Avalanche Forecast

Northwest Inland

Date issued

Dec 10, 2017 - 4:08 PM

Valid until

Dec 11, 2017 - 2:00 PM

Prepared by

jlammers

Warming and solar radiation will drive the avalanche danger on Monday. Use extra caution in sun-exposed terrain and limit your exposure to large overhead slopes.

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

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.

Monday - December 11, 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.

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: Loose Wet

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

With precipitation forecast for Sunday night (and then sun on Monday), I would expect loose wet avalanches to occur, especially in steep, sun-exposed terrain where dry snow is exposed to solar radiation for the first time

Travel and Terrain Advice
  • Avoid sun exposed slopes when the solar radiation is strong, especially if snow is moist or wet.
  • Use caution above cliffs where small avalanches may have severe consequences.

Avalanche Problem 2: Deep Persistent Slabs

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

The late October crust lies near the base of the snowpack with a thin layer of weak, sugary snow above it. This layer should be on a strengthening trend, but a heavy trigger in a thin snowpack area or warming may still be enough to cause it to react.

Travel and Terrain Advice
  • Be aware of the possibility for full depth avalanches due to deeply buried weak layers.
  • Be especially cautious in shallow snowpack areas where triggering a deeper layer may be easier.

Avalanche Problem 3: Wind Slabs

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

At elevations where Sunday night's precipitation falls as snow, thin yet dense wind slabs can be expected. Watch for triggering in higher elevation, wind-exposed terrain.

Travel and Terrain Advice
  • If triggered, wind slabs may step down to deeper layers and result in large avalanches.
  • Be cautious as you transition into wind affected terrain.

Avalanche Summary

No new avalanches have been reported, however it is expected that warm alpine temperatures and sunshine initiated natural activity on steep solar aspects at higher elevations on Sunday.

Snowpack Summary

With the recent temperature inversion, solar and temperature crusts are expected to have formed on sun-exposed slopes, while stiff, stubborn wind slabs linger in lee alpine terrain.

Approximately 30-50 cm of storm snow now overlies two crusts that were buried near the end of November. These crusts are widespread and extend into the alpine. Although snowpack tests have previously shown hard results at these crust interfaces, more recent observations have been limited. Below these crusts, a well settled mid-pack overlies the late October crust near the base of the snowpack. This "crust/facet combo" layer is widespread and has been reactive to rider and remote triggers. It also continues to produce sudden snowpack test results.

Average snowpack depths now total 80-140 cm at treeline elevations and up to 160 cm in the alpine. (Depths of up to 210 cm have been reported from the Howsons.)

Weather Forecast

Sunday night: 3-6mm of precipitation / Moderate southerly winds / Freezing level at about 2000m

Monday: Mainly clear / Moderate southerly winds / Freezing level at about 2000m

Tuesday: 2-5cm of new snow / Moderate southerly winds / Freezing level at 1700m

Wednesday: Mix of sun and cloud / Light and variable winds / Freezing level at 2300m

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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