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Issued
Wed, March 17th, 2021 - 7:00AM
Expires
Thu, March 18th, 2021 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains MODERATE at all elevations. A bump in northwest winds today may be enough to create some new shallow wind slabs in upper elevation terrain. Additionally, there is still a chance a person or snowmachine could trigger a slab avalanche that fails on weak snow buried between 1-3′ feet deep. Keep in mind these slabs may be triggered remotely. Assess the snowpack as you travel and evaluate terrain and consequences.

LOST LAKE/SNUG/SEWARD: The northwest winds are expected to be stronger in this region, increasing the likelihood of wind slab avalanches. Extra caution is advised.

Wed, March 17th, 2021
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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.
Recent Avalanches

Yesterday was a true Alaskan springtime day. Despite the cold temperatures, the sun was able to warm the surface snow enough to initiate natural rollerballs on southerly aspects. We also had reports of skier triggered sluffs on steep slopes. One of these was quite large and began as a dry sluff that transitioned to a moist sluff due to the warming surface snow lower on the slope.

We also had a report of a party on Orca near Girdwood (southerly face around 2,300′) that experienced a large enough ‘whumpf’ (collapse) that they turned around. The weak layer was facets under a sun crust just over a foot deep.

Roller balls on an upper elevation southerly face in the Placer Valley (Squirrel Flats) yesterday. 3.16.21. Photo: Graham Predeger.

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

While another sunny springtime day is in store today, it will be the winds that are the question. The northwest winds should pick up to 15-20mph along the higher peaks today; and 30mph tomorrow. For today, this may be enough to move snow into shallow wind slabs up to a foot thick in areas favored by this flow direction. These areas include Crow Pass, the mountains along Turnagain Arm, Portage and south toward Summit Lake and Seward. Turnagain Pass itself may escape the wind (cross you fingers!). The breezy conditions may also limit surface warming that was experienced yesterday, especially in the upper elevation terrain.

Paying attention to what the wind is doing and watching for active wind loading will be key. This should be fairly straightforward. Other signs of instability such as ‘whumpfing’ and cracks in the snow that shoot out from you are clues we may have found a wind slab or even a persistent slab (more on that in Problem #2).

Sluffs:  There is plenty of soft surface snow that triggering a loose snow avalanche (sluff) in steep terrain should be expected. These can gain enough volume to catch and carry a person on steep sustained slopes. Moist or wet sluffs may be possible to trigger as well on steep south slopes that are able to heat up during the day.

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

Any new wind slabs that form are likely to add to our persistent slab issue. Meaning, they could form over weak persistent grains as there are facets and buried surface hoar that sit under the storm snow from last Wednesday (1-3′ deep). On southerly slopes a sun crust is sandwiched in with this weak snow. As mentioned above, a group of skiers triggered a large collapse at 2,300′ in this layer yesterday (facets under a sun crust) and turned around. This not only points to an unstable snowpack, but that it’s possible to trigger an avalanche remotely (from the top, side or below).

On the flip side, we have had several reports of these layers not being reactive and that is good news. Hence, it is important to evaluate each slope as you come upon it. It is also good to remember this setup does not always give us warning signs. If we do get whumpfing and/or shooting cracks, that’s great info for going somewhere else. If we don’t, we need to decide if we are willing to expose ourselves to the potential consequences. If so, do so one at a time while having escape routes planned and watching our partners closely.

Photo of Graham Predeger’s snow pit in Squirrel Flats yesterday. The sun crust is just below his shovel. Although he did not find any weakness around the crust, we can’t rule out it’s not there just around the corner. Note the amount of soft surface snow available for the wind to transport. 

Weather
Wed, March 17th, 2021

Yesterday:  Springtime conditions were had yesterday with sunny skies and light westerly winds. Temperatures were in the teens above treeline and in the 20’sF below.

Today:  Sunny skies are expected again today. Ridgetop winds are forecast to increase to 10-20mph from the northwest through the day and into tonight. Temperatures are sitting in the single digits from sea level to the peaks this morning and should warm up through the day in the lower elevations to the 20’sF and mid elevations to the teens.

Tomorrow:  Partly cloudy skies and cold temperatures are expected tomorrow with a chance for full sunshine by the afternoon. The northwest ridgetop winds may increase to the 25-30mph range before letting up for a calm and sunny day Friday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 18 0 0 115
Summit Lake (1400′) 12 0 0 46
Alyeska Mid (1700′) 18 0 0 118

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 6 5 W 18
Seattle Ridge (2400′) 18 var 3 12 (NW)
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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.