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Issued
Wed, December 30th, 2020 - 7:00AM
Expires
Thu, December 31st, 2020 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains MODERATE at elevations above 1,000′. Triggering either a fresh wind slab from overnight or a lingering wind slab from earlier this week is possible. These slabs are around 1-2′ thick and found on wind loaded steeper slopes and in steep cross-loaded gullies. Additionally, between 1300-2500’, there is a small chance of triggering a deeper avalanche (3-5′ thick) on weak snow near a crust that formed Dec 1st.

The avalanche danger is LOW below 1,000′.

SUMMIT LAKE: With a thinner and weaker snowpack, there is still a small possibility of triggering an avalanche on a weak layer buried mid-pack or near the ground.

Wed, December 30th, 2020
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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

Another round of easterly ridgetop winds hit the region last night as a small weather system is moving through currently. Overnight, between 1-3″ of new snow has fallen and another 1-2″ is expected for today; this is above 400′ as below, light rain is expected. The ridgetop east winds were blowing 20-30mph from 8pm last night till around 2am this morning before quieting down to 10-20mph, where they should remain for the day. This said, we can expect some fresh wind slabs out there in exposed areas in addition to lingering wind slabs from earlier in the week.

Most of these fresh slabs are likely to be in the higher elevations, around a foot thick, and possible to trigger. With today’s poor light, travel into the Alpine may be somewhat thwarted, but finding these in the mid-elevations is also possible. While most older wind slabs seem to be fairly stuck into place, there is still a chance a few lurking stubborn slabs could pose a threat on just the right steep wind loaded slope. Keeping a close eye out for the classic wind slab tell-tale signs will be key.

Classic wind slab signs:

  • Rounded and pillow-like surfaces (wind deposited snow)
  • Hollow feeling or drum-like feeling on the snow due to stiff snow over softer snow
  • Cracks in the snow that shoot out from your machine, skis or board
  • Any collapsing in the snow or ‘whumpfing’
  • The skiff of new snow from today may be obscuring some wind slabs

A couple ‘Normal Caution’ avalanche issues: On steep slopes out of the winds where loose surface snow is found, keep an eye on your sluff. And, as always, give cornices a wide berth and limit time under them.

Impressive wind effect along Seattle Ridge around 2,500′. 12.29.20. Photo: Beau Gehler

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

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

On mid-elevation slopes (1,300′ – 2,500′) we have been tracking a crust that formed on 12/1. This crust is degrading, developing a thin layer of facets, and beginning to show signs of forming a reactive weak layer in some areas, but not all. The crust sits 3-5′ below the surface and has been responsible for large avalanches trigged with explosives and a large collapse at Pete’s North. Triggering an avalanche on this layer is unlikely but could have bad consequences. Signs of instability will not likely be present and stability tests could give unreliable data. Steep unsupported slopes in the 1300′-2500′ elevation range should be considered suspect and there may be multiple tracks on the slope before it fails.

The Summit Lake region to the south of our forecast area has a thinner snowpack with faceted snow at the ground that is a concern above 2500′.  This area also has the December 1st crust/facet combo below 2500′. Both these weak layers are worth keeping in mind before trying to push into bigger terrain in the Summit Lake area.

Snow pit at 2,300′ on Seattle Ridge. This is representative of the Repeat Offender slide path. No instability was found. The crust here, despite degrading and faceting, was not reacting in the pit the way some pits have been on the non-motorized side of the Pass. 

Weather
Wed, December 30th, 2020

Yesterday:  Mostly cloudy skies were over the area. Ridgetop winds were light easterly during the day and increased (20-30mph), with gusts to 51mph, overnight. Light snowfall fell overnight adding 1-2″ over most locations, including sea level. Temperatures were in the 20’sF in the mountains and in the mid 30’sF at sea level, before cooling a few degrees overnight.

Today:  A shortwave trough is moving the forecast area and bringing, cloudy skies, a bump in wind, and 1-2″ inches of additional snow today. Ridgetop winds should remain in the 15-25mph range with stronger gusts. Temperatures are expected to remain in the mid 20’sF along ridgelines and in the upper 30’s F at sea level. The rain/snow should be just above sea level (~400).

Tomorrow:  For tomorrow, skies should be partly cloudy, ridgetop winds look to back off and blow in the 5-10mph range from the east and temperatures cool into the teens at the mid and upper elevations. This will mark a several day period of clear weather and high pressure that is expected to last through Saturday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 32 2 0.1 81
Summit Lake (1400′) 31 0 0 31
Alyeska Mid (1700′) 31 1 0.1 84

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24 NE 23 51
Seattle Ridge (2400′) 27 SE 9 22
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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.