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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, December 25th, 2014 - 7:00AM
Expires
Fri, December 26th, 2014 - 7:00AM
Forecaster
John Fitzgerald
Avalanche risk The Bottom Line

The avalanche danger is MODERATE in the Alpine elevations today.   Dense slabs 2-3′ in depth could be triggered in steep terrain.   These slabs have the potential to propagate across large areas and injure or bury a person.

Pockets of wind slab up to 10″ deep will be sensitive to human triggers in upper elevation leeward starting zones today.

The danger is LOW at the Treeline elevations.   Despite this it will be important to pay attention to any groups above, as human triggered avalanches starting in the upper elevations have the potential to run down into this elevation band.

Below treeline (1,000′) there is not enough snow coverage to warrant a danger rating.

Special Announcements

All of us at the avalanche center want to wish everyone a Merry Christmas and a happy and safe holiday season.   We are grateful for all of the support in making our operation successful.   Thank you for your generous contributions, observations from the field and feedback, we couldn’t do it without you!

Thu, December 25th, 2014
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
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

It has been over a week since the last loading event placed 2-3’ slabs on top of a layer of surface hoar.  This layer was evenly distributed on Turnagain Pass from 1,500’ up to ridge tops.  Soon after this loading event a large avalanche was triggered by a skier resulting in a burial (and recovery).  Since that time we have not seen or received reports of any further avalanche activity.  

Despite the lack of activity, we are still left with a layer that is not trustworthy.  This layer of buried surface hoar has gained strength over the past week.  As it has gained strength it has also become more difficult to detect in our pits.  We are still finding it and it continues to show potential for propagation across slopes.  Add into the mix a slab that’s become more dense over time.  This slab is generally 2 feet thick in starting zones and has the ability to support a lot of weight.  While this may sound like a good thing, it can also be misleading in that you might be able to put many tracks on a slope without triggering an avalanche.  What our tests have shown, however, is that if you find the right trigger point and are able to initiate an avalanche, the outcome could be bad, as this slab + weak layer combo has the potential to produce high volume (relative to humans) avalanches.

During the last storm rain fell in the middle of the event which has now become a buried crust up to 2,500’ in elevation.  This rain and resulting crust has destroyed the buried surface hoar in the lower portion of the treeline elevations and added some strength to the slab in the upper portions of this elevation band.

What does this all boil down to?  Avoid steep terrain and trigger points above 2,500’.  Slopes over 35 degrees, convex rollovers, and areas where the snowpack is thinner are places where you could trigger an avalanche today.  Obvious signs like shooting cracks or whoompfing may or may not present themselves– this problem is better understood through investigating the snow below the surface.  Conservative terrain selection is the best way to manage this problem and have an enjoyable Christmas Day.

Avalanche Problem 2
  • Wind Slabs
    Wind Slabs
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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

Winds have picked up this morning along ridge tops with Seattle ridge showing wind speeds in the 25 mph range.  4-6” of low density snow sitting on the surface combined with the potential for another 5” to fall during the day could produce pockets of wind slab up to 10” in depth.  Due to the showery nature of the current local weather pattern, this will be a greater issue in areas receiving those 5”.  Be on the lookout for wind loading along ridge crests and in upper elevation starting zones.  Predominant wind direction will be out of the East and Southeast but know that wind direction will vary as wind interacts with terrain.  Pay attention to and avoid slopes that are actively being loaded by wind, have a rounded and pillowy look or produce shooting cracks as you step onto them.

Weather
Thu, December 25th, 2014

It looks like Santa was flying by braille last night, as a large band of low pressure moved through the region.   Light snow showers brought a trace of new snow to the Center Ridge SNOTEL and 2 € to the Alyeska Midway station.   Ridge top winds were light out of the East averaging 10 mph, making sleigh steering a breeze for the big man and his reindeer.   Temperatures remained in the 20s F at ridge tops and closer to the mid 30s F at rooftop level in the Girdwood Valley.

Ridge top winds have picked up this morning and are currently in the 20-25 mph range.   Today expect a continuation of snow showers as this large band of moisture continues to move through the area.   Snowfall amounts could be as much as 5 €.   Precipitation amounts will vary with this system-some areas could receive 5 € and other areas could receive only a trace.   Ridge top winds will be out of the East at 15-25mph.   Temperatures will be close to freezing (32F) at 1,000′.

The extended outlook is calling for clearing beginning tomorrow and lasting into Saturday.   A warm and wet pattern looks to potentially come our way as we head into the early part of next week.

* Seattle ridge wind data begins at 4am and is not a 24 hr average.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28  trace trace 28
Summit Lake (1400′) 25  2 .2  5
Alyeska Mid (1700′) 29  2 .2  23.4

RIDGETOP 24-hour data (6am – 6am)  

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  21  E  10  43
Seattle Ridge (2400′)  23  *SW  *28

 41

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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.