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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, April 4th, 2023 - 7:00AM
Expires
Wed, April 5th, 2023 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains CONSIDERABLE above 2,500′. Strong easterly winds continue to impact the higher elevations. Watch for fresh wind slabs along ridgelines and in cross-loaded gullies. These are likely to be up to a foot thick and easy to trigger. Additionally, there remains a chance a person could trigger a very large avalanche that breaks in a weak layer buried around 3-6′. The danger is MODERATE below 2500′ where wind slabs should be less likely to find, but we are still concerned with the potential for triggering a very large avalanche. Careful terrain selection is recommended, which includes limiting exposure to steep terrain and runout zones.

Tue, April 4th, 2023
Alpine
Above 2,500'
3 - Considerable
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

The last known avalanche within our advisory area was a week ago when a skier remotely triggered a large avalanche to the northeast of Girdwood. During the week prior, 18 very large human-triggered avalanches occurred that were between 3-6′ deep. All of these we suspect failed on faceted snow that was buried on March 14th. There have been relatively few people traveling in the forecast zone the past week.

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

A weak system is moving through today that will bring cloudy skies, breezy east winds, and a few snow flurries. Places closer to the Sound, such as Portage, may pick up a few inches of new snow but Girdwood Valley, Turnagain, and Summit Lake should only see a trace to an inch. We did not get out into the mountains yesterday yet we know from our trusty weather stations that the ridgetop winds have been blowing for 24 hours now in the 10-20mph range with gusts in the 30’s at times.

Fresh wind slabs have, and should continue, to form in the typical areas. These are generally along ridgelines and in cross-loaded gullies. They should be in the foot deep category and easy to spot by watching for active wind loading, cracking in the snow around you, and feeling for stiffer snow over softer snow. Even though new wind slabs should be touchy, they are not nearly as big and dangerous as our Problem 2, the Deep Slab… See below:

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.

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

As winds continue through today, they will be adding more snow on top of a snowpack that is very questionable. As I suspect many people that read this far down in our forecast know, we are still concerned about a person triggering a huge avalanche. It has been exactly a week now since the last one of these occurred. The problem ‘weak layer’ is old faceted snow that sits under all the March snow. This weak layer is roughly 3-6′ deep in the snowpack. Unfortunately, we cannot rely on snowpack tests because the weak layer is too deep for our tests. We simply have to remember it is lurking.

In short, we have entered the scary MODERATE phase where triggering a very large and deadly avalanche is unlikely, but not out of the question. If one occurs it can propagate across entire bowls and run through low angle terrain. This problem will be with us for a while and could rear its head when the snowpack starts warming up. This setup is still keeping us traveling in the lower angle terrain with a conservative mindset.

Weather
Tue, April 4th, 2023

Yesterday:. Overcast skies and breezy east winds were over the region yesterday. No precipitation. Ridgetops winds averaged in the 10-20mph range with gusts in the 30’s. Temperatures were in the low 30’sF below treeline and in the teens in the Alpine.

Today:  Cloudy skies will remain over the region a weak system pushes through. Light snow is expected along the western Prince William Sound with only a trace to a couple inches possible in Girdwood and Turnagain Pass. Ridgetop winds look to remain in the 10-20mph range with gusts in the 30’s. Temperatures should stay in the mid 30’sF at sea level and in the teens in the Alpine.

Tomorrow:  Skies look to start breaking up tomorrow, Wednesday, with some sunshine possible in the afternoon. Ridgetop winds are forecast to decrease and turn westerly (5-10mph). Daytime warming may bring temperatures to above freezing at the mid and upper elevations.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29 0 0 92
Summit Lake (1400′) 28 0 0 46
Alyeska Mid (1700′) 29 0 0 84
Bear Valley – Portage (132′) 34 0 0

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17 NE 14 31
Seattle Ridge (2400′) 22 SE 12 26
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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.