017 - 2:00 PM

Prepared by

mgrist

If the rain-soaked snow freezes as expected through the weekend, avalanche hazard will follow a decreasing trend until the next storm late Sunday. Please treat this forecast as an initial assessment, as we have little information at this time.

Regular season
Saturday - November 25, 2017
Alpine

3 - Considerable

Dangerous avalanche conditions. Careful snowpack evaluation,cautious route-finding and conservative decision-making essential.

Natural avalanches possible;human-triggered avalanches likely.

Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.

Treeline

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

Below Treeline

1 - Low

Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

Natural and human-triggered avalanches unlikely.

Small avalanches in isolated areas or extreme terrain.

Sunday - November 26, 2017
Alpine

3 - Considerable

Dangerous avalanche conditions. Careful snowpack evaluation,cautious route-finding and conservative decision-making essential.

Natural avalanches possible;human-triggered avalanches likely.

Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.

Treeline

3 - Considerable

Dangerous avalanche conditions. Careful snowpack evaluation,cautious route-finding and conservative decision-making essential.

Natural avalanches possible;human-triggered avalanches likely.

Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.

Below Treeline

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

Monday - November 27, 2017
Alpine

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

Treeline

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

Below Treeline

1 - Low

Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

Natural and human-triggered avalanches unlikely.

Small avalanches in isolated areas or extreme terrain.

Avalanche Problem 1: Persistent Slabs

What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?

We know there's a crust buried approx. 50cm below the surface, but don't know much about how likely avalanches are on it. Once there's a new thick crust on the surface, this lower layer may become less of a problem. Until then, assume it's a player.

Travel and Terrain Advice
  • Start with lower angle slopes before gradually working up to steeper objectives.
  • Watch for whumpfing, hollow sounds, shooting cracks or recent avalanches.

Avalanche Problem 2: Wind Slabs

What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?

5-15cm of new snow fell at the tail end of the storm, as temperatures cooled and the rain turned to snow. Watch for wind slabs from redistributed snow on north easterly slopes at higher elevations.

Travel and Terrain Advice
  • If triggered, wind slabs may step down to deeper layers including persistent slabs (see above).
  • Use caution in lee areas. Recent wind loading may have created wind slabs.

Avalanche Summary

There are few observers right now. Several loose wet avalanches to Size 1 were reported at the tail end of the rains (Thursday).

Some persistent slab activity to Size 2.5 was reported to the north in Kananaskis country. Elevations ranged from 2400 to 2700m on North through East aspects. I suspect avalanche activity will taper off with cooling temperatures through the weekend.

Snowpack Summary

Rain and warm weather have resulted in a widespread wet upper snowpack. The freezing level rose to approximately 2700 m on Wednesday and Thursday meaning very few places, if any, will have escaped the melt. The snowpack diminished by approximately 20-30 cm.

Some areas saw 5-15 cm of snow at the tail end of the storm and the possibility of thin windslabs exists. Expect treeline snow depths of around 45 cm in the Elk Valley and more like 80 cm further south in the Flathead in sheltered areas. A crust that formed around Halloween has been reported within the snowpack that may lie around 40-50 cm below the surface.

Weather Forecast

We're looking at relatively benign weather until Sunday afternoon, when a brief but intense cold front will bring 5-10 mm of rain and then 5-15 cm of snow by Sunday night.

Sunday: 5-10 mm of rain early afternoon changing to snow (5-15 cm possible overnight). Freezing levels 2200m decreasing to surface by the evening. Winds moderate from the south west.

Monday: Scattered flurries. Freezing level around 1200 m. Moderate westerly winds.

Tuesday: Isolated flurries. Freezing levels rising to 1700 m. Moderate southwesterly winds.

Danger Ratings Explained

1 - Low

Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

Natural and human-triggered avalanches unlikely.

Small avalanches in isolated areas or extreme terrain.

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

3 - Considerable

Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

Natural avalanches possible; human-triggered avalanches likely.

Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.

4 - High

Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

Natural avalanches likely; human-triggered avalanches very likely.

Large avalanches in many areas; or very large avalanches in specific areas.

5 - Extreme

Avoid all avalanche terrain.

Natural and human-triggered avalanches certain.

Large to very large avalanches in many areas.

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