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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, April 22nd, 2022 - 7:00AM
Expires
Sat, April 23rd, 2022 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is rising to CONSIDERABLE as strong wind, rain and snow impact the region. Up to a foot of new snow could accumulate by this evening in the high terrain while rain on snow will destabilize the mid and lower elevations. Natural avalanches are possible and human triggered avalanches are likely. These will be wet snow avalanches (wet loose and wet slabs) in the mid and lower elevations and dry snow avalanches (wind slabs, storm slabs and cornice falls) in the upper elevations. Cautious route-finding is recommended for anyone venturing out in the stormy weather.

*Avalanche danger could increase to HIGH tomorrow if the storm continues to peak overnight tonight. 

Special Announcements
  • End of Season Operations:  Forecasts will be issued on Tuesday, Thursday, Saturday and Sunday through the end of April. If a dramatic change in avalanche conditions occurs on an off day, like today, we will provide a forecast. The final forecast will be on Saturday, April 30th.
Fri, April 22nd, 2022
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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

Girdwood Valley:  A large natural wet slab avalanche released yesterday evening on the lower east shoulder of Raggedtop. Note the wide propagation along the terrain. See more photos HERE.

Large wet slab avalanche that occurred yesterday evening on the lower east shoulder of Raggedtop. Photo taken by George Creighton from the Crow Creek Road 4.21.22.

Avalanche Problem 1
  • Wet Slab
    Wet Slab
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Wet Slab
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.

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

After a brilliant 12-day stretch of clear weather, a good-sized low pressure is churning south of Seward and impacting the coastal mountains. This is bringing moderate to heavy rainfall up to 1,000′ coupled with strong ridgetop east winds. Around 3-5″ of snow has fallen overnight in the high elevations but another 5-10″ is expected today with another 5-10″ overnight tonight. The rain/snow line looks to stay near or just above 1,000′.

With springtime avalanches beginning to occur and a storm to add to the equation, we can expect a variety of avalanche problems in the mountains. The largest and most destructive will be wet snow avalanches that could occur in the mid and lower elevations. Rain, or wet snow, on the existing snowpack, along with cloud cover keeping the old surface from re-freezing, will both contribute to naturally occurring wet slab and wet loose avalanches. These could be very large and scour down to the ground, as seen in the wet slab that released on Raggedtop yesterday. These types of avalanches are common in the spring when the snowpack becomes saturated and wet. It looks as though we are headed into a period of the springtime shed cycle where the snowpack is literally falling apart and we should start to see more and more avalanches like this in the coming week. Something to be extra careful for when the skies clear again.

Another photo looking up the path of a portion of the wet slab avalanche from yesterday on Raggedtop. Photo George Creighton 4.21.22.

 

Glide Avalanches: In addition to the wet avalanche issues, glide avalanches are occurring as well. These can look similar at times when wet avalanches scour down to the ground giving the debris a dirty look. With the mountains starting to shed their snow right now, it’s good to be extra conservative and just avoid being under any glide cracks.

Glide crack on Cornbiscuit photographed by John on Wednesday 4.20.22.

Avalanche Problem 2
  • Storm Slabs
    Storm Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

In the higher elevations where dry snow is falling we can expect the typical wintertime storm snow avalanche problems to be forming. These are wind slabs on wind loaded slopes. They could be up to 2′ thick by this evening and very easy to trigger. Storm slabs are possible on slopes out of the wind in places where over a foot of snow piles up. And last, Cornice falls. I’d expect this storm to push some of the large cornices over the brink of failure.

This new snow is falling on a mixed bag of surfaces. On southerly facing slopes and all aspects below ~2,000′ a sun crust was present before the storm. On more shaded northerly slopes in the higher elevations there was a mix of soft snow and wind crusts with some surface hoar on top. The new snow may or may not stick to the crusts, but is most likely not going to stick that well to the loose snow and surface hoar. As with any rapid loading snowfall event, we should always be suspect of storm snow avalanches during and just after a storm. Looking ahead, once the sun comes back out all this new snow could become pretty touchy as the sun heats it up.

 

Sunburst 5am image of the snow storm up along the ridge. Images are found on the Sunburst weather station page HERE

Weather
Fri, April 22nd, 2022

Yesterday:  Mostly cloudy skies were over the area with light precipitation during the evening and overnight hours. The rain snow line was around 1,400′ with 3-5″ of snow above this. Easterly ridgetop winds increased through the day and picked up significantly overnight into the 25-35 mph range with gusts in the 50’s.

Today:  This will be a stormy day with up to .5″ of rain below 1,000′ and 5-10″ of snow above this in the higher terrain. Tonight, another .5″ of rain is expected with an additional 5-10″ of snow. Ridgetop winds will remain easterly in the 25-35mph range with gusts in the 60’s. Temperatures will remain near 40F at sea level and in the mid 20’sF along ridgelines.

Tomorrow:  The storm should begin to abate tomorrow with light rain and snowfall lingering. Models are showing around 3-5″ of snow could fall during the day above 1,500′. Ridgetop winds are forecast to lessen to around 15-25mph with gusts in the 30’s. Temperatures should remain in the 40’s at sea level in the mid to upper 20’sF along the ridges.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 35 3 0.3 105
Summit Lake (1400′) 34 0 0 35
Alyeska Mid (1700′) 34 1 0.16 N/A

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23 NE 22 57
Seattle Ridge (2400′) 27 SE 15 29
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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.