Avalanche Forecast

Northwest Coastal

Date issued

Dec 19, 2017 - 3:30 PM

Valid until

Dec 20, 2017 - 2:00 PM

Prepared by

jfloyer

Cool and stable weather should mean a gradually strengthening snowpack.

Regular season
Tuesday - December 19, 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.

Wednesday - December 20, 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.

Thursday - December 21, 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

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.

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: Wind Slabs

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

20 to 60cm of snow fell during the weekend with wind from the SW, W, NW and SE. As a result, wind slabs on a wide variety of aspects should be expected.

Travel and Terrain Advice
  • Watch for signs of instability such as whumpfing or cracking.
  • Be alert to conditions that change with elevation, aspect and exposure to wind.

Avalanche Problem 2: Deep Persistent Slabs

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

As recently as this weekend, large avalanches ran on deep persistent weak layers near the base of the snowpack. While it appears to be more likely in the north of the region, the possibility of triggering something big should remain on your radar.

Travel and Terrain Advice
  • Be especially cautious in areas with a thin or variable snowpack.
  • Storm slabs in motion could step down and trigger deep persistent slab avalanches.
  • Minimize exposure to overhead avalanche terrain, carefully select genuinely safe spots to re-group.

Avalanche Summary

The north part of the region reported up to size three natural avalanches over the weekend. Poor visibility limited observations in the south but no new avalanches were reported.

Snowpack Summary

The region picked up 20 to 60cm of storm snow over the weekend accompanied by winds out of the southwest, west, northwest, and most recently the east. This will have set up storm slabs in more sheltered areas and wind slabs on a wide variety of aspects in more exposed areas. These new storm and wind slabs may rest on a melt freeze crust (reported to be 3 to 15cm thick), and there are reports there also may be surface hoar resting above this crust in sheltered locations. Below this crust the mid-pack is reported to be generally strong. You can expect to find a second prominent crust, which was buried at the end of October, 100 to 200cm below the surface. This lower layer may have associated facets, particularly in more shallow locations.

Weather Forecast

Wednesday: Dry with some clear spells until late in the day. Strong northwesterly winds. Models indicate an inversion will set up for Wednesday afternoon with valley temps around -10C and above-zero between 1400m and 2000m.

Thursday: Light snow, 2-5 cm expected, clearing thorough the day. Winds initially strong northwesterly but diminishing through the day. Treeline temps around -8C.

Friday: Cold and dry. Moderate and locally strong easterly winds. Treeline temps around -12C.

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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