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Issued
Fri, December 27th, 2019 - 7:00AM
Expires
Sat, December 28th, 2019 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is MODERATE today above 1,000′. Human triggered wind slab avalanches between 1-2 feet thick will be possible. Steep windloaded slopes are the most likely places to find and trigger an avalanche. Watch for signs of instability and steer clear of cornices and glide cracks.

Another storm is on the way!  The NWS has issued another Winter Weather Advisory that begins at 6pm this evening and extends through tomorrow. Avalanche danger is expected to rise to CONSIDERABLE overnight with additional snow and wind.

Fri, December 27th, 2019
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. 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.
Recent Avalanches

Christmas night’s foot of low-density snow greeted many backcountry skiers at Turnagain Pass yesterday. The snow was quite light and though it was sitting on a weak layer, was too loose to act like and slab in popular areas with little wind effect. Despite this, a group skiing on Tincan did trigger this very small wind slab avalanche from a distance. With such poor visibility and folks staying away from the higher windloaded terrain, this was the only avalanche we know of.

Small and very soft wind slab avalanche, remotely triggered by skiers descending from the higher terrain under Common Bowl and into the Tincan Trees yesterday 12/26. Credit: Billy Finley 

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

For today, any fresh wind slab and/or lingering wind slabs from the past two days are all suspect for being unstable. This is due to either a layer of buried surface hoar or simply very loose weak snow that sits right under the slabs. High elevation slopes with the most wind effect are the most concerning.

If skies stay clear enough for travel to these higher slopes, keep a close lookout for windloading and areas where the top foot of snow is stiffer than the snow underneath. Any whumpfing or cracks that shoot out in the snow around are a clear sign that layer of snow could avalanche.

Video from Tincan 12/26, link HERE.

There are actually two layers of buried surface hoar, one buried on the Solstice and one on Christmas. Of these, the lower one (the Solstice Buried Surface Hoar) has been the most prevalent and could start causing us more grief in the days to come. So far, much of the 12-14″ of snow above it has not been consolidated enough to form a slab, once it does, we could start seeing avalanches in popular areas that we did not see yesterday.

Sluffs:  The very cold temperatures that moved in last night should keep much of the snow above these weak layers loose in areas out of the wind. Watch your sluff on the steeper slopes.

Cornices:  Natural cornice falls were noted during the Christmas night storm as well as a few from yesterday. These are worth giving a wide berth as they could be teetering on the balance.

Mid elevations and more snow on the way:  A heads up that with warming temperatures and more snow on the way, even slopes in the trees could become unstable and dangerous in the days to come. This is all due to that pesky solstice weak layer that is just waiting for a consolidated slab to start producing avalanches…

Aleph points out the Solstice Buried Surface Hoar sitting on a crust. The layer is under 14″ of loose snow at 1,200′, which is just above the road. 

Additional Concern
  • Glide Avalanches
    Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
More info at Avalanche.org

New snow and wind is likely filling in glide cracks throughout the region and they could be more difficult to see. Despite this, keep a close look out for them and remember glide avalanches are very unpredictable and it’s not a good idea to linger under cracks.

Weather
Fri, December 27th, 2019

Yesterday:  Cloudy and obscured skies were over the region. No snow has fallen in the past 24 hours. Ridgetop westerly winds were moderate (5-15mph) and in bringing in very cold air. Temperatures have been declining and sit this morning in the single digits and minus single digits at all elevations.

Today:  Mostly clear skies should transition to high clouds in the afternoon as the next wave of precipitation moves in tonight. Snowfall should begin around 6pm with 4-8″ forecast to fall tonight. Ridgetop winds will being a shift to an easterly direction and blow ~5-15mph today before increasing tonight up to 20-30mph with stronger gusts. Temperatures will remain cold, near 0F today before slightly increasing tonight.

Tomorrow:  Snowfall should continue through Saturday with an additional 4″ or so falling before the system pushes out in the evening. Ridgetop winds look to remain gusty, swing back westerly and blow in the 15-30mph range. Temperatures should bump back up the teens and 20F before the next front looks to swing in on Sunday evening bringing another round of precipitation.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 11 0 0 36
Summit Lake (1400′) 9 0 0 10
Alyeska Mid (1700′) 11 trace trace 27

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 3 W 7 20
Seattle Ridge (2400′) N/A* N/A* N/A* N/A*

*Rime has covered the wind sensor on Seattle Ridge.

Observations
Recent Observations for Turnagain Pass
Date Region Location
11/16/23 Turnagain Observation: Tincan Common
11/14/23 Turnagain Observation: Sunburst/Magnum
11/14/23 Turnagain Observation: Seattle Ridge
11/13/23 Turnagain Avalanche: Tincan Trees
11/12/23 Turnagain Observation: Sunburst
11/12/23 Turnagain Avalanche: Goldpan – avalanche
11/11/23 Turnagain Observation: Tincan Common
11/11/23 Turnagain Observation: Taylor Pass – Sunburst
11/10/23 Turnagain Observation: Tincan
11/10/23 Turnagain Observation: Tincan Trees
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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.