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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, January 2nd, 2013 - 7:00AM
Expires
Thu, January 3rd, 2013 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

The storm flow has backed off a bit last night and today.   A decrease in the storm intensity will decrease the avalanche danger, but does not take us away from “dangerous”.   Over the past 8 days we tripled the snowpack on the ground, placing a tremendous stress on a persistent weak layer.   CONSIDERABLE avalanche conditions can be found across a wide region today, meaning that steep slopes should still be avoided.

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Crews will be conducting avalanche hazard reduction work between Girdwood and Portage Wednesday Jan 2, 2013 between 9am-11am. Expect intermittent delays on the Seward Highway while this work is in progress.

Wed, January 2nd, 2013
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Avalanche Problem 1
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
More info at Avalanche.org

This deep slab problem has already shown itself to be very active during the storm cycle.  Now that the precip and wind has lessened, the likelihood of natural avalanche activity will decrease but human triggered avalanches will remain a serious problem.  The longer we wait to jump onto steeper terrain, the less likely we will be to trigger a deep avalanche.  As recently as yesterday we saw large avalanches triggered by the Department of Transportation.  Since that time another 10 inches of snow has fallen in some areas.  Some thoughts on the deep slab problem –

– whumpfing and shooting cracks may not be evident

– standard snowpit tests are not suitable for testing deeply buried weak layers

– a lack of observed avalanche activity is not a reliable indicator of potential hazard.

– Waiting 36-48 hours after significant weather changes will decrease the likelihood of triggering (but not eliminate it)

– In some places we have seen the same slope avalanche twice during the recent storm cycle

– Terrain management is the only reliable prevention tool when snow stability is a problem

Avalanche Problem 2
  • Wind Slabs
    Wind Slabs
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
More info at Avalanche.org

Wind slab may be a problem above treeline today.  We haven’t been able to see avalanche activity within the new storm layers because we haven’t been able to access higher elevation areas in the past 8 days.  Recent wind and forecasted wind today may be loading up lee oriented slopes.

Weather
Wed, January 2nd, 2013

It’s been a wet and wild week, and it isn’t finished yet.   The graphs below show Turnagain Pass snow depth and wind from the recent weather events.

snow graph

wind graph

Today’s weather looks to be mild by comparison.   Snow showers are expected today, with 2-3 inches of accumulation.   Rain line will be 500 feet.   Southeast wind from 30-40mph during daylight hours.   Increased precip and wind is expected again tonight.   The overall weather pattern is expected to remain active, although not as intense for the remainder of the week.


John will issue the next advisory on Thursday, December 3rd.

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This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area. This advisory does not apply to highways, railroads or operating ski areas.