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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, December 31st, 2021 - 7:00AM
Expires
Sat, January 1st, 2022 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 2500′ as a weak system brings snow showers and wind to the area today. It is possible a person could trigger an avalanche up to a foot deep on fresh wind slabs forming in the alpine. These will most likely be found just below ridgetops, steep rollovers, and in gullies. Pay attention to continued wind loading during the day, and be on the lookout for warning signs of instability like shooting cracks, collapsing, and fresh avalanche activity.

The avalanche danger is LOW below 2500′, where lighter snowfall and calmer winds will make avalanches unlikely.

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Fri, December 31st, 2021
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
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. Dangerous avalanche conditions. 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

Could all this rain crust be forgot, and never brought to mind?

Could all this rain crust be forgot and buried in no time!

Call me optimistic, but hopefully the 3-6″ snow expected during this New Year’s Eve storm is at least enough to improve riding and skiing conditions. Light snowfall with sustained winds at 10-15 mph began early this morning and is expected to continue until this afternoon before this system passes. This combination of wind and snow will make it possible to trigger an avalanche up to a foot deep on wind loaded slopes in the alpine. Fresh wind slabs are most likely to be found just below ridgelines, on the downhill side of convex rollovers, and in gullies. It is possible that these slabs might be forming on top of a recent round of surface hoar that formed in isolated pockets on top of the recent crusts, which will make them particularly sensitive to human triggers. From what we know so far, this appears to be the exception rather than the rule.

You might be able to recognize a fresh wind slab from a distance as a smooth, rounded pillow of drifted snow. You should also be able to feel the difference in the surface conditions on a freshly wind loaded slope- be on the lookout for a ‘punchy’ feeling, or stiff snow sitting on top of weaker snow. Wind slabs often give warning signs like shooting cracks, collapsing, and other avalanche activity when conditions are unstable. You can seek out more information by testing small terrain features before trying to move into steeper terrain, and step back from the bigger objectives if you notice any warning signs. Even though the potential size of avalanches will be relatively small today, the firm and low-friction bed surfaces will make avalanches fast-moving and make it difficult to get off of a slope if you do trigger something. Today it will be important to pay attention to how the weather is contributing to instability, dialing things back if we end up on the heavier end of the predicted snowfall totals or if you see any of the red flags mentioned previously.

It ain’t much, but it’s something. Let’s keep our fingers crossed that we hit the higher end of these storm totals today! Graphic courtesy of NWS Anchorage.

New snow and winds will be building fresh slabs on top of crusts like these throughout the area. Breakable rain crust on Goat Mtn. yesterday. 12.30.2021

 

Additional Concern
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
More info at Avalanche.org

At this point it is unlikely a person will trigger an avalanche on the weak layer of faceted snow buried 2-5′ deep in most parts of the advisory area, and shallower towards the south end of Turnagain Pass, Silvertip Creek, and the Summit Lake area. The light snowfall expected today will not be enough of a load to put any kind of significant stress on this weak layer, and in the aftermath of the melt-freeze cycle from earlier in the week it would be very surprising if we saw any avalanche activity on this layer. That said, the weak snow is still there, and it is a layer that we will continue to track. It is still worth considering this weak layer while making terrain choices, and avoiding consequential terrain that funnels into terrain traps like gullies or ravines, cliffs, or trees.

Weather
Fri, December 31st, 2021

Yesterday: Mostly sunny skies in the morning gradually became mostly cloudy later in the day. Winds were light at 5-10 mph out of the west for most of the day before switching easterly and picking up to 10-15 mph with gusts of 25-35 mph last night.

Today: Light snowfall began to trickle in early this morning, with 1-2″ as of 5:00 a.m. We should see another 1-3″ snow during the day today, with snow down to sea level. High temperatures are expected in the mid to upper 20’s F for most of the day before dropping into the single digits tonight. Easterly winds are expected to continue this morning at 10-20 mph with gusts of 25-25 mph, and gradually decrease during the day. Skies are expected to remain cloudy all day.

Tomorrow: Temperatures are expected to drop with northwest winds returning following this brief storm. Tomorrow should bring high temperatures in the single digits above 0 F in Girdwood and Turnagain, with negative single digits in Summit Lake. Winds will be around 10-20 mph for most of the core advisory area, and slightly stronger in the typical gaps like Portage, Whittier, and Seward. Skies are expected to be mostly cloudy.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28 1 0.1 65
Summit Lake (1400′) 15 1 0.1 22
Alyeska Mid (1700′) 26 3 0.12 38

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19 W-E* 10* 33
Seattle Ridge (2400′) 22 W-SE* 9* 21

*Winds shifted directions and increased yesterday evening.

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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.