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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, December 4th, 2023 - 7:00AM
Expires
Tue, December 5th, 2023 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 1000′ today, as east to north winds will still make it possible to trigger wind slab avalanches up to a foot deep. The most likely places to find unstable snow will be in upper elevations near ridgelines, convex rollovers, and steep gullies. Safe travel today will require taking the time to identify and avoid steep wind-loaded terrain features. We are also keeping an eye on some deeper layers that may be problematic, but for now it doesn’t seem very likely a person will be able to trigger a deeper avalanche. The danger is LOW below 1000′.

Mon, December 4th, 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
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
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
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

As skies cleared this weekend we saw plenty of evidence of big avalanches during last week’s storm. Yesterday skiers in upper Bertha Creek saw the aftermath of the widest propagating  avalanches that we’ve gotten photos of for this cycle, and they also noticed an avalanche on Granddaddy that looks like it may have failed close to the ground. All of those look like they failed as the storm was finishing last Thursday. The last known human-triggered avalanche was a small wind-loaded pocket on the south side of Tincan on Saturday.

The crown of a recent avalanche that propagated several hundred feet. This is at the top of the Bertha Creek drainage, on the saddle between Pastoral and Granddaddy. Photo: Adam Rothman, 12.03.2023

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

After three days in a row of quiet and sunny weather, an approaching low pressure system is bringing easterly to northerly winds with light snowfall today and tomorrow. For now it is looking like most of our forecast area will only get a trace to an inch of snow today, with 3-5″ possible in Portage and Placer. Easterly winds picked up yesterday evening and are expected to continue to blow at 10-20 mph with gusts of 20-30 mph for most of the day today. This will make wind slabs our main concern for the day once again.

Another round of wind slabs has likely formed in the alpine and upper treeline elevation bands, and will be most often found near ridgelines, convex rollovers, and in steep gullies. Be on the lookout for stiffer snow on top of softer snow, and avoid steep slopes with fresh wind slabs on the surface. Testing small but steep pieces of terrain can be a good way to assess this type of problem. If you notice any red flags like shooting cracks or collapsing, you know you have found a dangerous setup. Be careful of how you interpret those red flags- just because you haven’t seen any shooting cracks doesn’t mean the snowpack is good to go, that’s just one piece of information.

We’re still thinking avalanches similar to the human-triggered wind slab on this mid-slope convexity will be the main concern for today. Photo submitted anonymously, 12.02.2023

 

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

This weekend saw a ton of ski and snowmachine traffic at Turnagain Pass, with one small human-triggered avalanche that we know of. This is an encouraging sign. However, we’ve  been paying close attention to how the snow is behaving around the Thanksgiving crust (now 2-4′ deep) and we’re starting to see things we don’t like in snowpits. We’ve gotten reports of multiple test pits showing unstable test results on that layer, and suspect it might be headed in the wrong direction stability-wise. For now, it seems it will take more snow or continued faceting at the crust interface to make an avalanche, but it is something we will continue to track closely.

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 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 the 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
Mon, December 4th, 2023

Yesterday: Skies were partly sunny with a thin layer of high altitude clouds and some lingering valley fog. Winds were light out of the north before picking up out of the east at around 6 pm at 10-20 mph with gusts of 20-30 mph. High temperatures were in the mid 20’s to low 30’s F with lows in the mid teens to mid 20’s F. No precipitation was recorded.

Today: A weak system moves into the area today, with partly to mostly cloudy skies and some light snowfall possible. Most areas should only see a trace to an inch of snow today, but some places closer to the coast like Portage and Placer should see 3-5” snow with the rain line staying down around 700’. Easterly winds will continue at 10-20 mph for the first part of the day before backing off slightly this afternoon. High temperatures should be in the low 20’s to 30 F, dropping to the upper teens to mid 20’s F tonight.

Tomorrow: Winds should switch to the north tonight, staying light at 5-15 mph with gusts of 10-20 mph through tomorrow. Seward should expect to see slightly stronger winds, especially at higher elevations, with speeds of 15-25 mph and gusts of 20-30 mph. We may see a few more snowflakes tomorrow, with the rain line staying down near sea level. Temperatures will be slightly cooler with highs in the 20’s F and lows in the mid teens to 20 F.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27 0 0 49
Summit Lake (1400′) 19 0 0 20
Alyeska Mid (1700′) 27 0 0 34
Bear Valley – Portage (132′) 26 0 0 –

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 25 ENE 10 33
Seattle Ridge (2400′) 27 SE 5 17
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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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