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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, April 29th, 2023 - 7:00AM
Expires
Sun, April 30th, 2023 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 1000′. It is possible a person will be able to trigger an avalanche up to 1-2′ deep where new snow from the past week is sitting on top of weaker facets, especially where it has been blown into a stiffer wind slab. It will also be possible to trigger wet loose avalanches as temperatures rise later in the day. The danger is LOW below 1000′.

 

PORTAGE VALLEY: Avalanches failing at mid and upper elevations often impact the low-elevation hiking trails in the spring. Be aware of this overhead hazard even if you are not trying to get up high into the mountains.

Special Announcements

End of season operations: We will issue our final advisory of the season tomorrow (4/30). We will continue to issue periodic snowpack updates for the following week as active weather continues, and will post our final springtime tips at the end of the week.

Sat, April 29th, 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

We triggered two small wind slab avalanches with ski cuts in steep north-facing terrain near Taylor Pass yesterday. These were only around 6″ deep and 10-20′ wide, but they ran for a few hundred vertical feet.

Debris from a skier-triggered wind slab (left) and a natural dry loose avalanche (right) near Taylor Pass yesterday. 04.28.2023

Avalanche Problem 1
  • 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

The main concern for today will be triggering an avalanche in steep, upper elevation terrain where snow from the past week is sitting on top of weaker facets. We have seen reactive pit results on this layer over the past few days (details here and here), and we triggered some small avalanches with ski cuts in steep, wind-loaded terrain near Taylor Pass yesterday (details here).  These avalanches could be anywhere from 6″ to 2′ deep, and they are becoming less likely with more time to settle out. With limited observations over the past week, there is some uncertainty as to just how widespread this problem is. That means safe travel today will require constant assessment before committing to steep terrain. Be on the lookout for warning signs like shooting cracks and collapses, and seek out feedback from the snowpack as you travel. This can be as simple as some quick hand pits to see how well the newer snow is bonding to the older snow, or using smaller slopes to test the surface snow. If you have any doubt about the likelihood of triggering an avalanche, you can avoid the problem by sticking to lower angle slopes.

We’ve seen multiple snowpits with unstable results on Tincan this week. We haven’t yet seen any avalanches on this layer, but there has been very little traffic since it was buried. Photo: Andy Moderow. 04.28.2023

Avalanche Problem 2
  • Wet Loose
    Wet Loose
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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

In addition to the dry snow problems mentioned above, we will also be on the lookout for loose wet avalanches as things heat up today. The likelihood of these wet snow avalanches will depend on how quickly the clouds move in this afternoon. Pay attention to changing conditions and increasing danger as the snow surface heats up later in the day. If you start to notice wet and sloppy snow near the surface it is time to move to shady aspects or head back to the parking lot. There is a small chance that a wet snow avalanche failing near the surface may pull out a bigger slab on its way down.

Glide Avalanches: We are seeing more and more glide activity as the snowpack continues to heat up. The timing on these is impossible to predict, and they can be very large because they involve the entire snowpack. Be sure to avoid spending time below glide cracks, as they can release unexpectedly.

Rollerballs like this are often a precursor to wet snow avalanches. If you start to see these as things heat up through the day, it is time to move into shady slopes or head back down for the day. 04.28.2023

We have seen quite a few glide avalanches releasing over the past week, and similar activity is possible today. This glide avalanche on the south end of Seattle Ridge occurred sometime Thursday or Friday. 04.28.2023

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 are still tracking the weak layer of facets that was buried in the middle of March and was likely the culprit for a large skier-triggered avalanche near Girdwood 10 days ago (details in this near-miss report). The chances of triggering a very large avalanche on this layer are small, but the consequences are severe. These deep slab avalanches are very difficult to predict, and often catch people by surprise. Consider the consequence of triggering one of these large avalanches before moving into steep terrain. With this extra layer of uncertainty, we are avoiding some of the bigger objectives we might otherwise be considering this time of year. If you want to avoid the problem entirely, you can simply stay off of steeper slopes.

Weather
Sat, April 29th, 2023

Yesterday: Skies were partly to mostly cloudy with high temperatures reaching the low 40’s F at lower elevations and the mid to upper 20’s F at higher elevations. The day started with moderate easterly winds at 15-20 mph but those winds quickly backed down to 5-10 mph with gusts of 10-20 mph for most of the day. We did not see any precipitation in the past 24 hours.

Today: Skies should start off mostly clear with increasing clouds through the day. High temperatures are expected to get back into the low 40’s F at lower elevations and into the mid 30’s F at upper elevations. Winds should be light out of the north to northwest, and we are not expecting any precipitation today.

Tomorrow: We are looking at a return to more active weather starting later tomorrow afternoon, with easterly winds increasing through the day and getting up to 25 mph with gusts to 40 mph before sunset. Chances of precipitation increase later in the day, but we will most likely only see a trace of snow before sunset. Skies will start off partly cloudy with increasing clouds through the day. Look for continued stormy weather into the first part of next week.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 36 0 0 83
Summit Lake (1400′) 37 0 0 35
Alyeska Mid (1700′) 35 0 0 75
Bear Valley – Portage (132′) 36 0 0

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23 ENE 8 16
Seattle Ridge (2400′) 28 SE 8 11
Observations
Recent Observations for Turnagain Pass
Date Region Location
11/16/23 Turnagain Observation: Tincan Common
11/14/23 Turnagain Observation: Sunburst/Magnum
11/14/23 Turnagain Observation: Seattle Ridge
11/13/23 Turnagain Avalanche: Tincan Trees
11/12/23 Turnagain Observation: Sunburst
11/12/23 Turnagain Avalanche: Goldpan – avalanche
11/11/23 Turnagain Observation: Tincan Common
11/11/23 Turnagain Observation: Taylor Pass – Sunburst
11/10/23 Turnagain Observation: Tincan
11/10/23 Turnagain Observation: Tincan Trees
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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.