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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, November 24th, 2013 - 7:00AM
Expires
Mon, November 25th, 2013 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

We have a MODERATE avalanche danger above treeline on all aspects today. Human triggered slab avalanches 6-20+” deep will be possible on slopes over 35 degrees. The shallower slabs will likely be fresh wind slabs created by today’s wind and few inches of snow. The thicker, and more dangerous yet harder to trigger, slabs will be those breaking in the old weak snow underneath the recent 12″ from Nov 21st.

Below treeline the danger is LOW  where a crust caps the snowpack.

The next advisory will be on Tuesday November 26th.  Tomorrow’s avalanche concerns will be similar to those today.  

Sun, November 24th, 2013
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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

We heard of one human triggered avalanche yesterday. This was in Turnagain Pass on a SSW facing slope of Sunburst. It was triggered by a skier that was ski cutting the slope. The avalanche ran to the valley floor – around 1,000ft. A photo of the crown is below. A few more details HERE.


                                                                                                                            Photo: Corky Still

It has been just over two days now since we received a foot of heavy snow on top of a 10″ thick layer of faceted snow. There was widespread avalanching during and right after the storm on Friday along with one skier triggered avalanche on Tincan. The culprit weak layer was the Nov 10 faceted snow. Facets are a common persistent weak layer and hence the use of the ‘persistent slab’ icon above. From the report on Sunburst, it sounds like this was a classic case of a wind slab over facets.

Image below is the Tincan skier triggered slide from Friday, Nov 22nd. (more details with the above link)

Though the snowpack is stabilizing, it is doing so slowly. The addition of 1-3″ of new snow should not tip the balance but any recent wind loading could. For anyone getting out today onto the steeper slopes, I’d suggest having your escape route planned in case the slab releases. Steering clear of any recent wind deposited snow will be a good bet as well. The poor structure is there in most locations above treeline – as you can see by the snow profile below – and though the slab is slowly deteriorating, making it harder to trigger, it is still there for now.

 

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 are slated to kick back up today from the east/southeast – blowing moderate (15-30mph). This, combined with a possible 3″ of new snow as well as existing snow to transport, should be enough factors to form fresh wind slabs. These likely will be shallow and sitting on a variety of surfaces. As always, be suspect of areas with wind deposited snow. These often have the tell-tale signs of a smooth rounded surface and hollow feeling. 

Weather
Sun, November 24th, 2013

A somewhat weak weather system will be moving over us today from the southeast. Cloud cover has set in and between a trace to an inch of snow has fallen as of 6am this morning. Temperatures have increased overnight with mountain tops approaching 20F and sea level areas are in the low 30’s. Winds have increased from the east/southeast and are blowing ~15mph with gusts to 30mph.  

Today we are expecting 1-3″ of light snow (up to .3″ water) above treeline. Temperatures should remain in the mid 20’s at treeline and winds continue to blow in the 15mph range with gusts to 30 from the east. Basically, a mild snow/weather day with limited visibility. A chance for freezing rain exists at sea level.

Monday, we should see skies clear up as we have a break between systems. Winds should continue from the east but settle down and temperatures remain mild in the 20’s.

Observations
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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.
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