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Fri, August 8th, 2014 - 7:00AM
Sat, August 9th, 2014 - 7:00AM
Heather Thamm
Avalanche risk The Bottom Line

Over the past 48 hours we have received 1.5 – 3 feet of heavy storm snow sitting on a very unstable weak layer of buried surface hoar at elevations above 2500 ft. This is our first buried weak layer of the season causing us to keep the avalanche danger at HIGH today in the Alpine. Large human triggered slab avalanches are likely on steep slopes on all aspects. The danger rating at treeline will remain CONSIDERABLE due to the run-out of these slide paths and the possibility of triggering a slide from below. Travel in avalanche terrain is not recommended. The safest travel will be in simple terrain without steeper slopes above.

Fri, August 8th, 2014
Above 2,500'
4 - High
Avalanche risk
3 - Considerable
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

Persistent slabs above 2500ft – 

A persistent slab problem requires three ingredients; a slab sitting on weak layer on top of a bed surface. We have all three! 18 – 30 inches of recent snow has fallen over the last 48hours, enough time for the new storm snow to become cohesive enough to form a slab. This slab is sitting on buried surface hoar, ranging in size from .5cm to 2cm long (3/16in– 13/16in). These collapsible crystal formations can remain intact under a large load of snow for weeks or until the right trigger comes along to produce an avalanche. The bed surface varies with elevation ranging from a rain crust below 3000ft to an old snow interface at higher elevations. Both surfaces are smooth and surface hoar was observed at all elevations as it was forming December 11 -13th before the storm arrived.

We were able to safely travel on low angle slopes to about 2700ft on sunburst yesterday during the storm where we observed widespread buried surface hoar in most places above 2000ft.  Visibility was very limited and no natural avalanches were observed. We preformed two extended column tests both with an easy score of ECTP7 (Q2) at 35cm (14”) deep. This is an indication of poor stability – The snow pack has poor structure combined with enough energy to propagate a large slab avalanche. Triggering a persistent slab is most likely in areas where the slab is thinner like around steep rocks or along the edges of a gulley. Triggering a persistent slab can be tricky and can give a false sense of security. Initiation is usually in places where the snowpack is thinner and where the buried surface hoar is still intact like below scoured ridgelines. A human could tip the balance and current snowpack is deep enough to connect an entire slope and could easily bury a person. 

Snow pit at 2700' on Sunburst West Ridge

3 -4mm Buried Surface Hoar

Avalanche Problem 2
  • Wet Loose
    Wet Loose
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
More info at Avalanche.org

Wet Avalanches were observed below 2500ft on steep terrain.Due to warm temperatures and moderate to heavy precipitation and snowfall loose wet avalanches are likely in mid elevations below 2500ft. Temperatures are expected to remain warm with the chance of rain/snow line dropping slightly throughout the day.


Fri, August 8th, 2014

Over the past 24 hours a large low-pressure system dumped ­ ­ ­ ­ ­ ­ ­1.2 inch of water on Turnagain Pass in the form of rain below 1500 feet and 1.5 – 3 feet of new snow above 2500ft. During the most intense part of the storm winds were in the mid 30- 40’s with gusts in the 60’s.

Overnight €“ Temperatures remained above freezing at lower elevations and rain/snow line remained around 1500ft. Precipitation continued throughout the night with .7 inches of water over the last 12 hours at Center Ridge SNOTEL, mostly in the form of slush and ridgetop winds backed off slightly averaging in the mid 20’s from the East.  

Today €“ Light precipitation is expected to continue throughout the day and Temperatures are expected to decrease slightly and could bring a mix of light rain and snow at lower elevations.   Easterly ridgetop winds are predicted to stay in the 20 €“ mid 30’s throughout the day.  

Wednesday into Thursday – A series of lows is stacked up and will continue to bring warm air and light precipitation over the next few days.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 33   1.2   1.2   30  
Summit Lake (1400′) 34   0   .4   4  
Alyeska Mid (1700′) 35   .4   1.99   12.5  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  27 E   27   54  
Seattle Ridge (2400′) 29   N/A   N/A   N/A  
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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.