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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, February 27th, 2013 - 7:00AM
Expires
Thu, February 28th, 2013 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE today on all slopes above and below treeline. New snow and strong wind continues to impact the mountains in the Eastern Turnagain Arm region. Natural avalanches are possible while human triggered avalanches are likely. During the past 48 hours we have seen between 1-2′ of snow with more expected today. Wind slab avalanches 1-3′ deep will most likely affect the near and above treeline elevations. Below treeline, the potential exists for avalanches to fail in mid-pack weak layers creating larger slides.

Safe travel in the backcountry will require conservative terrain choices and expert level snow assessment skills if entering avalanche terrain.

Special Announcements

Placer/Skookum and 20 mile are now OPEN to snowmachines, with adequate snow cover in those areas.   Watch for areas of thin snow cover, open water, and other hazards that you won’t find at higher elevations.   The avalanche center staff have not been to this zone yet, so we can only speculate on the snowpack at this time.

AK DOT avalanche hazard reduction work:   There will be intermittent road closures at the Sterling Wye on the Seward highway.   Motorists should expect delays of up to 45 minutes between 11:00 AM and 2:00 PM. See 511.alaska.gov for updates.

Wed, February 27th, 2013
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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

Today’s main concern will be wind and storm slab avalanches formed by the new snow. These are expected to be 1-3′ deep, possibly releasing naturally, and not something to mess with. For the past two days the new snow has been deposited in rapid loading events. One occurred Monday night, another last night and a third is on the way today. The strong east winds have been sustained through these loading events and blowing to lower elevations. Avalanche activity is expected to be a bit more active than yesterday (see below). Once the wind and snowfall lessen, the mild temperatures will act to stabilize the new snow – but we should be in the thick of it today.

Yesterday’s avalanche activity:
We saw a handful of natural avalanches yesterday including this one from southwest facing Sunburst pictured below. There were also several artillery triggered avalanches by the AK DOT. These were all confined to the 10-12” of new snow from Monday night. More details HERE.

 

Additional concerns:

Cornices deserve a wide berth as they have been building steadily recently and breaking fairly far back.

Loose snow avalanches are expected in the dryer snow above treeline as well as damp point release avalanches below treeline.

 

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 additional load of new snow (1 to 2” of water weight) onto the pack has the potential to reawaken buried weak layers. The mid and lower elevations harbor various facet/crust combos (weak snow on top of a crust) that so far seem to be pretty stubborn to trigger. The most concerning combo is at the lower elevations, below 1,500’ where it only sits about 2’ deep. Another one is the late January combo which is 3-6’ deep and reaches up to 3,000’.

With regard to our old friend the deep slab, we received a report of collapsing in the weak early season snow yesterday in the Summit Lake region. The interior Kenai has a shallower snowpack and triggering a slide breaking at the ground in this area is not out of the question. This weak layer may be 2 months old but a snowmachiner did trigger it 9 days ago.

Weather
Wed, February 27th, 2013

Snow totals from Monday night’s storm turned out to be a bit more than forecast in the Turnagain area. We saw 10-12 € above treeline at Turnagain Pass and 8 € in the parking lots yesterday (Tuesday) morning. Snowfall gave way to clearing skies for an hour or two before firing back up in the evening. Winds on the other hand didn’t take a break. These have been relentless from the east for two days now averaging 20-35mph with gusts in the 50’s. Temperatures have been holding steady with ridgetops near 20F while sea levels are hovering in the mid 30’s. We have been at a rain/snow mix at sea level with moist snow at 1,000′.

Today, another round of snow and wind is moving through. As of 6am this morning Center Ridge SNOTEL  at 1,900′ is reporting 7 € of medium density snow overnight with another 4-8 € expected today. Wind will remain from an easterly direction with averages near 20-30mph and gusts to 50. Temperatures look to stay near 20F on the ridgelines and the mid 30’s at sea level. A rain/snow mix should continue at sea level.

Thursday intermittent snow showers and partly clearing skies should be over us as this system heads out.

 Graham will issue the next advisory tomorrow morning, February 28th.

Observations
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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.
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