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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, February 16th, 2020 - 7:00AM
Expires
Mon, February 17th, 2020 - 7:00AM
Forecaster
Ryan Van Luit
Avalanche risk The Bottom Line

The avalanche danger is MODERATE at all elevations. It’s possible for a human to trigger a large slab avalanche 1-4′ below the snow surface. Although the likelihood of triggering these slab avalanches is slowly decreasing, the consequences could be deadly. Conservative terrain choices and decision making are necessary to limit the danger. Additionally, cornice breaks remain a concern, and if triggered, could initiate deeper layers.

Sun, February 16th, 2020
Alpine
Above 2,500'
2 - Moderate
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. 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
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.

Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

Today the weather forecast calls for sunny skies, temperatures in the teens and calm winds.

Right now, various weak layers ranging from 1-4′ deep in the snowpack are concerns at all elevations, and on all aspects.  Earlier this week, additional stress from storm snow and wind loading events tipped the balance of our precarious snowpack.  We had many reports of natural and human triggered avalanches throughout the advisory area and beyond.  Some of these avalanches were large and ran far.  The same weak layers still exist and are slow to adjust.  Although the chances of triggering these weak layers is decreasing, the consequences remain significant if a person is caught and carried.

Observers continue to report red flag information such as whumpfing and shooting cracks.  These are obvious signs of instability but may not exist as clues in all areas.  Signs of instability may not occur before a slope releases.  It’s possible for the current persistent slab problems to be remotely triggered from the sides, below, or above.

If you want to reduce exposure to this snowpack beast, choose to travel in lower angle terrain outside of runout zones.  As always, use good travel protocol – travel one at a time in steeper terrain, and spot partners from areas with limited exposure.

This avalanche on Spirit Walker was not observed in action, but reported yesterday.  2.15.2020 . Photo: Ben Walker

 

Cornices:  As always, give cornices a wide margin and limit time moving through and under them.

Weather
Sun, February 16th, 2020

Yesterday: Partly cloudy skies with some clear periods.  Winds were calm to light and  Temperatures stayed in the teens to 20°F.

Today: Today will be mostly sunny with temperatures ranging from the teens to twenties °F. Winds will be calm to light from the west and will shift to easterly tonight with moderate gusts.  A chance of flurries this evening with 1″ of accumulation.

Tomorrow:  A large low-pressure system is moving in Monday and Tuesday. As of now this next storm looks to be relatively warm, wet and windy. Models are showing up to 2-3″ of SWE (2-3′ snow) and a rain line that could approach 1,500′ by Tuesday evening.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 10 0 0 62
Summit Lake (1400′) 8 0 0 26
Alyeska Mid (1700′) 10 0 0 60

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 11 W 6 18
Seattle Ridge (2400′) 11 N 2 7
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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.