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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, February 9th, 2014 - 7:00AM
Expires
Mon, February 10th, 2014 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

Areas of CONSIDERABLE avalanche danger exist again today at and above treeline. Soft slab avalanches between 16-24″ deep may be triggered on slopes over 35 degrees. These areas are most pronounced on South, West and Northerly aspects as well as cross loaded terrain (such as the East face of Seattle Ridge). Friday’s new snow is sitting on a thin layer of either surface hoar or facets over a crust. Though the new snow was mostly loose and unconsolidated yesterday, it has likely settled into more of a ‘slab’ today. These avalanches could be triggered remotely. A MODERATE danger exists below treeline for soft slabs around a foot deep.

Sun, February 9th, 2014
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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

Recent Avalanche Activity:
There were several small to medium sized human triggered wind/storm slab avalanches yesterday. There was also evidence of a handful of natural slabs and sluffs occurring mid-storm on 2/7. Several observations were sent in with more detail on yesterday’s activity – check them out!

Sunburst

 

Tincan – At least 5, but up to 11, small storm/wind slab pockets remotely triggered by skiers. (See photos below)

  • 2 small soft slabs in Tincan Common Bowl (Kitchen Wall) remotely triggered from up track
  • 2 small soft slabs on lower south facing rolling terrain below up track, also thought to be remotely triggered
  • 1 small wind slab on CFR 
  • 1 larger soft slab in Hippy Bowl with several small slabs releasing below the lower rollover

Seattle Ridge

  • Several natural storm slabs and sluffing occurring mid-storm

Sunburst skier triggered soft slab (WSW, 3,650′). Thanks to the party for sending in this photo – check out their additional photos with a write up on triggering the slide HERE.

 

Tincan small soft slabs on the lower south facing rollovers below the common up track (SW facing, 2,500′). *Note the crack extending from one pocket to the other intersecting the ski track. (Photo: Mark Norquist)
 

 

Tincan’s Hippy Bowl larger soft slab (SW, 3,300′). Note the several pockets below the rollover. These are believed to be remotely triggered from above.

 

Today’s primary concern – Persisting storm/wind slabs:

There is much uncertainly with how the storm snow is bonding with the pre-existing surfaces. There were areas yesterday (i.e. Tincan) where the new snow was quite active and people were able to remotely trigger avalanches from over 100′ away. Yet on Sunburst, only one slide occurred even though many people were skiing/snowboarding. At elevations around 3,000′ and below surface hoar exists between the old hard crust and the storm snow. Additionally, above 3,000′ there is a facet/crust combination that exists below the storm snow (responsible for the Sunburst slide). The trick is the new snow was still unconsolidated enough yesterday that slides were mainly confined to small pockets. With settlement overnight allowing the new snow to become more of a slab over these persistent grain types, we could see remote triggering again today along with wider propagation and subsequent larger avalanche activity. 

Quick hand pits and keeping an eye out for cracking and collapsing are good ways to assess the new snow/old snow bonding. These clues are hard to get while on the up track so stepping off it now and then may tell you something. To avoid any avalanche issues today you can steer clear of slopes steeper than 30 degrees.

 

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
More info at Avalanche.org

Once again, we are mentioning the deep slab concern that exists at the upper most elevations (above 3-3,500′). This problem has shown signs of inactivity for the time being and the probability of triggering a deep slab is low yet, the consequences are high. It’s possible another load, or just the right trigger (cornice, a person on a thin spot or a large group of people), could initiate a failure in the weak snow at the base of the pack (basal facets) causing a large full-depth slide.

Below is a cross section of the pack at the site of the Sunburst avalanche mentioned above. You can see the basal facets sitting under the pack that are responsible for the lingering deep slab concern.

Weather
Sun, February 9th, 2014

Sunny skies prevailed yesterday. Temperatures hovered around 20F above treeline and the mid 20’sF at sea level. Winds were light from the Northwest on the ridgetops. Overnight,  VERY cold air has moved in and all elevations are reading in the single digits.

Storm totals from 2/7:
Turnagain Pass above treeline – 16-18″
Turnagain Pass below treeline – 12-16″
Summit Lake above treeline – 6-8″
Summit Lake below treeline – 3-5″

Today should be another blue bird day – but cold. Temperatures may warm up to the teens by mid-day above treeline but likely stay in the single digits at valley bottoms. Ridgetop winds look to slowly switch around to the east but remain light in the 5-10mph range.

Clear and cold weather is forecast to persist into the middle of the week with a chance for snow later in the week.

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.