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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, December 20th, 2019 - 7:00AM
Expires
Sat, December 21st, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

MODERATE avalanche danger exists above 2500′ for lingering wind slab avalanches in steep wind-loaded terrain and cornice falls. Triggering a wind slab 1-3′ thick on an unsupported slope or cross-loaded gully is still possible. Give cornices and glide cracks a wide berth. Keep the chance of triggering a persistent slab avalanche in high elevation terrain on your radar.

 

Special Announcements

The final accident report from the first US avalanche fatality of the 2019/20 season that happened in Colorado is available and is worth a read: https://avalanche.state.co.us/caic/acc/acc_report.php?acc_id=726&accfm=inv. Our deepest condolences to family, friends and all involved.

Fri, December 20th, 2019
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
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

With improving visibility and surface conditions more folks have been venturing into higher terrain. Temperatures are dropping, there is settlement in the snowpack and more time has passed since the last loading event. These factors are generally positive for the overall stability and triggering an avalanche is trending towards unlikely. However, in the Alpine caution is still advised on specific terrain and there is some lingering uncertainty about persistent slab potential above 3000′ (See Avalanche Problem 2). As you travel today there are a few things to keep in mind.  Steep (35°+) unsupported slopes or wind-loaded gullies may still harbor stubborn wind slabs near ridgelines. Feel for stiff snow over soft snow and listen for hollow sounds. Remember even a small avalanche can be serious in high consequence terrain.

Cornices: Cornices may still fail under the weight of a person and often break farther back than expected.  Pay attention to other groups traveling above or below you.

Recent cornice fall on the south side of Magnum, 12.19.19.

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

So… maybe you are using this weather window to go for a bigger objective or to explore up high? We have limited information about the snowpack above 3500′. Every time we dig a pit above 3000′ we find the Veterans Day facets sitting on a crust under hard snow. The reactivity of this layer has been variable and the depth the layer is buried is also variable. What we do know is that facets are a persistent weak layer and present a lingering concern, like a little voice saying what if? The takeaway for today and into the weekend is that there is still a chance that someone traveling in upper elevation terrain may find the wrong spot and trigger a large avalanche. The mostly likely scenario for triggering this type of avalanche would be hitting a thin spot in the snowpack or near a rock poking through that initiates failure in the weak layer. The overall snowpack is shallower towards the southern end of Turnagain Pass, in Summit Lake and in the Crow Pass terrain just north of Girdwood. 

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

Keep your eyes peeled for glide cracks in the terrain that you are traveling in. Steer clear and avoid lingering in the runout of this unpredictable hazard. There is a chance we may see some more glide cracks open and release as temperatures drop or the glide activity may completely stop.

Glide crack on Cornbiscuit, 12.19.19

Weather
Fri, December 20th, 2019

Yesterday: Skies were partly cloudy and there was lingering valley fog. Temperatures were in the high to low 20°Fs depending on elevation. Winds were light and westerly. In the evening temperatures began to drop and went down 10 degrees at most area weather stations. Winds remained light and skies cleared overnight.

Today: Mostly sunny skies are on tap for today. This morning sea level stations are in the low 20°Fs and ridgetops are around 10°F. Highs today are forecast to be in the mid teens to single digits. Overnight skies will be partly cloudy and temperatures will be in the single digits. Winds will be light and westerly.

Tomorrow: Partly sunny with temperatures in the teens and single digits and light winds. The cold temperatures and sunshine continue through the Sunday. Looking forward the models show some potential for wintery precipitation during the holiday week! Fingers crossed. Stay tuned.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 21 0 0 29
Summit Lake (1400′) 15 0 0 7
Alyeska Mid (1700′) 20 0 0 16

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17 W 4 14
Seattle Ridge (2400′) NA* NA* NA* NA*

*Seattle Ridge weather station is 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.