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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, March 1st, 2013 - 7:00AM
Expires
Sat, March 2nd, 2013 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

Still within 48 hours of a significant storm event, caution is warranted in the backcountry.   Some areas received over 2 feet of snow in the latest storm.   A handful of large and destructive avalanches were caused by this storm in the last few days.   Above treeline the danger will be MODERATE for storm snow, wind slab, and cornices.  

The backcountry stability is on a steady improving trend.   Natural avalanches are unlikely, human triggered avalanches are possible in specific steeper terrain.  

Watch for lower elevation south facing slopes to become active if temperatures rise this afternoon.

Fri, March 1st, 2013
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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. Dangerous avalanche conditions. 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
  • Storm Slabs
    Storm Slabs
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

The recent storm had peak intensity 2 days ago, and dropped more than 2 feet of snow in some areas including Turnagain Pass.  It was deep enough on Wednesday to make travel downhill quite difficult.  After yesterday’s relatively clear weather that snow should be settled and consolidated to the point that travel is easier. 

A handful of large avalanches were caused by that storm, including a large natural that crossed the Portage road.  All the major avalanche programs in our region were doing explosive avalanche reduction work.  After the storm ended yesterday, explosive triggers were only causing small to medium avalanches occasionally, with nothing deeper than the recent storm snow.  The picture below shows one of the larger avalanches produced by Seward highway work yesterday. 

Colder temperatures and calmer weather will be acting to bring stability to the recent storm snow, but it still deserves a cautious approach today.

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
More info at Avalanche.org

The larger avalanches that we’ve seen this week are suspected to be a result of deeper weak layers at the late January crusts.  The good news is it remains a less likely problem to encounter.  The bad news is it remains a tough problem to predict and avoid 100% of the time, even for the experts.

The buried crusts can be found in some areas at mid elevations between roughly 1900 and 3000 feet.  As those layers get deeper with each storm, the likely places to trigger deeper layers will be from shallow points that have been partially scoured by wind.

 

Weather
Fri, March 1st, 2013

Storm totals yesterday morning reached near 2 feet of snow and up to 2 inches of water equivalent in some areas.   Areas such as Girdwood, Grandview, and Portage got more snow.   Summit lake got less, but still enough to cause concern.  

In the last 24 hours temperatures have cooled off a few degrees, wind has dropped, and precipitation ended by the afternoon.  

Today, a weak low pressure is spinning in Prince William Sound which may bring a few inches of snow.   Snowfall will end tonight and the weekend will bring a clearing trend.   Expect temperatures in the 30s and light wind.  


Fitz will issue the next advisory Saturday morning, March 2nd.

Observations
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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.