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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, February 24th, 2024 - 7:00AM
Expires
Sun, February 25th, 2024 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 2500′. New snowfall combined with NW gap winds this afternoon are likely to cause natural and human triggered avalanches up to 1′ deep. Larger avalanches on a buried weak layer 1.5-3′ deep are possible on isolated slopes above 2000′.

The avalanche danger is MODERATE below 2500′. Natural and human triggered avalanches are possible in areas exposed to wind. Loose snow avalanches are likely on steeper terrain.

Special Announcements

NWS Special Weather Statement: Heavy snowfall this morning in Girdwood, Portage, Whittier, Moose Pass, and Seward.

NWS Winter Weather Advisory: Heavy snowfall with 4-10″ of new snow in Anchorage, Eagle River, Indian, and Eklutna.

John Mtn Avalanche Report: The full accident report for the recent avalanche fatality on John Mtn is available on our accidents page.

Sat, February 24th, 2024
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
Sun, February 25th, 2024
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
Sun, February 25th, 2024
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. 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

No new avalanches were reported yesterday. The most recent known avalanche activity was on Thursday after 6-12″ of new snow fell on Wednesday night. There were several small human triggered avalanches in the Seattle Ridge area. In Lynx creek there was a large natural avalanche that likely released on a persistent weak layer due to the way it connected across multiple gully features.

Large natural avalanche in Lynx Creek on a W aspect at 3000′ that likely released sometime Wednesday night or early Thursday morning. Photo 2.22.24

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

Snowfall is expected across the region today, with 4-6″ of new snow falling in Turnagain Pass and Girdwood and closer to 8-12″ in Portage and Placer. Winds are starting out light out of the SE but should switch to NW around noon and increase to averages of 15-25 mph with stronger gusts. With light new snow on the surface and increasing winds we are expecting wind slabs around 1′ deep to form quickly at upper elevations.

Winds from the NW are typically strongest along Turnagain Arm and other gap wind locations, where higher local wind speeds gusting to 50 mph are likely. To identify areas where you could trigger a wind slab look for active wind loading along ridgelines and step off the beaten path to feel the consistency of the surface snow. Using small, safe test slopes to check for shooting cracks or small avalanches is a great way to determine if wind slabs are reactive in the area you are travelling.

If the snowfall forecast of 8-12″ for more coastal areas like Portage and Placer verifies, it is possible that storm snow avalanches could occur in the new snow even in sheltered areas. Using hand pits or small test slopes is a good way to check how well the new snow is bonding to the old snow surface. Dry loose avalanches (aka sluffs) are also likely in steep terrain where the new snow might not readily bond with the old snow surface.

Snowfall totals from Saturday at 3am to Sunday at 3am. Graphic from NWS Anchorage 2.24.24

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

A weak layer of facets buried 1.5-3′ deep is still cause for concern above 2000′. This weak layer formed during the cold spell in January and has likely been responsible for some recent large natural avalanches as well as being the weak layer involved with the avalanche fatality on John Mtn in the Summit Lake area. For the most part the weak layer does not appear to be as unstable in the Turnagain Pass area as it is in Summit lake, but we have considerable uncertainty about the specific locations where this weak layer could still be an issue. Areas like Lynx Creek, Silvertip, and Johnson Pass which typically have a thinner snowpack compared to the rest of the Turnagain Pass forecast area could be more similar to the Summit Lake snowpack.

To avoid this problem entirely we recommend sticking to lower angle terrain and being aware of overhead avalanche paths that could runout to lower elevations. Snow pits are the best tool to identify and assess buried persistent weak layers, but they can provide false stable results and we do not recommend making decisions based on test results from a single snowpit. With the new snow today adding some stress to the weak layer we recommend giving the snowpack some time to adjust before entering consequential terrain.

Below 2000′ there is a stout crust and moist snow on top of the January facets, making it very unlikely to produce an avalanche. Photo 2.22.24

Weather
Sat, February 24th, 2024

Yesterday: Skies were mostly sunny yesterday until the late afternoon when cloud cover started to move into the area. Temperatures remained in the teens F at upper elevations and low to mid 20s at lower elevations. Winds were remarkably light at upper elevations, with averages of 0-10 mph and gusts into the 20-25 mph range picking up overnight. Some snow showers started overnight, but only 0-1″ of snow has fallen so far.

Today: Snowfall is expected today, with 4-6″ of accumulation throughout the day. Winds should be light this morning with averages of 5-10 mph out of the SE, followed by a switch to stronger NW winds this afternoon averaging 15-25 mph with gusts of 35 mph. Temperatures will remain in the teens F at upper elevations and low to mid 20s at lower elevations. Snow is expected to fall down to sea level.

Tomorrow: Skies should clear overnight tonight, bringing mostly sunny skies on Sunday. Temperatures will start to drop into teens to single digits F under the clear skies on Sunday. Winds will remain out of the NW, with averages of 15-25 mph and stronger gusts. With this wind direction gap winds tend to be stronger along Turnagain Arm. No new snow is expected on Sunday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 20 2 0.1 89
Summit Lake (1400′) 17 0 0 47
Alyeska Mid (1700′) 24 1 0.1 94
Bear Valley – Portage (132′) 24 0 0
Grouse Ck (700′) 21 0 0 69

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 14 ENE 3 23
Seattle Ridge (2400′) 17 SE 7 16
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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.