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Issued
Sat, December 8th, 2012 - 7:00AM
Expires
Sun, December 9th, 2012 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

AVALANCHE WARNING BEGINNING TODAY AS STORM SNOW BUILDS

The game changing storm is due to arrive today with peak intensity sometime this evening.   This combination of a big snow and wind event over the top of a shallow and extremely weak snowpack will turn the backcountry dangerous very quickly today.   We know avalanches will happen from this cycle, it’s just a question of how bad it will get and when the peak danger will happen.   Through the daylight hours as the new snow builds, the danger rating will be CONSIDERABLE.   Later this evening as the new snow depth gets beyond 1 foot the danger rating will increase to HIGH with very dangerous avalanche conditions.   All avalanche senses need to be on peak alert if you plan to travel in the mountains this weekend.

Sat, December 8th, 2012
Alpine
Above 2,500'
3 - Considerable
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

Snowfall totals in the mountains are expected to be 10-18 inches today, with another 10-16 inches tonight.  East wind at ridgetops will be 50-75mph.  If this proves true, avalanche danger will reach HIGH sometime late today.  Fresh snowfall and high wind will create wind slabs at upper elevations.  The timing of the storm may catch people off guard as they try to use the Saturday opportunity before the accumulation gets too deep.  With the blizzard warning in effect and a very weak base snowpack, it will not take a lot of snow for avalanche conditions to change quickly.  Watch the changes as they happen today and be prepared to find a safe route back to the car before it gets too dangerous.

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

As the storm snow and wind slabs build, they will be resting on top of all the junky facets and surface hoar that built over the last month of cold dry weather.  This is a really bad combination, and the first test in quite a long time.  Everything about this setup is screaming bad news.  All the new snow will fall on 2 feet of collapse-prone ball bearings.  We can expect avalanches to behave more dangerously than normal, breaking at the new/old snow interface or at the ground and running far and fast.  Propagation potential today will depend on the temperature of the storm (the density of the snow) and the intensity of the wind to build cohesive slabs.  Even a small amount of cohesiveness in this new layer will allow for widespread connectivity of the slab. 

If you can’t already tell, I’m pretty worried about the storm today!

Weather
Sat, December 8th, 2012

Yesterday was a cloudless and calm day in the mountains.   This morning, temperatures have risen up to 10 degrees in some areas and wind is reaching into the 30s at the ridgetops.   As of 6am snowfall has not started in Turnagain Pass or Girdwood, but radar in Prince William Sound is starting to show the precipitation approaching.  

The blizzard warning is active across our region starting at 10am, so confidence in this storm is high.   Expect snow – 10-18 inches today, and 10-16 inches tonight with 1.5 inches of water equivalent.   East wind is expected to blow 50-75mph at the ridgetops.   Temperatures will remain below freezing today, with slightly warmer temperatures tomorrow in coastal areas such as Seward and Whittier.  

The weather that happens today will drive the overall avalanche danger.   What you see is what you get, so pay attention if you want to play in the mountains today.   New snow approaching 1 foot depth will take us into dangerous avalanche conditions where trying to outsmart the problem will become impossible.  


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).  

Wendy will issue the next advisory Sunday morning, December 9th.

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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.