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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, January 1st, 2014 - 7:00AM
Expires
Thu, January 2nd, 2014 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

A warm winter storm overnight will ring in the New Year with CONSIDERABLE avalanche danger today where human triggered avalanches will be likely.   Wind slab and storm slab avalanches will be of great concern throughout the day as this new weight overloads buried weak layers.   Cautious route finding and conservative decision-making will be essential if travelling in avalanche terrain.    

Wed, January 1st, 2014
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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
  • 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

This latest storm to hit eastern Turnagain Arm is the first substantial shock to our snowpack this season.  With .9 inches of snow water equivalent (SWE) and 90+ mph gusts at ridge top weather stations in the Turnagain pass area, rapid loading will create dangerous avalanche conditions.  Above treeline, wind slab avalanches up to 3’ deep (unmanageable for a skier or snowmachiner) are likely to be triggered by a human today.  In areas below treeline there is some uncertainty as to how reactive this storm snow will be in areas protected from the wind.  Do not ignore obvious red flags today.  Recent avalanches, whoomphing of the snowpack and shooting cracks area all bulls eye clues of dangerous avalanche conditions!

                              Shooting cracks: An obvious red flag!

Of note, the Girdwood Valley looks to have received significantly more water weight that Turnagain Pass with Alyeska’s weather station (at 2800 feet) reading over 2” of water in the last 18 hours since the storm began.  

 

Avalanche Problem 2
  • 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

Our snowpack to date is comprised of weak faceted snow above the ground, a series of crust/ facet combinations and a 12-20” slab that encompasses the entirety of December’s meager snowfall. This weak foundation of snow that has continued to prove reactive, just received a substantial shock with last nights storm over a short timeframe.  Like a hung-over NYE partygoer, the mountains will be grumpy and unstable today.  It is very likely that the December drizzle crust/ facet combination will either be overloaded by new snow and wind or, brought dangerously close to its tipping point today.  It will be wise to avoid avalanche terrain and let the mountains adjust accordingly over the next 24-36 hours.  

Weather
Wed, January 1st, 2014

Don’t let the relative lack of weather in Anchorage fool you.   Yesterday was an active day in the eastern Turnagain Arm region.   Above freezing temperatures and rain at sea level transitioned to a rain/snow mix at 1,000 feet during the day light hours.   After dark Easterly winds began to pick up in earnest with the Sunburst weather station registering a 93mph gust at 6PM.   As of 6AM this morning we have about 8 € of heavy, wet snow on the ground at Turnagain Pass.   This should bode well in weighting those pesky alder down!

Today we can expect continued unsettled weather as this latest storm dissipates.   Winds will be moderate out of the southeast and temperatures look to be similar to yesterday with rain at sea level, gradually cooling with elevation.   Another 2-8 € of heavy, wet snow is expected above about 600′.

Some December stats below:

We end 2013 with a  SWE  that is only 20% of average for December.   With yesterday’s precip we narrowly missed the podium ending the month with 3.3 € of water for the month, good enough for 4th place. Numbers below are from the Turnagain Pass SNOTEL Station (1880′).

Top 5 lowest December SWE from 1983 till 2013:

1st    1984 €“ 0.5 €
2nd    2011 €“ 1.3 €
3rd    1985 €“ 3.1 €
4th   2013 €“ 3.3 €
5th   2009 €“ 4.2 €

Average December SWE is 16.2 €

Snow Depth:  
Dec 31 2013 at midnight was 30 €
Average for past 10 years is 64 € (Data only goes back to 2004)

*The above numbers were calculated with  SNOTEL data from the NRCS.

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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.