Avalanche Forecast

Northwest Coastal

Date issued

Dec 9, 2017 - 3:39 PM

Valid until

Dec 10, 2017 - 2:00 PM

Prepared by

jlammers

Forecast freezing levels and precipitation amounts are highly uncertain for Sunday. Pay close attention to how much rain or snow falls in your area and be prepared to back off to simple terrain.

Regular season
Sunday - December 10, 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 - December 11, 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.

Tuesday - December 12, 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: Storm Slabs

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

Fresh storm slabs will begin to build as a storm pulse brings snowfall with strong winds. Poor bonding may occur at the surface, so slabs may be touchy. Expect thicker, more dangerous slabs in coastal areas and on lee aspects.

Travel and Terrain Advice
  • Use extra caution in lee areas in the alpine and treeline. Storm snow will form touchy slabs.
  • Use ridges or ribs to avoid pockets of wind loaded snow.

Avalanche Problem 2: Loose Wet

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

Rain on Sunday may cause small loose wet avalanches to occur, mainly in steep terrain and where rain or moist new snow accumulates.

Travel and Terrain Advice
  • Use extra caution on slopes if the snow is moist or wet.
  • Be cautious of sluffing in steep terrain, particularly where the debris flows into terrain traps.

Avalanche Problem 3: Deep Persistent Slabs

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

Weak layers buried near the base of the snowpack can't be dismissed while new snow or rain load the surface. Releasing a thick storm slab or wind slab in the right location may provide enough of a trigger to step down a deep early season weakness.

Travel and Terrain Advice
  • Take extra caution around areas with a thin or variable snowpack.
  • If triggered, storm slabs may step down to deeper layers and result in large avalanches.

Avalanche Summary

On Friday, warm temperatures and sunshine initiated numerous loose wet avalanches to Size 1.5 on steep solar aspects in the alpine around the Terrace area. Warming also triggered a few deep persistent slabs to size 3 out of steep south facing alpine terrain. Basal facets were though to be the culprit in these events. Looking forward, forecast snow and wind will likely promoted a round of storm slab activity in higher elevation terrain. In areas where rain falls, loose wet avalanches may continue to run in steep terrain .

Avalanche and snowpack data is extremely sparse at this time. Please submit your observations to the Mountain Information Network. (MIN)

Snowpack Summary

Up to 5mm of rain fell in the Skeena corridor on Friday night. At higher elevations I suspect snow fell and was shifted by strong winds into wind slabs in lee terrain. In the Bear Pass area where temperatures were cooler, around 15cm of new snow fell down to near valley bottom. These accumulations overlie melt freeze crusts and possibly surface hoar.

Another layer of large surface hoar that was recently observed in the Shames area is now buried about 15 cm below the surface. Information on the distribution and reactivity of this layer is limited. Below it, storm snow received last week is gradually settling above a 5-10 cm thick crust (November 23) down roughly 70-100cm. Another layer of surface hoar (November 11) continues to be observed down approximately 140 cm in the Bear Pass area, showing some signs of improved bonding. The widespread crust/facet interface that was buried at the end of October can now be found 90-160 cm deep.

The base of the snowpack is reported to be a mix of facets and melt-freeze crusts.

.

Weather Forecast

Saturday night: 5-20cm of new snow with rain at lower elevations / Strong to extreme southerly winds / Freezing level at 1800m

Sunday: 5-30mm of precipitation / Strong to extreme southerly winds / Freezing level at 2400m

Monday: Clear skies / Strong to extreme southerly winds / Freezing level at 2400m

Tuesday: Trace amounts of new snow / Light variable winds / Freezing level at 1200m

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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