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Wed, February 20th, 2013 - 7:00AM
Thu, February 21st, 2013 - 7:00AM
Kevin Wright
Avalanche risk The Bottom Line

The blizzard overnight dropped enough snow to raise the danger rating in some areas today.   Weather stations are showing 24 hour snow totals ranging from a couple inches in Summit lake, to 10 to 12 inches at Grandview.   Turnagain Pass and Girdwood got at least 8 inches at mid elevations.  

The danger rating has risen to CONSIDERABLE above treeline where storm snow and wind slab is deep enough to be a problem.   Tender wind slab at high elevations may be the most likely problem today, but we have recently seen deeper avalanches fail both at a 3 week old crust interface and full depth to the ground.

Wed, February 20th, 2013
Above 2,500'
3 - Considerable
Avalanche risk
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. 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
  • 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

Storm snow combined with wind overnight with gusts to 70 mph will make dangerous avalanche conditions in higher elevation terrain.  Today, as the snowfall continues to build, we can expect wind slabs 1-2 feet deep to be reactive to human triggers.  It may be difficult to travel above treeline due to poor visibility…

Below treeline, a low to moderate danger exists today.  Some areas in the trees that are more prone to wind loading may have shooting cracks and small reactive wind pillows.  Larger runouts are worth avoiding today until the new snow has time to settle.

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 persistent slab is the enigma that is keeping us on our toes.  Despite the old age of these weak layers, they remain active on an infrequent basis.  Two specific issues are at play right now –

1.  A crust formed in late January is now 2-6 feet deep.  There was a report of a slide, possibly on this layer in Junior’s bowl on Seattle ridge earlier this week.  This layer seems to be a problem only at a focused elevation band between 1900 and 3000 feet, which is mid-elevation for most of our local mountains.

2.  The old October/November facets that have been dormant for the past 6-8 weeks finally showed themselves at Carter lake on Monday.  A snowmachine triggered avalanche slid to the ground on a south facing slope, narrowly missing people watching at the bottom and damaging a couple sleds that were parked. This problem is more likely to be found in the central portion of the Kenai peninsula where the snowpack is thinner and the deeper layers are more easily influenced by the weight of a person or snowmachine.

Wed, February 20th, 2013

The blizzard warning overnight seemed to bring more snow to Grandview than to Girdwood or Turnagain Pass.   Snowfall is likely more than a foot in some areas over the last 24 hours, with an average closer to 8 inches.   It is still snowing as of 7am this morning.   Wind peaked around 5pm last night and gradually diminished.   Gusts are still reaching into the 30s, which is ideal to move snow around and create wind slabs and cornices.   Temperatures rose from the single digits yesterday morning to the mid to high 20s today.

3-6 inches snow is expected today, with continued moderate to high wind.   Precipitation should remain as snow to sea level.

Graham will issue the next advisory on Thursday, February 21st.

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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.