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Issued
Wed, February 25th, 2015 - 7:00AM
Expires
Thu, February 26th, 2015 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

Today in the Alpine the avalanche danger is CONSIDERABLE where a slab 1-3′ thick if triggered is likely to have high consequences. It is in the Apline where it will be important to avoid large slopes steeper than 35 °. Below 2500′ the danger is MODERATE where an avalanche if triggered from above could run into this elevation band.

Cautious route-finding and conservative decision making should be at the forefront of our minds today, especially if being tempted by sucker tracks. This is the kind of avalanche problem where a person may ski/ride a steeper slope and nothing happens, but the 2nd or 3rd person could trigger a large avalanche.  

Wed, February 25th, 2015
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
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
  • 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

Yesterday was the first day of clear sunny weather following a warm and wet weekend of rain to 2500’ and 3-6” of new snow at higher elevations. Cooler temperatures have created a crust layer along the surface that is adding strength to the snowpack at mid elevations. Above 2700’ where this surface crust does not exist a softer slab 1-3’ thick is sitting on a widespread layer of facets.

This new snow looks very much like the classic Maritime snow we so often associate with stable conditions, but lurking below is an already stressed weak layer + thick slab that formed on Feb.16.

We currently don’t have a lot of information about how much force it will take to trigger an avalanche in the upper elevations. What we do know is that the weak layer is widespread throughout the region and test pit scores are showing moderate strength with high propagation potential. Meaning – if a slab is triggered an avalanche of its size is likely to have high consequences. This is why the danger for the Alpine will remain at CONSIDERABLE for today.

As we move away from this most recent loading event obvious signs like collapsing and shooting cracks will become less common. This could be deceiving because this also means that the weak layer will become harder and harder to trigger yet the consequences will remain high for awhile. This is a good reminder to be patient and stay off of steep slopes until conditions become more stable. 

This is a picture of the surface crust at 2000′ on Sunburst yesterday. This crust gradally gets thinner as you gain elevation and disappears completely at 2700′. 

Weather
Wed, February 25th, 2015

Yesterday morning a trace of new snow fell at Turnagain Pass, just enough to cover the bare ground for a few hours before mid day temperatures (low 30’s F) melted most of it away. At higher elevations temperatures stayed in the mid 20’s F and North winds averaged 5-15mph along ridgetops. Skies were sunny and with thin cloud cover later in the day.

Overnight temperatures remained in the low to mid 20’s F. Ridgetop winds were from the North and averaged 5-15 mph. No new precipitation was recorded.

Today light cloud cover and patchy fog will become mostly clear by this afternoon. Temperatures will be in the 20’s F and are likely to increase into the low 30’s F mid day with sunny skies. Ridgetop winds should remain light to moderate from the North, but winds are expected to increase near Whittier throughout the day into the evening.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29   trace .01 40  
Summit Lake (1400′) 29   0   0   6  
Alyeska Mid (1700′) 30   trace .04 23  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 21   N/A   N/A   N/A  
Seattle Ridge (2400′) 24   NNW   9   21  
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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.