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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, February 19th, 2015 - 7:00AM
Expires
Fri, February 20th, 2015 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains CONSIDERABLE at the higher elevations above 2,000′ where very weak snow sits under a hard slab 1-3+’ thick. Because of this poor snowpack structure, human triggered large slab avalanches 1-3′ deep continue to be likely on steep slopes (> 35 degrees) and may pull back further than expected. Triggering an avalanche remotely, from a ridge or from below for example, is possible. Signs of instability such as collapsing/whoomphing may not be present until it’s too late.  

The avalanche danger is MODERATE below 2,000′ where the moist snowpack is re-freezing and the greatest hazard is an avalanche from above running into this zone.

*Sticking to lower angle terrain is again the best strategy for a safe day in the backcountry.

Thu, February 19th, 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. Dangerous avalanche conditions. 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

The last two days have brought warm, sunny spring-like conditions to Turnagain Pass and today will be much the same. After several human triggered avalanches on Tuesday, there were no reported slides yesterday – however, very few people were out. Although the weather is inviting, the snowpack is not. A very poor structure exists currently and is one that will likely persist for the foreseeable future. The set-up is simple: a hard slab 1-3′ thick over weak faceted (sugary) snow on top of a hard bed surface. This is textbook for human triggered slab avalanches. 

The slab on top of the weak layer is quite variable due to the strong winds during Monday’s storm. There are portions of terrain that have been scoured to the point of uncovering old tracks (anti-tracks) and areas where a slab thickens from 0 to 30″ over just 10′. These are extreme examples, but my point is – sussing out trigger points and how one triggers an avalanche today. Thin areas of the slab, roughly 1′ or less, are the most likely places for a person to trigger an avalanche. The thicker areas of the slab will take a group of people near each other, such as a large party congregated on a ridgeline, something we call large triggers. Large triggers also include snowmachines – yet our unprecedented lack of snow has kept motorized use closed.

Not only is sussing out trigger points a hard thing to do and a bit too much to gamble on right now, but another thing to consider is the potential lack of obvious signs of instability. Collapsing or whoomphing was widespread two days ago, but less so yesterday. Don’t let an absence of these signs lure you into thinking the snowpack is stable until it is too late. All you have to do is dig in a couple feet and you’ll see for yourself the weak faceted snow that is plaguing the mountains. 

Photo and Video – Textbook slab avalanche ingredients:

Windloaded slopes vs. scoured slopes. Avoiding steep windloaded areas is a good strategy right now.

Weather
Thu, February 19th, 2015

A nice day was had in the backcountry yesterday with mostly sunny skies and light Northerly ridgetop winds. Temperatures were in the low to mid 20’s F on the ridgetops and in the low 30’s at 1,000′.

Today looks to be similar – another warm spring like day – with clear skies and light variable ridgetops winds. Ridgetop temperatures should be in the low 20’s F with valley bottoms in the low 30’s F.  

Looking ahead to Friday through Sunday, another juicy North Pacific storm is on tap. Once again this low pressure will pump very warm air from the South along with the moisture. Models are showing a rain/snow line around 2,000-2,500′ at this point with accumulated precipitation in the 1-1.5″ range beginning Friday afternoon through Sunday.

*There is no wind information for both Sunburst and Seattle Ridge. The next closest stations near Turnagain Pass are Max’s Mountain (Alyeska) and Penguin Peak (above Bird) and Fresno Ridge down near Summit Lake. We will be fixing these as soon as possible!

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29    0    0 43  
Summit Lake (1400′) 24   0  0 8  
Alyeska Mid (1700′) 29   0    0 26  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  21 Wind   sensor destroyed  
Seattle Ridge (2400′) 25   Wind   sensor is   rimed  
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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.