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Issued
Sun, February 28th, 2021 - 7:00AM
Expires
Mon, March 1st, 2021 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

Strong winds today will raise the avalanche danger to CONSIDERABLE at elevations above 2500’, where it is likely a person could trigger an avalanche in steep, wind-loaded terrain. An avalanche triggered on sensitive wind slabs near the surface may step down to multiple buried weak layers in the upper 2-3’ of the snowpack, making larger avalanches. Dangerous avalanche conditions warrant cautious route finding today, which means identifying and avoiding steep, wind-loaded slopes.

The avalanche danger remains MODERATE at elevations below 2500’, where it will be possible to trigger smaller wind slab avalanches, or avalanches failing on those weak layers in the upper snowpack.

Summit/Seward/Portage: These areas tend to see the strongest winds during these northwesterly wind events, resulting in more wind loading and increased likelihood of natural and human-triggered avalanches.

Sun, February 28th, 2021
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.
Recent Avalanches

Winner Creek: Skiers triggered avalanches failing on a layer of buried facets sitting on top of a crust. The slabs were 1′ deep on average, with some pockets as deep as 3′. Some of the slabs pulled out on slope angles less than 30 degrees. Nobody was caught.

Skier-triggered avalanches failing on buried facets in the Winner Creek drainage yesterday. Photo: Mike Welch

Avalanche Problem 1
  • 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

We are in the midst of another northwest wind event as high pressure moves into the area. Strong winds are expected to continue through today, with sustained ridgetop speeds of 25-40 mph and the strongest winds towards the south end of Turnagain Pass. These winds will be blowing snow into sensitive slabs up to a foot deep, making it easy to trigger an avalanche on wind-loaded slopes today. These northwesterly winds often get channeled up the Sixmile creek drainage near Hope, and get funneled around the south end of Turnagain pass, resulting in strong southerly winds at mid- and lower elevations on the skier side of the pass. This makes it tricky to identify a consistent loading pattern during these wind events, since wind directions will usually change drastically with elevation.

Today it will be important to pay attention to how winds are redistributing snow. Some signs may be obvious from a distance, like wind blowing off ridgelines or across slopes, or fresh avalanche activity. Others may be more obvious immediately around you, like cracks shooting out from your machine or skis, or the feeling of a stiff, punchy slab at the surface. We saw avalanche activity during a similar wind event last Thursday (details here and here), and can expect to see more of the same today.

Avalanche Problem 2
  • 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

Multiple weak layers in the upper 3 feet of the snowpack continue to produce avalanches and show warning signs in our area. Strong winds today will place an additional load on these weak layers while building a stiffer slab on top of them,  increasing the likelihood of a person triggering an avalanche. These persistent weak layers are tricky, because they have become stubborn enough that they are not always giving us direct feedback like cracking and collapsing before avalanching. But with multiple avalanches failing on these weak layers in the past few days, they are clearly still cause for concern.

These layers are widespread through our area, so navigating avalanche terrain right now requires elevated caution. If you are trying to access steep terrain (after assessing the wind slab problem mentioned above), consider the consequences of getting dragged down a slope in an avalanche. Give yourself and your partners an extra safety margin by avoiding consequential terrain above rocks, cliffs and trees, or features that will collect avalanche debris and pile it up deep. Be diligent with safe travel protocol by only exposing one person at a time to steep terrain, and watching your partners from safe locations out of the potential avalanche path and runout zones. These problems are not easy to deal with, and now is not a great time to be pushing the needle into big terrain.

Poor stability test results on a layer of buried facets from a test pit on Raggedtop yesterday.

Weather
Sun, February 28th, 2021

Yesterday: Temperatures reached the mid 20’s to mid 30’s F under cloudy skies. Periodic light snowfall brought a trace of snow. Winds were light and out of the east during the day yesterday, and have shifted to the northwest around midnight.

Today: Strong northwesterly winds will return today, with sustained speeds of 25-40 mph at ridgetops. Cloud cover is expected to decrease during the day, with mostly sunny skies by this afternoon. Temperatures will drop drastically today, reaching single digits F by the afternoon and approaching 0 by tonight.

Tomorrow: Clouds build back in tomorrow with a chance of 1-2” of snow. Winds will switch back to the southeast, blowing 10-20 mph at ridgetops. High temperatures are expected in the mid teens to low 20’s F.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 30 0 0.2 114
Summit Lake (1400′) 29 0 0.1 44
Alyeska Mid (1700′) 29 0 0.01 117

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20 NE-NW* 9 31
Seattle Ridge (2400′) 23 SE-NW* 8 32

*Winds shifted directions around midnight last night, with increasing speeds this morning.

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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.