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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, February 17th, 2021 - 7:00AM
Expires
Thu, February 18th, 2021 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE today, and it is likely a person could trigger an avalanche on fresh wind slabs up to a foot deep, which could step down to deeper weak layers now buried 1-3’ deep. Strong winds overnight into this morning, along with up to 8″ of upside-down storm snow since early yesterday morning have increased the avalanche danger. Cautious route finding will be the name of the game today, which means avoiding steep terrain and paying attention to the slopes above you.

PLACER/PORTAGE: These areas are expected to see heavier snowfall today than the rest of the advisory area, and already have more snow sitting on top of the persistent weak layers. This makes larger avalanches easier to trigger, and extra caution is advised in these areas.

Wed, February 17th, 2021
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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
  • 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

Unfortunately, our latest round of stormy weather is proving to be mainly a wind event. Weather stations are showing sustained ridgetop wind speeds of 15-35 mph and gusts to 58 mph since around midnight last night. Along with these strong winds, we have gotten up to 9” of snow since Tuesday morning, with stations recording the highest totals near Girdwood. There are a few factors that make it likely that a person could trigger an avalanche today. First, the strong winds have created sensitive slabs of snow up to a foot thick that will be easily triggered. Second, temperatures are warming as the snow continues to trickle in. This means we will see an upside-down setup on the surface, making for sensitive avalanche conditions even in areas sheltered from the winds. And finally, we know there are multiple layers of weak snow buried a little deeper in the snowpack (more on this below). Even a relatively small avalanche that may be easily triggered near the surface could step down to these persistent weak layers, resulting in a large avalanche.

Today will not be a good day to try to get into steeper terrain. Our snowpack will need some time to adjust to the new load from the wind and fresh snow, and until it does, we will be dealing with dangerous avalanche conditions. Conservative decision making will be key today- this means sticking to low-angle terrain and keeping some distance from steep slopes overhead.

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

As mentioned above, we are also concerned with multiple buried weak layers in the upper 2-3’ of the snowpack. We have found layers of buried surface hoar (more info here) and near-surface facets (details here), and we recently saw several natural and human-triggered avalanches failing on these weak layers after our last wind event (details from Comet/Squirrel Flats, Notch, and  Eddies). These layers have been becoming more stubborn with time, but the strong winds today are pushing them close enough to their breaking point that it will be likely a person could trigger a large avalanche, and we might even see some natural activity. These layers are a little more sensitive at lower and mid-elevations, where they are sitting on top of a rain crust that goes up to around 1200-2000′.

Cornices: Today’s strong winds will make our large cornices a bit larger. Be sure to give them plenty of space, and minimize the amount of time you spend traveling below them.

Natural avalanche on Comet. The avalanche likely occurred during the 2/11 wind event. Photo submitted by Travis Smith, taken on 02.13.2021

Multiple buried weak layers in the upper snowpack at Eddies, just before the latest round of wind and snow. Photo: CNFAIC 02.15.2021

Weather
Wed, February 17th, 2021

Yesterday: We received 2-5” low density snow, with mostly cloudy skies throughout the day. High temperatures were in the mid 20’s to lower 30’s F, with easterly winds blowing 5-10 mph during the day. Winds picked up around midnight, with sustained speeds of 15-35 mph and gusts to 58 mph.

Today: Continued strong easterly winds are expected until later this morning, with sustained speeds of 30-40 mph. Winds are expected to decrease to 10-15 mph by this afternoon. We might see 2-4” new snow today, with the highest chance for precipitation this morning and higher totals in the Portage and Placer valleys. The rain level is expected to rise to 500’ today, with highs in the low 30’s F at low elevations and in the mid 20’s F at upper elevations. Skies are expected to remain mostly cloudy through the day.

Tomorrow: There is another chance for snow late tonight into tomorrow morning, with another 3-7” possible by the end of the day. Easterly winds are expected at 10-20 mph at the ridgetops. Low temperatures will hover in the mid 20’s to 30 F tonight, and remain in the low 20’s to low 30’s F tomorrow.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27 1 0.2 115
Summit Lake (1400′) 28 2 0.2 44
Alyeska Mid (1700′) 28 4 0.3 114

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19 E 13* 58*
Seattle Ridge (2400′) 22 SE 11 28

*Winds have been averaging 20-35 mph since 2:00 a.m., with gusts 35-58 mph.

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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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