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Issued
Mon, March 15th, 2021 - 7:00AM
Expires
Tue, March 16th, 2021 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger remains MODERATE today. With an early morning wind shift from east to west/northwest and snow available for transport, watch for small newly formed wind slabs in the upper elevation terrain and in exposed areas in the trees.  Additionally, triggering a slab avalanche that fails on weak snow that is buried between 1-3′ feet deep is still possible at all elevations. Keep in mind these slabs may be triggered remotely. Assess the snowpack as you travel, identify areas of concern and evaluate terrain consequences.

LOST LAKE/SNUG/SEWARD: The northwest winds are forecast to be stronger in these areas. Watch for blowing snow and pay attention to terrain selection.

Mon, March 15th, 2021
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
  • 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 winds were easterly and were strong enough to move snow around in wind exposed terrain. Observers in Girdwood reported shooting cracks and small skier triggered wind slabs. This morning winds are shifting to the west and light snow is falling. Winds today will likely move snow around and will continue form shallow wind slabs. Because of the change in wind direction these new slabs could be on all aspects. Pay attention to blowing snow and recent loading. Fresh wind slabs will feel stiffer than snow that has been sheltered, and may appear as a smooth, rounded pillow. Look for cracking and be suspect of the terrain features that commonly harbor fresh wind slabs– just below ridgelines, below convexities, or in cross-loaded gullies. Remember triggering a shallow wind slab avalanche may step down to the layer of  buried weak snow (more info in Problem 2) and cause a larger, more dangerous avalanche.

Sluffs: Steep slopes that have been sheltered from the wind have loose snow sitting on top of firm surfaces. It will be easy to trigger dry loose avalanches today, and they can pick up enough volume and speed to carry a person. While it is unlikely they will be big enough to bury you, they can be dangerous if they drag you into terrain traps like cliffs, trees, rocks, or gullies.

Wind texture on Petes North yesterday, 3.14.21. Look for signs of recent wind-loading 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

While small fresh wind slabs may be easy to trigger today, triggering a larger avalanche the fails on buried weak snow is still possible as well.  Last Wednesday’s snow is sitting on a weak layer of facets and/or surface hoar as well as a slick sun crust on steep southerly slopes. Don’t forget that persistent weak layers need more time to heal than a typical wind or storm slab. This facet/surface hoar layer is buried anywhere from 6″ to 3′ deep and will see additional loading today. In some terrain it is sitting under stiff wind affected snow (due to the NW wind event Thursday and Friday). Where the weak snow is associated with a sun crust, in steep southerly terrain, appears to be particularly suspect.  We saw human triggered avalanches on this set-up and some avalanches were triggered remotely.  This means it is possible to trigger an avalanche from below, to the side of, or above steep terrain, and requires an additional level of awareness while traveling in the mountains.

Andrew’s list from Saturday of what to keep in mind works well for those heading out today.

  • The most likely place to trigger an avalanche will be on slopes that were wind-loaded in the past few days. Slopes with stiff snow at the surface should be treated with caution today.
  • Unstable snow will sometimes, not always, give you clear warning signs. If you see shooting cracks, collapsing, or other avalanches, you can bet the snowpack is capable of avalanching. However, the absence of these signs does not necessarily imply stable conditions.
  • It is possible to find the perfect trigger point on a slope even after there are several sets of tracks on it. Previous tracks on a slope do not equal stability.

Just like we saw last month, these persistent weak layers will need some time to gain enough strength to be trusted. For now, the safe bet is to recognize the increased hazard and choose terrain carefully.

Buried surface hoar/facets in a snow pit on Petes North yesterday, 3.14.21. Remember this layer is buried deeper in the snowpack on the north end of the Pass and in Skookum/Placer and Girdwood.

Weather
Mon, March 15th, 2021

Yesterday: Skies were cloudy with very light snow showers in the afternoon ending in the evening. Upper elevation temperatures were in the single digits to mid teens and it was in the upper 20°Fs at sea level. Winds were easterly 5-15 mph with gusts into the 20s. Overnight temperatures were in the low teens to low 20°Fs.

Today: Very light snow showers resumed early this morning and are forecast to continue throughout the day, ending in the early afternoon/evening. Winds are shifting to the west/northwest this morning and will blow 10-15 mph with gusts into the 20s. Temperatures will be in the teens to high 20°Fs. Overnight skies will be mostly cloudy with light west winds and temperatures in the teens to mid 20°Fs.

Tomorrow: Skies will be partly cloudy with light northwest winds and temperatures in the teens to mid 20°Fs. Mostly sunny skies are in the forecast for Wednesday and Thursday, possibly continuing through the weekend.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 21* trace* trace* 114*
Summit Lake (1400′) 22* trace* trace* 45*
Alyeska Mid (1700′) 19 2 0.08 118

*The SNOTEL sites stopped reporting at 9 pm last night.

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 10 NE 10 32
Seattle Ridge (2400′) 13 SE 8 19
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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.