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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, February 19th, 2013 - 7:00AM
Expires
Wed, February 20th, 2013 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

There is a MODERATE avalanche danger today for fresh wind slab avalanches above treeline. These will be building through the course of the day with an increase in easterly wind as the next system moves in. Steering clear of areas where the wind is visibly loading slopes will be your best bet for avoiding a fresh and sensitive slab – not to mention maximizing the good powder conditions. Additionally, loose snow avalanches will continue to be easy to initiate in the steeper terrain. Below treeline there is a LOW avalanche danger.

Tue, February 19th, 2013
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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
  • 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

For anyone stricken with Powder Fever – President’s Day was just what the doctor ordered. It was one of those great days to enjoy the backcountry. The only avalanche activity seen/reported in the Turnagain area was confined to loose snow sluffs and very small and manageable wind slabs. However, outside of the Turnagain area there was one significant avalanche in the Carter Lake region south of Summit Lake toward Seward. This is outside of our forecasting zone but in our neck of the woods nonetheless.  Alex Mclain, Summit Lake’s CNFAIC forecaster, will try to get a closer look this afternoon.

Today, fresh wind slabs will be our main concern as the east winds ramp up. There is a plethora of loose snow out there to be blown into soft and sensitive slabs. These should be easy to identify by watching where the wind is blowing and visibly loading slopes. Cracking in the snow around you will likely be your next clue. Wind patterns get pretty complex around Turnagain but suspect areas are upper elevation ridgelines and over rollovers on south, west and northerly aspects.  

Loose snow avalanches in areas out of the wind will be an additional concern. With the decrease in temperature overnight, these could run further and faster than they did yesterday. This issue pertains to below treeline elevations as well.

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

There remain two layers buried below the surface we still have our minds on:

First, a thin layer of weak snow that sits on a late January crust. This is now buried 2-6’ deep and we have not seen avalanche activity within this layer in the Turnagain region. This has been more of an issue in the Girdwood Valley where avalanche activity has been seen and possibly in other surrounding locations (i.e., Placer Valley and 20-mile).

Second, the weak snow near the ground from our early season. This is the deep slab problem we have not been talking about much lately. It has gone dormant for now in the Turnagain and Girdwood areas, but we are concerned with the possibility of it coming back to life with a large multi-day storm cycle and/or in the spring. However, in shallower snow pack areas, such as Summit Lake, it is still more up front in our minds. It looks as though the Carter Lake avalanche mentioned above broke in the weak snow near the ground. Hopefully we will get more information on this today.

Weather
Tue, February 19th, 2013

Once again our break between storms is short lived. Yesterday’s mostly clear skies, light northerly winds and temps in the teens are being replaced by another system headed this way. Winds have already shifted to the east overnight but are still light (~10mph).  These will pick up through the day into the 20-40mph range. Temperature is on rise at the ridgetops, going from the single digits to the mid-teens. An inversion has set in at the lower elevations where temps are hovering around 10F. Clouds may hold back enough for good visibility for the first part of the day but look to fill the skies by this afternoon.

Tonight and into tomorrow another quick hitting system will move through. It looks as though we will pick up another 6-12 € with snow to sea level and strong east winds on the ridgelines. Stay tuned.


Kevin will issue the next advisory tomorrow morning, February 20th.

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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.