Avalanche Forecast

Purcells

Date issued

Apr 16, 2018 - 4:36 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

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.

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

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.

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?

Snow and wind Sunday night and Monday will likely form touchy storm slabs in the alpine and at treeline.

Travel and Terrain Advice
  • Use caution on leeward and cross-loaded terrain near ridge crests.
  • Choose well supported terrain without convexities.
  • Be cautious of sluffing in steep terrain.

Avalanche Problem 2: Loose Wet

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

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

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

Avalanche Summary

Sunday there was a report of a near miss for skiers who triggered a size 2 wind slab on a north aspect at treeline in the northern part of the region. While in the northwestern part of the region saw a warming-induced, natural avalanche cycle up to size 3 on large alpine features in the recent storm snow.

Saturday, explosive control work in northern parts of the region produced several storm slab releases size 2-2.5 on predominantly treeline to alpine, west aspects with wide propagations up to 200 m. And ski cuts produced size 1-1.5 results on cross-loaded features.

Friday several skier-triggered, size 1.5 wind slab releases were reported on north aspects at 2300 m. As well as a few solar-triggered storm slab releases (size 1.5) on steep, unsupported. west-facing features at 2200 m.

Wednesday there was a report of an icefall-triggered size 1.5 wind slab (50 cm deep and 50 m wide) on a north aspect at 2400 m.

Snowpack Summary

About 20 cm on new snow overlies a crust on all aspects to at least 2000 m (and possibly higher on south-facing slopes), while 30-40 cm can be found at higher elevations (and up to 60 cm in some areas in the northwest of the region). Below 2200 m this new snow has thin surface crust, breaking down to moist or wet snow in the afternoons.

Within the upper snowpack there a now several buried crusts with only the mostly 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 50 to 90 cm below the surface. This buried crust/surface hoar interface 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 step-down from a surface avalanche or cornice collapse)

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

Weather Forecast

TONIGHT: Mostly cloudy, flurries. Accumulation 10-15 cm. Ridge wind moderate to strong, southwest. Alpine temperature near -5. Freezing level 1000 m.

TUESDAY: Mostly cloudy, light flurries. Accumulation up to 3 cm. Ridge wind moderate to strong, west. Alpine temperature near -6. Freezing level 1700 m.

WEDNESDAY: Mix of sun and cloud. Ridge wind light, southeast. Alpine temperature near -4. Freezing level 1800 m.

THURSDAY: Increasing cloud. Ridge wind light, southwest. Alpine temperature near -1. Freezing level 2200 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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