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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, December 4th, 2019 - 7:00AM
Expires
Thu, December 5th, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

A MODERATE avalanche danger exists above 2,500′.  Triggering a large, dangerous avalanche remains possible due to a suspect weak layer/ bed surface combination near the ground. In addition, be on the lookout for lingering small wind slabs particularly along or just below leeward ridges.

Special Announcements

Join CNFAIC Forecaster Aleph Johnston-Bloom at Blue & Gold Boardshop Monday, Dec 9th, 7:00-8:30 for a FREE evening avalanche discussion on patterns in Alaskan avalanche accidents with practical takeaways to use this season. There will also be an avalanche gear demo outside in the snow (weather permitting).

Headed to Hatcher Pass? Don’t forget to check hpavalanche.org and their Facebook page!

Wed, December 4th, 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. 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
  • 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

With clearing skies today and improving surface conditions, if you travel farther and higher into favorite zones across the advisory area, please use caution. There is still limited data above 3000′ and evidence that there maybe a dangerous facet/crust combination near the base of the snowpack in the Alpine. Forecasters and observers have found this set-up to be reactive in some snow pits and it was responsible for a very large whumpf with shooting cracks on Sunburst ridge last weekend. The buried crust seems to be widespread throughout the Turnagain area but the presence of the weak, faceted snow above remains the question mark. How widespread the facet layer is such an unknown and due to the reality that there has been essentially zero slope testing, higher elevation slopes should not be trusted. Signs of instability may not be present, this type of avalanche may let you get out onto the slope before it fails and unfortunately it might not be the first rider that triggers it.

Spooky set-up in the Alpine. How widespread is this? Caution is advised!

 

 

Avalanche Problem 2
  • 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

If you travel into the Alpine be on the lookout for wind effect in along ridgelines, especially in areas that load with a west wind. The frontside a.k.a the Sunnyside of Seattle ridge is one of those zones. Lingering wind slabs on the leeward slopes are still possible today. Winds were mostly light yesterday but the previous day they were stronger. There may be hard snow under the fluff. Look for cracking, listen for hollow sounding snow and pole probe for hard snow over soft snow. Even small winds slabs can have high consequences in steep terrain.

Weather
Wed, December 4th, 2019

Yesterday: Skies were mostly cloudy with light snow showers. Temperatures were in the high teens to mid 20Fs. Winds were light and westerly.

Today: Mostly cloudy to partly sunny skies with a chance of light snow showers in the morning. There maybe patchy valley fog as skies clear. Temperatures will start in the high teens to mid 20Fs and drop throughout today. Overnight temperatures are forecast to be in the low teens and single digits. Winds will remain light and westerly. Freezing valley fog may develop overnight.

Tomorrow: Partly sunny skies with temperatures in the high teens and mid 20Fs. Light variable winds. Clouds move back in overnight with a chance of snow as the next front approaches.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 18 trace 0.02 16
Summit Lake (1400′) 17 0 0 10
Alyeska Mid (1700′) 19 0.5 0.04 18

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 12 W 2 5
Seattle Ridge (2400′) NA* NA* NA* NA*

*Seattle Ridge wind sensor is rimed over and the temperature sensor is not functioning.

Observations
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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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