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Tue, January 8th, 2013 - 7:00AM
Wed, January 9th, 2013 - 7:00AM
Wendy Wagner
Avalanche risk The Bottom Line

A CONSIDERABLE avalanche danger exists above treeline today where human triggered avalanches are likely on slopes steeper than 35 degrees. We have a complex avalanche problem in the backcountry and any avalanche triggered today has the potential to be large and dangerous. Below treeline the danger is MODERATE where the likelihood is lower but the possibility remains for triggering a slide.

Tue, January 8th, 2013
Above 2,500'
3 - Considerable
Avalanche risk
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
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
More info at Avalanche.org

It seems we are on the doorstep for clearing skies and a break in the past 15 days of stormy weather. If we don’t see the sun this afternoon we should by tomorrow morning. The good visibility should allow for travel out of the trees and into an enticing couple feet of fresh powder, which brings us to our primary concern – triggering a deep and dangerous slab avalanche.

To recap, we have had 122+” of snow fall in just over two weeks on very weak faceted snow formed in October and November. Large natural avalanches have been widespread along with one human triggered avalanche during these two weeks – all failing in the weak early season snow. Two deep slabs were pulled out yesterday with explosives and the low visibility kept any information hidden as to larger avalanches triggered by the 2-3’ of load added the past two days. The weak layer is now 4-8+’ deep and rarely giving us clues as to its instability – until it fails catastrophically. There is a lot of uncertainty as to how hard these deep slabs are to trigger at this point and we have very little information from above treeline. That said, these are potentially unsurvivable slides and warrant respect.

As we go into this clear period we cannot forget about this deeper weakness. Keeping terrain choices conservative, steering clear of large open committing slopes and slopes steeper than 35 degrees is recommended. Watching for and avoiding trigger points is also key – check out John’s great write up on that HERE. It is very easy to focus on the light powder, new wind slabs and forget about the dragon lurking below. This snowpack deserves patience.

Avalanche Problem 2
  • 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 winds that have been blowing out of the east have died down overnight and should switch to a northwest direction today. There are likely a few lingering wind slabs from yesterday that have not stabilized yet to watch out for. These could be in the 1-4’ deep range. Also, due to the shift in wind watch for fresh slabs on easterly slopes (i.e., Seattle Ridge). Keeping an eye out for stiffer feeling snow as well as cracking and collapsing in the new snow are good clues as to finding a wind slab. There is also the possibility a wind slab could step down and trigger a deeper slab so be aware of the terrain you are in and how committing it is.

Tue, January 8th, 2013

Snow totals at Turnagain Pass in the last 48 hours are 22 €, the past 24 hours has seen roughly 8-10 €. Expect higher amounts above treeline. Winds during the snowfall have been easterly averaging 20-30mph, gusting to the upper 40’s during this period while temperatures have decreased a few degrees and are near freezing at sea level this morning.

Today snow showers will continue to taper off and skies begin to clear as the low pressure it the Gulf moves east. There is a chance for an additional trace-2 €. Winds have died down dramatically overnight and will shift to the NW through the day picking up to 20mph on the ridges by this afternoon. Temperatures look to remain in the low 20’s F above treeline and low 30’s below.

A short lived ridge moving over us tomorrow will bring a break in the weather and our first dose of sunshine since just before Christmas Eve. An unsettled westerly flow will bump us back in the clouds and snow for Thursday and through the weekend.

Kevin will issue the next advisory on Wednesday, January 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.