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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, February 9th, 2023 - 7:00AM
Expires
Fri, February 10th, 2023 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 1000′. Triggering a wind slab 1-2′ deep that is actively forming today or lingering from earlier this week is possible at upper elevations. These are most likely to be found along ridgelines and cross loaded gully features. A weak layer of buried surface hoar 2-4′ deep also exists and could still be reactive to human triggering in isolated areas. Below 1000′ the avalanche danger is LOW. 

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Thu, February 9th, 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. 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

There were no known avalanches triggered yesterday. During a very brief clearing period yesterday we saw part of an avalanche crown that likely released earlier this week at about 3400′ in Todd’s Bowl on the N aspect of Tincan. The avalanche released mid slope about 200′ down from the ridge and looked to be about 1-3′ deep but did not appear to have very wide propagation. This avalanches looked very similar to some other recent wind slabs and storm slabs that released during or just after the last significant wind and snowfall event on Sunday and Monday.

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

Today should be a beautiful day to be in the mountains, with light snow on the surface, mostly sunny skies, and light winds. Winds are expected to be 5-15 mph out of the NW, which might be enough to form some small fresh wind slabs up to 1′ deep at upper elevations due to the very low density snow on the surface right now. These fresh wind slabs could be touchy if they are actively forming, so keep an eye out for snow transport along ridgelines and signs of recent wind transport on the snow surface. There is a chance of finding a larger wind slab 1-2′ deep lingering at upper elevations in steep terrain from the snowfall and strong winds earlier this week.

At lower elevations we have seen some evidence that the new snow from this week is not bonding very well to a melt freeze crust that formed on 1/25 and exists up to about 1500-2000′. For now the snow on top of this weak layer is pretty low density and doesn’t seem to be causing any avalanche activity, but it could become more of a concern with additional snowfall and wind on the way tomorrow and into the weekend. In general the snowpack seems to be adjusting well to the new snow load from earlier this week and avalanche conditions are much less reactive than what we saw during and immediately after that last loading event.

Cornices are getting quite large now and the potential exists for accidently breaking off a large piece and taking a ride down a steep slope. Give these a wide birth along ridgelines and be aware that they are more likely to fail when getting direct sun. Loose snow avalanches (aka sluffs) are also likely in steep terrain with low density dry snow on the surface. Be aware of where your sluff if building up and try to avoid being pushed into unsavory terrain.

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Aspect/Elevation
  • 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.

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

Keep in mind that there is a layer of buried surface hoar about 2-4′ deep from 1/10 that exists across the forecast area but has been showing signs of becoming less likely to produce avalanches. We found this layer in all three of our snow pits yesterday on Eddies, but did not have any significant results on this layer in our stability tests. It is an encouraging sign that this layer is becoming much less reactive, but it is still feasible that this weak layer could be preserved in isolated sheltered areas or where the weak layer is more developed and could still cause large avalanches. It is worth taking some time to dig down and check for this layer in the snowpack before committing to larger terrain features. If you see large upright surface hoar grains in the snowpack or have reactive stability tests, like a compression test or extended column test, then it a sign that the weak layer could be more active in your area.

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

We have lingering concerns about the potential for very large avalanches on the Thanksgiving facet/crust combo towards the base of the snowpack. In Turnagain Pass, where the snowpack is generally thicker, we think this layer is unreactive and in a dormant phase. This weak layer is most concerning in areas with a shallower overall snowpack depth, like near Silvertip and the southern end of the forecast zone in general. An unexpected avalanche on a buried weak layer is more likely in these areas with an overall weaker snowpack structure.

Weather
Thu, February 9th, 2023

Yesterday: Periods of moderate to heavy snowfall mixed with brief clearings in the cloud cover. About 2-3″ of very low density new snow fell on the N end of Turnagain Pass during the day yesterday. Winds were calm, with averages less than 5 mph at ridgeline weather stations. Temperatures stayed in the teens to single digits at upper elevations and mid 20s at sea level.

Today: Cold temperatures in the single digits to teens F with partly sunny skies. Winds are expected to remain light to moderate out of the NW at 5-15 mph. No new snowfall expected today.

Tomorrow: A storm system is moving into the forecast area tomorrow. Strong winds are expected to pick up with averages of 40-60 mph at upper elevations. Moderate snowfall will accompany the windy conditions with 4-10″ of new snow expected on Friday. Temperatures will increase rapidly during the day on Friday with snow line expected to remain below 500′.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 19 2 0.1 67
Summit Lake (1400′) 16 1 0.1 37
Alyeska Mid (1700′) 18 2 0.1 72
Bear Valley – Portage (132′) 24 3 0.3

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 11 W 3 22
Seattle Ridge (2400′) 13 N 1 5
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.