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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, December 5th, 2019 - 7:00AM
Expires
Fri, December 6th, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

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!

Thu, December 5th, 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

Yesterday sunshine and soft snow above the fog allowed for further travel into the Alpine. With a similar day on tap caution is still advised as more slopes are explored. Forecasters and observers continued to investigate the suspect facet/crust combination (weak snow over very hard snow) near the base of the snowpack. Yesterday a series of snow pits above 3000′ on Sunburst found this set-up in every spot. In the snowpack tests conducted it varied in reactivity, suggesting that it may be more stubborn to trigger but that an avalanche may still be initiated. If you venture out today remember that signs of instability may not be present, this type of avalanche may let you get out onto the slope before it fractures, and it might not be the first rider that triggers it. The snowpack depths are quite variable across the terrain. Avalanches are often triggered from thinner areas where it is easier to affect the weak layer.

Sunburst snowpack 3200′

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 leeward slopes are still possible today. There may be hard snow under the fluffy surface snow. 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
Thu, December 5th, 2019

Yesterday: Partly cloudy skies with mid level valley fog. Temperatures were in the mid 20Fs in the valley bottoms to single digits along ridgetops. Winds were light and westerly. Overnight temperatures dipped into the low teens and single digits with Summit Lake getting down to 4F at road level.

Today: Partly sunny skies with valley fog and increasing clouds in the afternoon. Temperatures will slowly rise to the mid to upper 20Fs at lower elevations and mid teens at upper elevations. Winds will be light and variable. Overnight will be mostly cloudy with a chance of snow showers and winds will become easterly increasing into Friday. Temperatures will continue to rise as warm air moves into the region.

Tomorrow: Mostly cloudy with snow showers. Temperatures in the mid 20Fs to mid 30Fs with easterly winds 10-20 mph. Unfortunately the forecast is for a warm and wet storm over the weekend. The models are still struggling with the details of this. Stay tuned and fingers crossed for some of the cold air to linger…

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 15 0 0 16
Summit Lake (1400′) 13 0 0 10
Alyeska Mid (1700′) 16 0 0 18

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 9 W 4 15
Seattle Ridge (2400′) NA* NA* NA* NA*

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

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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.