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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, December 26th, 2015 - 7:00AM
Expires
Sun, December 27th, 2015 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

After an intense period of heavy snowfall added 12-18″ of new snow overnight, we have a HIGH avalanche danger above treeline this morning and  a CONSIDERABLE  avalanche danger at treeline. A MODERATE danger exists below 1,000′ where avalanches occurring from above may run. The new snow fell in conjunction with strong Easterly winds and was deposited on a very weak pre-existing surface. Natural avalanches 1-3′ thick have likely been occurring through the night and may persist today.  HUMAN TRIGGERED AVALANCHES 1-3′ thick WILL BE LIKELY today on slopes steeper than 30 degrees.  

**Travel in avalanche terrain is not recommended until the snowpack has had time to adjust. Choose mellow terrain and avoid all runnout zones. If skies partially clear, the Alpine avalanche danger will trend to CONSIDERABLE, yet patience is still warranted due to a slow to stabilize snowpack.  

Sat, December 26th, 2015
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
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

Today will be a tricky day in the backcountry: It’s Saturday after Christmas, many folks may be itching to get out and snowmachine, ski, snowboard, etc., we just had 12-18″ of new snow and…. skies may clear up. The problem is, and the tricky part, will be keeping our terrain choices in check since HUMAN TRIGGERED AVALANCHES ARE LIKELY. Not only are we just hours away from a ‘rapid loading’ event, but all this new snow fell, with wind loading, onto a weak surface. Hence, bonding between the new and old snow is not expected to be quick. Reports from yesterday, just as the storm was ramping up, were that avalanche conditions were rapidly increasing and shooting cracks were seen at treeline. 

Storm snow totals (10am yesterday to 6am this morning – rain fell before 800′):
Turnagain Pass:  15″ snow at 1,880′ and likely 16-18″ at 2,500′ (higher amounts on North end of Pass and lesser amounts on the South end)
Girdwood Valley:  ~ 12″ snow at 1,700′, likely more at the higher elevations
Summit Lake:  Only 1″ of new snow…

Avalanche problems expected today:

WIND SLABS: With strong Easterly wind overnight and moderate Easterly winds remaining through today, we can expect windloaded slopes to be prime to avalanche. Triggering a wind slab 1-3′ thick is likely and these may be triggered remotely from the ridgeline.

STORM SLABS: Temperatures increased between 10-15 degrees F during snowfall. This is a perfect set up for an “upside-down” storm; depositing denser snow on top of less dense snow, creating a slab avalanche problem in areas sheltered from the wind.

LOOSE SNOW AVALANCHES: Sluffs due to heavy snowfall likely occurred last night and again will be likely today. These have the ability to trigger a slab avalanche on their way down.

What this means is very conservative choices are necessary for a safe day, and a safe Christmas weekend, in the backcountry. Choosing to ride your snowmachine in the flats and skiing/snowboarding on slopes 30 degrees or less is recommended. If skies do clear today, or tomorrow for that matter, very careful snowpack assessment is needed before venturing into the steeper and avalanche prone terrain. 

 

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

Cornices are expected to have grown and be teetering on the brink of breaking today. Some of these likely broke off during the night, which in turn is one of the main triggers of naturally occurring avalanches. These may break naturally today, triggering an avalanche below or easily triggered by people on a ridgeline.

For a look at the rapid loading overnight watch this loop of the Turnagain Pass DOT weather station snow stake! Photo is a still image from 2:42am on December 26th.

Weather
Sat, December 26th, 2015

Light snowfall began yesterday morning along with moderate winds from the East along the ridgetops. Late in the day, snowfall intensified, temperatures warmed ~10F and the Easterly winds began to cranked (averages in the 30 and 40’smph). Additionally, the rain snow line climbed from sea level to ~800′ during the day. Snowfall totals at the mid-elevations are in the chart below.

Today, the brunt of the storm is through and we will have intermittent snow showers and partly cloudy skies throughout the region. In favored areas another 2-4″ of snow is possible. The Easterly winds are expected to be 15-25mph on the ridgetops and temperatures in the low 20’s F above treeline and near 30F at 1,000′.

Another intense Pacific storm is on track for Monday. Weather models are showing this system bringing the warmest air we have seen yet this season; along with intense precipitation and wind, we should see a rain/snow line between 1,000-2,000′.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 26   15   1.2   62  
Summit Lake (1400′) 28   1   0.1   17  
Alyeska Mid (1700′) 27   12   0.83   44  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19    NE 33    62
Seattle Ridge (2400′) 22   SE   24    55
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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.