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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, December 13th, 2012 - 7:00AM
Expires
Fri, December 14th, 2012 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

Avalanche danger today remains CONSIDERABLE above tree line where the core advisory area received 4-6 € of new snow.   Increasing westerly winds overnight have transported this snow into tender wind slabs where it will be likely for a skier to trigger an avalanche.   Below tree line where the surface has generally been unaffected by wind there is a MODERATE danger, primarily for loose snow avalanches.

Also of note: Virtually everywhere else in the region (Front Range, Girdwood, Summit Lake, and Hatcher pass) received more snow than Turnagain pass yesterday so avalanche potential outside of the core advisory area is expected to be higher.

Thu, December 13th, 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. 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
  • 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

The modest weight of yesterday’s snow in Turnagain pass (.5” water) probably isn’t enough to affect last Saturdays wind slab on its own.  Add a skier to the equation and you will likely be able to trigger wind slabs on terrain steeper than 35 degrees.  We experienced substantial whumphing at tree line yesterday where the snowpack structure begins to change (weak and unconsolidated below tree line changing to a denser slab or wind scoured above).  This whumphing is a big auditory red flag that if the slope were steep enough (greater than 35 degrees) to slide, it probably would have.  Below tree line we found very little to no wind affected snow.

Avalanche Problem 2
  • Dry Loose
    Dry Loose
Dry Loose
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.
More info at Avalanche.org

Yesterday’s storm snow appeared to be bonding fairly well to last Saturday’s accumulation.  Snowpit test results below tree line consistently failed at the interface between Saturday’s storm snow and the November facets.  Though a slab is beginning to amalgamate at these lower elevations, my greater concern today is sluffing.  Our snowpack is still very loose in wind protected areas and a skier on steep terrain has the potential to entrain a significant amount of sluff in the form of storm snow and facets.  Just how weak this lower elevation snow is becomes quite obvious as your ski tips continually submarine into the October and November facet layers. 

Weather
Thu, December 13th, 2012

Yesterday’s storm roared through south central with a vengence leaving upwards of 15″ in favored areas such as Hatcher pass.   Unfortunately Turnagain pass found itself in the “Donut hole” with a meager 4-6″ of accumulation.   Overnight winds have picked up from the west and will continue to blow today in the 20-40mph range, tapering this evening.   We can expect a trace amount of snow to fall today over the advisory area as temps look to stay below the freezing mark at all elevations.  

This evening winds will back to the east in advance of a low pressure system building in the gulf.   As this low moves into Prince William Sound tomorrow, the Chugach and Kenai mountains should experience some snowfall Friday, though highest accumulations will likely be east of our area in the Sound.

Storm totals for 12/12/12:

Turnagain pass: 4-6″

Summit Lake: 8″

Anchorage bowl: 5-11″

Girdwood: 8-12″

Hatcher pass: 12-15″


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 Friday morning, December 14th.

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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.