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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, December 12th, 2020 - 7:00AM
Expires
Sun, December 13th, 2020 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 1,000′. Strong easterly ridgetop winds today are expected to build fresh wind slabs, up to a foot thick, on leeward slopes and in cross-loaded gullies. Watch for fresh wind deposited snow. Additionally, on the southern end of Turnagain Pass, there is a remote chance that a large avalanche could break near the ground.

The avalanche danger remains LOW below 1,000′.

SUMMIT LAKE:  The snowpack is only 3-4′ thick in this area and weak faceted layers sit in the mid and base of the pack. Above 2,500′ there is still a chance a person could trigger a large avalanche and extra caution is advised.

Special Announcements
Sat, December 12th, 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
  • Aspect/Elevation
  • 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.
Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.
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

I hate to say it, but it looks like a wind event is on the doorstep. Weather models are forecasting easterly ridgetop winds today to be in the 30-40mph range with stronger gusts and even increase into the 50’s mph tomorrow. If this verifies, we can expect fresh wind slabs to be forming. Winds also have a chance at making it into the trees, but we can cross our fingers it doesn’t ruin too much of the good riding conditions. Interestingly enough, only a few inches of snow at most should accompany the winds, so new slabs will primarily be composed of the existing loose snow on the surface.

For today- watch for active wind loading and any stiff snow over softer snow. Cracking in the snow around you is a sure bet you’ve found a slab. It should be relatively easy to suss out any new wind slabs as conditions should be changing in front of our eyes through the day. Areas spared from the winds have a generally LOW avalanche danger.

Winds transporting snow along the top of Tincan’s Hippy Bowl yesterday. 12.11.20. Photo: Eric Roberts

Plenty of loose snow on the surface to blow into slabs… The good news, the new crop of surface hoar will hopefully get blown over ahead of the next snowfall event. 12.11.20. Photo: Alan Abel

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Aspect/Elevation
  • 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.
Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.
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

In the higher Alpine elevations on the southern end of Turnagain Pass and in the Summit Lake area, we are still concerned about the weak facets on the ground. These areas have a much shallower and weaker snowpack than the core of Turnagain Pass. In order for an avalanche to be triggered, we need a slab as well as a weak layer. Elevations below 2,500′ the pack is littered with mid-pack crusts and is lacking that ‘slab’, at the higher elevations it’s not necessarily. Hence, if traveling in the higher terrain be suspect of a hard shallow snowpack over weak snow where a deep slab could be lurking.

Regarding the mid-pack faceted layer that sits 1-3′ below the surface (below the last storm snow from Dec 6-7), this continues to show bonding and good stability. This is a layer we were watching for the past few days.

Weather
Sat, December 12th, 2020

Yesterday:  Mostly blue skies were over the region. Winds along the ridgelines were from the east in the 10-20mph range with gusts into the 30’s mph after sunset. Temperature were in the single digits in creek bottoms and warmed to near 20F at 1500′ above the inversion.

Today:  Clouds, wind and some slow flurries are moving in as a front passes over associated with a low-pressure system in the Gulf. Ridgetop winds are expected to increase from the east up to 40-50mph with stronger gusts. Only a meager amount of snow is expected, up to an inch at most today and a couple inches tonight. Temperatures should remain just cool enough for snow to sea level.

Tomorrow:  Strong easterly winds are expected to remain through tomorrow with little chance for precipitation. Skies do have a chance for some clearing along with the strong winds. A better chance for snow is headed in on Monday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 19 0 0 62
Summit Lake (1400′) 10 0 0 24
Alyeska Mid (1700′) 21 0 0 57

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17 E 15 37
Seattle Ridge (2400′) 19 SE 13 26
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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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