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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, February 11th, 2020 - 7:00AM
Expires
Wed, February 12th, 2020 - 7:00AM
Forecaster
Ryan Van Luit
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 1000′.  Triggering a large and dangerous slab avalanche 2-4′ thick on buried weak layers is possible and these avalanches may be triggered remotely. Wind from yesterday likely added to the formation of wind slabs on the leeward terrain throughout Alpine and Treeline elevations.  A conservative mindset is called for while evaluating route and terrain choices.

REGION-WIDE: Heads up! Strong winds have created dangerous slab avalanche conditions from Seward to Hatcher Pass. Extra Caution is advised. Choose terrain very carefully. Check out these recent observations for more information.

Special Announcements

Our deepest condolences go out to the family, friends, and everyone affected by the a fatal avalanche that occurred near Cooper Landing yesterday. Details are limited but CNFAIC forecasters will share pertinent information as it becomes available and will attempt to visit the site today. Media link HERE.

Tue, February 11th, 2020
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
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.
Recent Avalanches

Yesterday, a slab avalanche was reportedly triggered by snowmachine in 2nd Bowl off Seattle Ridge.  The rider was caught but not buried in the avalanche.  Details are unknown at this time. In addition, there was another slab in Widowmaker that was possibly remotely triggered and there was very large remotely triggered avalanche in Placer Valley.

A snowmachine triggered slab avalanche in 2nd Bowl off Seattle Ridge. 2.10.2020 . Photo T. Smith

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

Very large avalanches were triggered yesterday including at least one remotely triggered.  This data should be front and center in your decision-making today if you venture out into the mountains.  It’s possible for a human to trigger a persistent slab avalanche which sits over a weak layer 2-3+ feet beneath the surface.  If this slab is triggered, we have evidence to indicate that it could be large to very large.  This means that an avalanche of this type could have the volume or force to bury a car, destroy a small building, or break trees, i.e. be unsurvivable.  Triggering this type of avalanche is a concern throughout the region right now, on all aspects, and especially on steeper slopes above 1000′.

One of the challenging parts of this problem is that clues to indicate the concern may not exist.  Evidence of shooting cracks or other recent avalanches may not be found.  It’s possible to trigger these avalanches from remotely – this means a person could initiate the avalanche from the side of the slope, from above, or even below.  Due to the potential size and volume, these avalanches could run into lower angle slopes far below in the runout. If you’re in the mountains today, it’s recommended to stick to lower angle slopes and choose routes carefully and conservatively.  Remain aware of steep slopes above, and reconsider your choice if you’re in the runout.

Skies will be mostly sunny today and temperatures are forecast to range in the teens F. Calm to 5mph winds are expected to be westerly and increase to 15mph tonight.  A chance of 1-2″ of snow is possible this afternoon into the evening.  If the forecast verifies, the weather will have little impact on our current avalanche concerns.

 

Avalanche Problem 2
  • Wind Slabs
    Wind Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.
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

Yesterday, moderate north winds transported snow to the lee aspects in the Alpine and Treeline elevations.  Although our winds have calmed, recently built wind slabs could be likely for a human to trigger.  If triggered, a small wind slab could tip the balance to initiate a persistent slab deeper within the snowpack.

Rapid wind loading near Mt Ascension in the Lost Lake Area toward Seward.  2.10.2020 . Photo A. McLain

Consistent wind loading began yesterday morning in the Crow creek drainage near Girdwood.  2.10.2020 . Photo CNFAIC Archive

Cornices:  Cornices have grown over the past few days. Avoid travel on or underneath them and remember a cornice fall could trigger an avalanche on the slope below.

Weather
Tue, February 11th, 2020

Yesterday:  Mostly sunny skies. Temperatures were in the 20Fs to teens and dropped into the single digits by evening. Winds  shifted to northwest and increase from 10-20 mph with gusts into the 30s.  Skies were mostly clear overnight and winds decreased by morning.

Today:  Mostly sunny today, with a high near 19°F and low around 14°F. Winds are forecast to be westerly around 5 mph becoming calm. There’s a chance of snow this evening with accumulations of 1-2″ and wind will become easterly from 5-15mph.

Tomorrow:  Cloudy skies with a high near 28°F and a low of 10°F.  Winds are forecast out of the north from 5 to 10 mph, and will shift to westerly at 5 mph by this evening.  A chance for intermittent snow showers may accumulate 1 to 2 inches.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 18 0 0 55
Summit Lake (1400′) 10 0 0 22
Alyeska Mid (1700′) 18 0 0 58

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 9 W 10 28
Seattle Ridge (2400′) 13 WNW 10 23
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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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