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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, February 26th, 2021 - 7:00AM
Expires
Sat, February 27th, 2021 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains MODERATE at all elevations. Human triggered wind slab avalanches up to a foot deep are possible in the mid and upper elevations. These were formed by the strong winds over the past two days. Additionally, slab avalanches failing in buried weak layers 1-3′ deep are possible at all elevations. Pay close attention to signs of instability and any changing weather later in the day.

* Stormy weather is headed in later this afternoon and the avalanche danger is expected to rise to CONSIDERABLE by tomorrow.

SUMMIT LAKE/LOST LAKE/SNUG:  The mountains south of the forecast zone saw significant wind yesterday. Heads up for triggering wind slab avalanches in these areas as well.

Special Announcements

This coming stormy weather is headed more for Hatcher Pass. Be sure to keep tabs on the HPAC website and Facebook pages!!

Fri, February 26th, 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. 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

Several shallow wind slab avalanches were triggered yesterday. We also saw some evidence of natural small wind slabs from either Wednesday or yesterday.

North face of Cornbiscuit:  A widely propagating wind slab was triggered near the ridge; it is believed to have been human triggered. The slab was roughy 300′ wide and between 4-10″ thick. Debris was reported to be up to 3′ deep (photo below).

Northeast face of Groundhog Ck off the Johnson Pass trail:  Several small (not large enough to bury a person) wind slabs were triggered by snowmachiners (photo of one below).

Tincan Proper?:  We had a report of a crown visible from a distance. It was 200-300′ below the top of Tincan proper on the SW face. If you know anything about this avalanche, please send us a quick report/photo. Thank you!

Wind slab on the N face of Cornbiscuit that propagated fairly wide under the rocks. This is thought to have been triggered from the ridge in the upper left of the photo. 2.25.21.

 

Small wind slab, snowmachine triggered, around 3,800′, NE aspect, in the Johnson Pass area. 2.25.21. Photo: Travis Smith. 

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 it is exciting that we could get a refresh of snow over the next couple days, it won’t be until later this afternoon that we are expecting light snow to start falling and a bump in east winds. Most of the daylight hours today should be fairly mild with cloudy skies, temps in the 20’sF and light ridgetop winds from the east.

The main concern today will be triggering a wind slab. Wednesday the mountains around Turnagain Pass saw strong easterly winds and yesterday strong northwesterly winds. Slabs have formed on a variety of aspects at the mid and upper elevations. These are up to a foot thick and were quite touchy yesterday. They are likely to be more stubborn today, yet something to really pay attention to. Be sure to consider the type of terrain you are getting into and the consequences if a wind slab is triggered.

Slabs should be fairly easy to identity. Look for signs of prior wind loading; smooth rounded surfaces and hollow/punchy feeling snow. Cracks that shoot out from you and a general stiffer feeling snow over softer snow are clues you have found a wind slab. These could be lower on the slope, in cross-loaded gullies, and on the steep side of rollovers.

Along ridgelines, be sure to give cornices a wide berth and limit travel under them. If you find yourself on a steep slope with soft snow and without wind effect, be sure to watch your sluff.

Wind transporting snow along the Cornbiscuit ridge and loading the North aspect. This photo was taken prior to the avalanche that is photographed above on the same face. 2.25.21.

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

Buried in the top three feet of the snowpack are a couple layers of buried surface hoar and/or facets. At elevations below 2,000′ a crust sits under the weak layers, making the lower elevations concerning as well. These layers are still showing signs of being reactive in some pits, yet not in others. This makes for a tricky situation and is reason enough to be cautious. The last known avalanche failing in one of these layers was five days ago on the steep SW face of Eddies Ridge. Slopes that have seen little traffic this winter and that have not been severely wind affected are the most suspect for triggering a slab.

Weather
Fri, February 26th, 2021

Yesterday:  Mostly sunny skies, moderate to strong NW winds and mild temperatures were over the region. Ridgetop weather stations averaged in the 15-20mph, however higher speeds were reported by observers. Temperatures were in the teens along the high terrain and in the low 30’sF at sea level.

Today:  Clouds are moving in this morning ahead of a two part storm cycle. The first pulse will push in this afternoon with light snow falling to sea level. By early tomorrow morning we could see up to 4″ of snow. Ridgetop winds are swinging around to the southeast this morning and should pick up into the 15-25mph range by this evening. Temperatures look to remain in the 20’s at the mid elevations and near 32F at sea level.

Tomorrow:  The second part of the storm cycle heads in Saturday with warmer air. It is looking like we could see another 4-8″ of snow. This will be higher density snow with a rain/snow line ~1,000′. Ridgetop winds will remain easterly, yet climb into the 25-35mph range. By Sunday morning the system heads out and temperatures cool back down.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 26 0 0 114
Summit Lake (1400′) 25 0 0 43
Alyeska Mid (1700′) 26 0 0 115

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17 W 15 41
Seattle Ridge (2400′) 22 NW 7 18

 

Observations
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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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