Avalanche Forecast

South Columbia

Date issued

Apr 16, 2018 - 4:46 PM

Valid until

Apr 17, 2018 - 2:00 PM

Prepared by

jmcbride

More snow Monday night will add to the storm slabs at upper elevations. These may become more reactive as the day warms or if the sun appears on Tuesday. Also watch for loose, wet activity on sunny and lower elevation slopes.

Regular season
Monday - April 16, 2018
Alpine

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.

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

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.

Tuesday - April 17, 2018
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

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.

Wednesday - April 18, 2018
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

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.

Avalanche Problem 1: Storm Slabs

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

New snow and strong winds have formed widespread, touchy storm slabs. Avoid wind-loaded and cross-loaded slopes at treeline and above where, these slabs sit on buried crusts and are especially deep and reactive.

Travel and Terrain Advice
  • Avoid steep lee and cross-loaded slopes
  • Use conservative route selection, choose moderate angled and supported terrain with low consequence.

Avalanche Problem 2: Loose Wet

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

Warming temperatures or a bit of sun may destabilize surface snow and initiate loose wet avalanche activity, especially below treeline and on steep south facing features. The likelihood of cornice failures also increases with sun and warming.

Travel and Terrain Advice
  • Cornices become weak with daytime heating. Avoid traveling on slopes below them.
  • Avoid sun exposed slopes when the solar radiation is strong, especially if snow is moist or wet.

Avalanche Summary

On Sunday warming and sunny breaks initiated widespread natural storm slab and loose, wet avalanche activity in the recent storm snow, up to size 2, on all aspects from 1500-2300 m.

Saturday saw widespread avalanche activity with natural, cornice, skier, and explosive triggered storm slab releases (size 2-2.5) on all aspects in the alpine and in immediate lee areas at treeline.

Friday there were reports of several natural and skier-triggered storm snow releases (size 1) in steep and leeward alpine terrain. As well as several natural storm slab releases up to size 2 on alpine, northeast through southeast aspects.

Thursday pockets of reactive wind slab up to size 1 in alpine lees were reported, as well as skier-triggered, loose, wet avalanches to size 1.5 up to 2100m on solar aspects and below 1900m on north aspects.

There were also reports from neighboring Glacier National Park on Thursday of a loose wet size 2 on a south aspect at 2100 m, and a size 4 slab that is thought to have been cornice triggered on a north aspect at 2800 m.

Tuesday there was a report of a natural size 2 wind slab avalanche that was observed on a north facing feature at 2800 m.

Snowpack Summary

About 30-40 cm of new snow overlies a crust on all aspects to at least 2300 m (and possibly higher on south-facing slopes), while 50-60 cm of storm snow can be found at higher elevations. Below 2400 m this new snow has a surface crust becoming moist or wet by the afternoon.

Within the upper snowpack there are now a few different crusts with only the most recently buried posing a concern with the recent storm snow.

There is some lingering concern around the mid-March Persistent Weak Layer (PWL) in the alpine where it is found 80 to 120 cm below the surface. This buried crust/surface hoar interface was widely reactive but has not produced any avalanche activity in the past week. It is likely trending towards dormancy, but we need to keep it in the back of our minds as it is capable of producing large avalanches should it be triggered (think larger triggers such as; sleds, step-down from a surface avalanche or, a cornice collapse).

Deeper persistent weak layers from December and January are considered dormant at this time.

Weather Forecast

TONIGHT: Cloudy, flurries. Accumulation 15-25 cm. Ridge wind light to moderate, southwest. Alpine temperature near -5. Freezing level 1000 m.

TUESDAY: Mostly cloudy, flurries. Accumulation up to 5 cm. Ridge wind moderate to strong, west. Alpine temperature near -6. Freezing level 1600 m.

WEDNESDAY: Mix of sun and cloud. Ridge wind light, south. Alpine temperature near -4. Freezing level 1700 m.

THURSDAY: Increasing cloud. Ridge wind light, southwest. Alpine temperature near -2. Freezing level 2100 m.

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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