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Sun, February 10th, 2013 - 7:00AM
Mon, February 11th, 2013 - 7:00AM
Wendy Wagner
Avalanche risk The Bottom Line

A CONSIDERABLE avalanche danger exists above treeline this morning as yesterday’s storm slowly exits the Eastern Turnagain Arm area. Triggering a wind slab avalanche 1-2+’ deep formed during very strong east wind over the past 36 hours is the main concern. Additionally, watch for looming cornices that could be teetering on the brink. Below treeline there is a MODERATE danger where warm temperatures may loosen the snow surface and wet loose and wet slab avalanches could be possible.

Sun, February 10th, 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
  • 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

Yesterday’s storm turned out to be a Girdwood Special with storm totals above treeline between 1-2’ while Turnagain Pass picked up only half that, 6-12” depending on elevation. A hand full of natural avalanches were seen and reported despite low visibility. These were above treeline and mostly garden variety wind slabs in the small to medium category. Seattle Ridge’s cross-loaded gullies on the east face flushed out small slabs and sluffs but debris stopped before hitting the lower angle runnout zones. Below treeline no avalanche activity was seen as the new snow was quite warm and sticky.  Even though Summit Lake only squeaked out a few inches that area was still able to produce natural avalanches thanks to the strong NE winds.

Today, the strong east wind is on the decline but still blowing moderate which is enough to build slabs. Watch for these to be fairly touchy and just beginning the mending process. They most likely will be 1-2+ feet in depth and found off ridgelines, rollovers and cross-loaded in gullies and sub-ridges. If you see cracking in the snow around you it will be a bull’s eye clue that slab is unstable. With the potential for lifting clouds and travel in the upper elevations, remember even a small avalanche triggered can be dangerous if one gets washed into a terrain trap (i.e., over a cliff or into a gully).

Below treeline:  There is between 5-10″ of medium to dense powder on top of multiple crust layers. Yesterday the powder was sticking quite well to the crust, but if the dense snow becomes saturated today with rising temperatures and a creeping up of the rain/snow line, wet loose and wet slab avalanches will be possible above the crust.

Deep slab:  We are continuing to track the weak snow near the ground. This is buried 4-8’+ deep, gaining strength and not likely to be triggered by a person in many areas in the Turnagain Pass and Girdwood Valley. But that said, shallow areas remain in these regions and Summit Lake, being shallower in general,  harbors more of a concern. There are a couple great observations demonstrating these locations HERE and HERE.


Avalanche Problem 2
  • 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

With over 24 hours of warm temperatures, snow and strong east wind cornices continue to grow. A few of these broke off yesterday triggering shallow wind slabs below. Keeping an eye for what is above you and giving these guys a wide berth will be prudent as they could be close to the tipping point.

Sun, February 10th, 2013

Obscured skies, snow and strong wind that dominated Saturday has persisted overnight. Sunburst weather station at 3,800′ recorded hourly averages in the 40’s and 50’s mph from the east with gusts in the 90’s. Storm totals (with a rain/snow line ~300′) from the past 36 hours, ending this morning at 6AM are:

Turnagain Pass (1880′)         6″ snow – 0.6″ water eq.
Alyeska Mid (1700′)           18″ snow – 1.3″ water eq.
Alyeska Top (2800′)         18-24″ snow – 2.2″ water eq.
Summit Lake (1400′)         2-3″ snow – 0.2″ water eq.

Today the strong east ridgetop winds will back down significantly to the 20-30mph range and shift to a more south and southwesterly direction. Snowfall should linger off and on adding around 1-3 additional inches. Temperatures are mild, 32 at 1000′ and 23 at 3800′, and should decrease slightly through the day.

Tomorrow we have a break in clouds and precipitation. Temperatures should cool off and skies clear. Could be a very nice day before another system looks to push clouds and flurries our way Tuesday/Wednesday.

Fitz will issue the next advisory tomorrow morning, February 11th.

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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.