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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, February 23rd, 2016 - 7:00AM
Expires
Wed, February 24th, 2016 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

A CONSIDERABLE avalanche danger exists today in the alpine where fresh wind slabs will be of concern after a potent storm delivered 3-4+ feet of new snow in the alpine between Saturday and Sunday.   Cautious route finding and conservative decision-making will be essential today if venturing into the alpine.   Today will warrant hyper-vigilance if moving around cornices.   These are under tremendous stress right now in the wake of our most recent storm and will be even more so with a bump in easterly winds today.    

At treeline the danger is MODERATE where glide cracks are lurking under this most recent dump of snow.   Though many are not currently visible, make no mistake; these cracks litter the Turnagain pass zone between 1500 €“ 2500 feet.

Special Announcements

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Tue, February 23rd, 2016
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
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

A quick tour to treeline on Tincan yesterday found significant avalanche carnage from the much publicized weekend storm.  Unfortunate for surface conditions, we found a breakable crust entombing 2 feet of settled storm snow up to 2,000 feet.  This crust appeared on the tail end of the storm as temperatures dropped Sunday night, and is acting to promote more stable snow in the mid-elevations (at treeline).  In the alpine however, soft, surface snow is abundant and will be ripe for transport today with winds forecasted in the 20 to 45 mph range.  Fresh wind slabs are likely to form on south and west aspects and may be 1-3 feet deep.  If travelling in the alpine, pay attention to changing surface conditions and drum-like or hollow sounding snow as this is indicative that you’ve found a wind slab. 

With limited visibility yesterday and no observations from the alpine since before the quick-hitting weekend storm, confidence is low that the storm snow is settling without incident.  Careful snowpack evaluation will be essential today if moving through the upper elevations to ensure you are managing terrain appropriately.

Storm slab/ wind slab just lookers left of Tincan proper looks to have avalanched at the tail end of the weekend storm.

Avalanche Problem 2
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Again, visibility hampered a good look into alpine start zones yesterday but given the state of cornices (large and mature) before this storm, you can bet that 4+ inches of water weight from the weekend and winds averaging in the 30’s mph have brought these backcountry bombs ever closer to their tipping point.  Avoid time spent below cornices and keep a healthy distance (and then some) away from the edge if travelling along a corniced ridge today.  Cornices now are as large as we’ve seen all season and have a tendency to break much further back than one might anticipate.  Given the sheer size and weight of any cornice failure today, it’s quite possible a cornice fall will initiate a slab on the slope below.  

Additional Concern
  • Glide Avalanches
    Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
More info at Avalanche.org

I spotted one new glide crack that appeared to have released mid-storm and maybe a hundred others that ‘disappeared’ during the storm.  Unfortunately glides are going to be difficult to see right now due to last weekends fresh snow, but make no mistake, these cracks are still scattered about in the backcountry, mainly in the 1500 – 2500’ elevation band in popular ski/ snowmachine terrain such as Seattle ridge, Main bowl, Tincan and others.  Glide cracks are growing larger by the day but still prove very difficult to forecast for.  Your best bet continues to be to map out where these cracks exist and limit you exposure time below. 

Pre-storm and post-storm photo of Seattle ridge from the motorized parking lot.  Notice the glide cracks in the upper image have all been buried with the subsequent storm.  Make no mistake, glide cracks do still exist and threaten a lot of well-travelled terrain on both the motorized and non-motorized side of the highway.

Weather
Tue, February 23rd, 2016

The short but intense storm that impacted south central Alaska wrapped up on Sunday night after depositing 2-4.5 inches of water weight from Summit Lake to Girdwood.   Yesterday, the Turnagain pass area was under a showery regime most of the day with just a few short breaks where the sun managed to poke through.   1-3 € of new snow fell throughout the day as temperatures hovered right around 32F at 1,000 feet.   Winds were out of the NE in the teens and gusting to 30’s mph at ridgetops.  

Today we can expect ridgetop winds to increase in the 25-45mph range from the East.   Temperatures will be in the mid-30’s at 1,000′ promoting 2-4 inches of snow below about 1,300 feet.  

Yet another warm front spinning off a Gulf of Alaska low is expected to impact our area overnight tonight and through the remainder of the workweek, bringing warmer temperatures and more unsettled weather.  Stay tuned!

PRECIPITATION 24-hour data (6am – 6am)

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  30  3 .3   121  
Summit Lake (1400′) 29    3 .4  40
Alyeska Mid (1700′)  32  2 .2    98

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  22 ENE    18 45  
Seattle Ridge (2400′)  25 n/a * n/a * n/a *

*Seattle ridge anemometer (wind) appears to be rimed up.

Observations
Recent Observations for Turnagain Pass
Date Region Location
11/16/23 Turnagain Observation: Tincan Common
11/14/23 Turnagain Observation: Sunburst/Magnum
11/14/23 Turnagain Observation: Seattle Ridge
11/13/23 Turnagain Avalanche: Tincan Trees
11/12/23 Turnagain Observation: Sunburst
11/12/23 Turnagain Avalanche: Goldpan – avalanche
11/11/23 Turnagain Observation: Tincan Common
11/11/23 Turnagain Observation: Taylor Pass – Sunburst
11/10/23 Turnagain Observation: Tincan
11/10/23 Turnagain Observation: Tincan Trees
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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.