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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, January 20th, 2013 - 7:00AM
Expires
Mon, January 21st, 2013 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

With the expected addition of new snow and continued wind over our region today the avalanche danger should rise to CONSIDERABLE above treeline. Upper elevation slopes may see up to a foot or more of new snow by this afternoon which will bring the threat of wind slab and storm snow avalanches. Expect these to be easily triggered by a person on slopes 35 degrees and steeper and in the 1-2′ deep range. There also remains the possibility a deeper and much larger avalanche could be triggered by a failure in the weak snow near the ground. A MODERATE danger will be found below treeline and in areas receiving less than a foot of new snow.

Sun, January 20th, 2013
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
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
  • 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

With a few inches of snow overnight, plus the addition of 4-8” at treeline today, the upper elevation slopes could see a total of a foot of new snow by this afternoon. There are two main things that will keep this new snow from bonding quickly: (1) The storm will be “upside-down” – due to a rise in temperature with snowfall. This creates a situation where more dense snow is deposited over less dense snow forming a slab/weak layer combo. And (2), there are several slopes that received enough wind over the past few days to create a hard wind crust/slab that will act as a bed surface for this upside-down storm to fall on. A few of these hard surfaces had a couple inches of weak snow on top that will also help to keep the new snow from bonding right away.

In addition to the Storm Snow concerns we also have Wind Slab avalanches. East winds have ramped up overnight and should continue to be in the moderate to strong range on the ridgelines. Even if snow totals do not amount to much there is still plenty of snow available for transport to form 1-2’ deep wind slabs. These will likely be soft and easy to trigger.

Watching for all the obvious signs of instability will be key today. These are:
Recent avalanches
Shooting cracks, whoomphing and collapsing in the new snow
Heavy snowfall
Wind loading

Below treeline we will likely see wet snow falling on a crust. Watch for any new snow in these areas to have a hard time sticking to the crust as there will be 2” of weak snow sandwiched in between. This weak snow is the few inches that fell on Wednesday which began to facet with the cold temperatures Thurs/Fri. We may not accumulate enough snow at these elevations for it to become a problem but something keep in mind.

Avalanche Problem 2
  • 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

With warmer temperatures, the addition of a foot or so of snow (~1” of water equivalent ) and strong wind there is the possibility of adding enough load on certain slopes to wake up a deep slab here or there. This storm does not look big enough to induce a deep slab avalanche cycle similar to last Sunday/Monday, but we can’t forget the possibility remains that one of these larger slides could occur. The 4-8” of well developed faceted snow near the ground from our early season is still weak enough and present in our start zones to warrant concern.

Weather
Sun, January 20th, 2013

Cloudy skies gave way to light snowfall yesterday as a low pressure system is pushing a warm front over our region currently. We have seen around 3″ of snow overnight on Turnagain Pass, 4″ at Girdwood Valley mid elevations and 2-3″ down at Summit Lake. Temperatures have warmed to the mid-upper 30’s at sea level, where there is currently rain/snow mix, and mid 20’s F on the ridgelines. Winds have increased from the east averaging 35mph overnight and gusting over 60mph on the peaks.

Today we can expect another 4-8 € of snowfall with the rain/snow line creeping up to 800′. The rise in temperature should allow ridgetops to get as warm as the upper 20’s by this evening. The strong east winds look to slowly taper off and blow around 25-30mph with gusts near 50mph through the day.

Tomorrow we should see a break in the precipitation and wind with another shot for Tuesday.


Fitz will issue the next advisory tomorrow morning, January 21st.

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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.