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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, December 3rd, 2019 - 7:00AM
Expires
Wed, December 4th, 2019 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

A MODERATE avalanche danger exists in the Turnagain Pass region above 2,500′.  Small wind slabs will be possible to trigger in higher elevation terrain, particularly along or just below leeward ridges. Furthermore, a suspect weak layer/ bed surface combo exists above 3,000’ where triggering a much larger, more dangerous avalanche remains possible.

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Tue, December 3rd, 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
  • 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

Slowly but surely we are building our snowpack in the Turnagain region with ~5” of new snow falling on Sunday and another 2-4” yesterday and overnight.

With a shift in winds to NNW yesterday, this new snow is forming fresh wind slabs in the upper elevations and along ridges yesterday and today. These are expected to be on the smaller side and relatively manageable. However, even with small avalanche problems, it’s good habit to obsess with the consequences of where the snow will go (in relation to you and your party) if a slope does slide.  Keep eyes and ears out for drifting, cracking and hollow sounding snow as these are all indicators of wind slabs.

Screen capture from windy.com (NAM model) showing westerly wind vectors today through the advisory area.

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

With surface conditions improving and fair weather setting in this week, we expect folks will be traveling farther and higher into favorite zones across the advisory area. Please do so with some trepidation. Forecasters and observers have all found a layer of faceted snow that sits on a hard melt freeze crust near the base of the snowpack around 3,000’. This combo has shown to be reactive in snow pits and was responsible for a very large whumpf on Sunburst ridge last weekend. Though the buried crust seems to be widespread throughout the Turnagain area, the weak, faceted snow above remains the question mark. It’s a spooky enough combo this early season, and shown reactive to human triggers that it should be treated as “guilty until proven innocent”. Continue to tune in to the forecast as we track this avalanche problem and please submit an observation as to what you’re finding (or not finding) in your travels this week!

Suspect persistent slab set up to be on the lookout for.

Weather
Tue, December 3rd, 2019

Yesterday: Mostly cloudy skies and intermittent snow dominated as a cooler, drier air mass funneled in from the north yesterday morning. Snow moved back in late in the day with 2-4” accumulating thru the Pass over the last 24 hrs. Winds were westerly in the single digits gusting into the teens with temps at 1,000’ on a steady decline toward the low 20’s F.

Today: Isolated snow showers to sea level are expected to taper off by mid-day. Temps will be in the low 20’s F at 1,000’ and single digits on ridge tops. Winds look to be generally light from the NW though may be gusting into the 20’s mph on ridge top locations.

Tomorrow: A ridge of high pressure will continue to build in tomorrow. Temps will remain more seasonably normal (cold!) with clear skies through at least Wednesday and Thursday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 23 2-3 .2 16
Summit Lake (1400′) 23 2 .1 11
Alyeska Mid (1700′) 22 1-3 .1 18

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 13 W 5 17
Seattle Ridge (2400′) N/A* N/A* N/A* N/A*

*Seattle Ridge is not recording temperature and wind stopped recording at 10 pm on 11/28.

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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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