Avalanche Forecast

Northwest Inland

Date issued

Nov 23, 2017 - 5:17 PM

Valid until

Nov 24, 2017 - 2:00 PM

Prepared by

cgarritty

A complex snowpack has begun to take shape in the Northwest. Choose terrain that is unlikely to release a storm slab and you'll likely avoid triggering the more dangerous persistent slab lurking beneath it.

Regular season
Thursday - November 23, 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.

Friday - November 24, 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

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.

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

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.

Avalanche Problem 1: Storm Slabs

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

Recent stormy weather has been building touchy storm slabs. The problem is widespread but you can expect slabs to be especially reactive in wind affected areas with more variable snow distribution

Travel and Terrain Advice
  • The new snow will require several days to settle and stabilize.
  • Avoid steep convexities and areas with more variable snow distribution.
  • Use conservative route selection, choose moderate angled and supported terrain with low consequence.

Avalanche Problem 2: Deep Persistent Slabs

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

A crust that formed in October exists near the base of the snowpack and a layer of weak, sugary snow has developed above it. This layer may react to a heavy trigger such as a storm slab release. This would produce a very large, destructive avalanche.

Travel and Terrain Advice
  • If triggered, storm slabs may step down to deeper layers and result in large avalanches.
  • Be aware of the potential for large avalanches due to weak layers at the base of the snowpack.

Avalanche Summary

One report from Wednesday discussed several estimated Size 2.5-3 avalanches observed on the Kathlyn face of Hudson Bay Mountain. No other new avalanches have been reported, which almost certainly owes more to a lack of observers than a lack of avalanche activity and potential. Please post your observations to the Mountain Information Network. (MIN)

Snowpack Summary

Snowpack depths over the region are above average for late November, with approximately 150 cm of settled snow present at treeline elevations and above. Stormy weather over the mid-week delivered about 30 cm of new snow to the region, forming storm slabs which are expected to be particularly reactive in open terrain where wind effects promote more variable loading. Mid-treeline and lower elevations did receive some precipitation as rain toward the end of the storm, but his thin rain layer is not expected to offer much bridging strength to the snowpack. About 20-30 cm of recent storm snow lies below this week's accumulations and above the widespread late October crust. The October crust is present near the base of the snowpack at all elevations and features a layer of weak, sugary grains above it. This "crust/facet combo" layer remains an ongoing concern for step-down potential from large triggers such as a storm slab release. The crust is less of a concern below treeline where it is broken up by vegetation near the ground.

Weather Forecast

Friday: Mainly cloudy with scattered flurries and a trace to 5 cm of new snow. Moderate to strong southwest winds. Alpine temperatures around -5.

Saturday: Mainly cloudy with scattered flurries and a approximately 5 cm of new snow. Moderate south winds. Alpine temperatures around -4.

Sunday: Cloudy with isolated flurries and a trace to 5 cm of new snow. Moderate south winds. Alpine temperatures of -3.

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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