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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, February 28th, 2020 - 7:00AM
Expires
Sat, February 29th, 2020 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is ‘scary MODERATE‘ due to possibility that a person could trigger a very large and dangerous slab avalanche. These slabs are 3-6′ thick and sitting on weak faceted snow. They could be triggered on, near or even below a slope. Although the likelihood of triggering is slowly decreasing, the consequences could still be deadly. Continued conservative decision making and terrain choices are necessary to limit this hazard.

Pay attention to blowing snow and loading if the northwest winds kick up today. Watch for lingering wind slabs on steep wind loaded slopes and cornice breaks along ridgelines. On steep slopes getting direct sun, watch for roller balls and loose snow sluffs with daytime heating.

SUMMIT LAKE TO SEWARD REGION: The likelihood for triggering a large slab avalanche is higher in this area due to a weaker snowpack structure and may see more wind effect today from increasing northwest winds. Extra caution is advised.

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Fri, February 28th, 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. 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

Small sun initiated sluffs were observed in Lynx Creek yesterday afternoon on a steep south facing slope.

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

As my colleague Wendy likes to say, “Don’t forget the bad news in the basement!” If you head out today to enjoy the sunshine and powder please remember that there are facets (weak sugary snow grains) that formed in January, buried 3-6′ deep that have been responsible for countless large slab avalanches. If you forgot what those avalanches look like, take a quick scroll back through the observations. Imagine the slopes you like to travel, having a slide like one of those while you are on it. If you polled avalanche professionals this might be one of the the scariest set-ups they can think of. Hence the ‘scary MODERATE’ avalanche hazard. The likelihood of triggering an avalanche is decreasing because the thickness of the slab makes it harder to affect the weak layers, time is allowing the snowpack to slowly adjust to the weight of the slab and signs of instability have stopped. However, the consequences of triggering one of these very large avalanches could be deadly. It’s pretty much a gamble and you know what you are wagering. Areas with a shallower snowpack, such as the Summit Lake area, have a higher likelihood for triggering.

Heading out today? Here are a few things to consider.

  • Areas that have seen little to no traffic this year are more likely to have a large slab release.
  • Signs of instability are not likely to be present before a slope releases.
  • It could be the 10th person on the slope before it avalanches.
  • Remote triggering a slab from below, the side or on top is possible. The Grandview avalanches on Monday were triggered from 1000′ away.
  • Consider the consequences if the slope slides, will debris take you into a terrain trap or fan out?
  • This is difficult low probability / high consequence situation.

*If you’d rather leave these issues behind, sticking to slopes 30° and less, with nothing steeper above you, is a great way to enjoy the excellent powder.

Deep slab over weak snow in Lynx creek yesterday at 3,000′.

Video link HERE.

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

Wind slabs:  Northwest winds are forecast to kick up today. These tend to have less of an impact in Turnagain Pass but can load slopes in Crow Pass, Summit Lake and terrain around Seward. Pay attention to blowing snow and loading. In addition, old lingering wind slabs from loading events earlier this week are still possible to trigger on steep loaded slopes. Remember, even a small wind slab triggered in the wrong place can take you for an unpleasant ride.

Other concerns for today:

Sun Effect:  Solar heating in the afternoon may cause loose snow sluffs again today on steep southerly aspects (especially near rocky terrain).

Loose snow avalanches: Watch your sluff on steep wind protected slopes. Cold temperatures are making surface snow looser in areas out of the sun and wind.

Cornices:  As always, give cornices a very wide berth and limit exposure under them.

Keep in mind, any one of the above issues on the surface has the potential to ‘step down’ and trigger a large slab.

Weather
Fri, February 28th, 2020

Yesterday: Skies were mostly sunny with some patches of valley fog. With an inversion in place temperatures ranged from 20°Fs and high teens at upper elevations to single digits and low teens in the valley bottoms. Winds were westerly 5-10 mph with gusts into the 20s.

Today: Sunshine and patchy valley fog with temperatures in the teens and low 20°Fs. Winds will be westerly 10-20 mph with gusts into the 30s.  Skies will be mostly clear overnight with winds decreasing and temperatures in the single digits.

Tomorrow: Partly cloudy skies becoming mostly cloudy with snow in the forecast starting in the afternoon. Light northwest winds winds will shift to the east in the morning blowing 10-20 mph with gusts into the 30s. Temperatures will be in the 20°Fs. Snow continues overnight into Sunday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 16 0 0 77
Summit Lake (1400′) 10 0 0 30
Alyeska Mid (1700′) 17 0 0 83

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16 W 6 20
Seattle Ridge (2400′) 16 *NA *NA *NA

*Seattle Ridge anemometer (wind sensor) is heavily rimed and not reporting.

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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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