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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, December 5th, 2023 - 7:00AM
Expires
Wed, December 6th, 2023 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

The avalanche danger is MODERATE today above 1,000’.  It will be possible to trigger a wind slab avalanche up to 2 feet deep as North and Northwest winds continue to redistribute the soft surface snow from last week’s storm.  Upper elevations near ridge lines and convex rollovers on the lee side of terrain are where we’d expect to find tender wind slabs.  It’ll be worth identifying and avoiding areas that harbor wind slabs, particularly in consequential terrain where even a small slab can knock you off your feet or push you into a terrain trap. 

There are some deeper layers in the snowpack that we’re keeping an eye on but it seem unlikely for a person to trigger today.  Below 1,000’ the avalanche danger is LOW. 

Special Announcements

Join Alaska Safe Riders and professional snowmobilers Dan Adams, Matt Entz, and Dustin Pancheri for an Avalanche Awareness Open House to prepare for your next mountain adventure.  Wasilla tonight and Anchorage on Friday.  All the details can be found here.

Tue, December 5th, 2023
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
Wed, December 6th, 2023
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
Wed, December 6th, 2023
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. 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.
Recent Avalanches

The last known human-triggerd avalanche reported was a small wind slab pocket on Tincan, on Friday (12/1).  Beyond that, observers have been reporting evidence of a natural avalanche cycle that likely ramped up as last week’s storm was winding down last Thursday (11/30).  Crowns were reported from Main bowl and Warm up bowl on the moto side of the highway to Sunburst and upper Bertha Creek on the non moto side.

Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Aspect/Elevation
  • 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.

Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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 ample snow available for transport in the wake of our late November storm last week, it doesn’t take very strong winds to build touchy wind slabs.  Winds were primarily from the East/ Northeast yesterday and stronger at higher elevations.  We can expect a slight wind shift today to a more West/ Northwest direction.  Ridgelines, steep gullies and convex rollovers should be approached with caution and with the expectation that you may trigger a wind slab. Based on new snow yesterday/ overnight and snow available for transport, wind slabs are likely to be in the 6-18″ deep range. 

Keep eyes and ears out for surface conditions changing under your skis or snowmachine.  If the surface all of the sudden feels stiff or drum-like, you are likely flirting with a wind slab.  Shooting cracks or whumpfing are obvious signs of instability but may not present before an avalanche.  Small, non-consequential test slopes are also a good way to suss out just how reactive this avalanche problem may be today.  Another good option could be simply to seek out protected terrain in the trees where winds have had less effect on the snow surface.  We found really good, dry snow with minimal wind effect yesterday in ~1,500-2,500 elevation band in Lynx Creek.  Bonus: visibility was waaaay better in the trees!

Currently, the Summit Lake zone has a similar snowpack setup as the Turnagain area and was seeing active wind loading yesterday.  More info on current conditions here.

This is what active wind loading looks like courtesy of the Sunburst webcam at 3,800′. Image taken at 6:30p last night.

Additional Concern
  • Persistent Slabs
    Persistent Slabs
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.
More info at Avalanche.org

Forecasters and observers have found the Thanksgiving crust to be widespread throughout the forecast area.  The good news is that literally thousands of slope testers over the past four days since it was buried have put countless tracks on all aspects and elevations throughout the advisory area with minimal avalanches on this layer.  Snowpit test results have been mixed, with some pits showing the potential to propagate a crack and others showing no concern.  The bad news is that crusts are notorious for ‘waking back up’ down the road as the snow weakens and facets out above and below these layers.

It’s quite easy right now to dig down into the mid-snowpack to get yourself acquainted with this usual suspect unaffectionately referred to as the “Thanksgiving Crust”.  We’ll continue to keep tabs on this layer as the season progresses.  And for you daily readers, this certainly won’t be the last you hear of the TC.

Snow grains pulled from the layer directly above the Thanksgiving crust.  No obvious faceted grains in either of our pits yesterday but we’ll be keeping close tabs on this layer in the days and weeks ahead. 12.04.2023

We’re also still paying attention to the layer of facets at the ground in the highest elevations (above around 3500-4000′). This layer is now buried 6-8′ deep in that upper elevation band, so you’d need to find the perfect thin spot to trigger an avalanche on it. That said, if someone were to trigger an avalanche on this layer it would be huge. The layer seems to be very isolated, but the most problematic terrain will be high elevation, steep, rocky slopes with a thinner overall depth.

Weather
Tue, December 5th, 2023

Yesterday: Skies were mostly obscured with moderate to light snow falling throughout the day and into the evening hours.  Temperatures were in the mid to high 20’s throughout the day with winds in the teens and gusting into the 30’s from the NE at ridgetops.  Total snow accumulation was 2-3″.

Today: It looks to be another grey-bird day on tap.  We can expect mostly cloudy skies, another 1-3″ of snow with a slight wind shift to the Northwest.  Winds will be in the teens and gusting to 20s mph at ridgetop locations.  Temps will start out in the mid to high 20’s at 1,000′ and drop to the low 20’s throughout the day.

Tomorrow: Temps will continue to slide a bit as colder air fills in behind the current low pressure that is tracking north.  We can expect our moderate winds to continue from the North and West with a chance for a few more inches of snow.  Temps look cold enough to support snow at sea level!

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 31 3 .3 50
Summit Lake (1400′) 29 0 0 22
Alyeska Mid (1700′) 30 3 .4 33
Bear Valley – Portage (132′) 28 3 .35

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24 NE 14 37
Seattle Ridge (2400′) 27 E 3 12
Observations
Recent Observations for Turnagain Pass
Date Region Location
02/24/24 Turnagain Observation: TinCan Backdoor/ Center Ridge
02/22/24 Turnagain Avalanche: Lynx Creek
02/22/24 Turnagain Observation: Turnagain, Seattle, Mt Ascension
02/21/24 Turnagain Observation: Tincan Trees
02/21/24 Turnagain Observation: Sunburst
02/20/24 Turnagain Avalanche: Tincan
02/20/24 Turnagain Observation: Seward Highway across from Johnson Pass TH
02/19/24 Turnagain Avalanche: Base of Seattle Ridge
02/18/24 Turnagain Observation: Lynx creek
02/18/24 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.