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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, February 27th, 2024 - 7:00AM
Expires
Wed, February 28th, 2024 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 2,500′ due to strong winds overnight creating wind slabs in the higher elevations. New wind slabs between 1 to 2 feet deep may be easily triggered on slopes with recent wind loading. The chance of finding a wind slab in the mid elevations is lower, but still exists in exposed areas. Additionally, give cornices a wide berth and keep an eye on the sunshine on south slopes in case some wet loose avalanches occur.

SUMMIT PASS:  Wind slab avalanches are a concern in the Summit area as well. There is also a buried weak layer 1-2′ deep that has us on guard. If a wind slab is triggered it has the potential to step down into this layer, creating a larger slide. Extra caution is recommended.

SEWARD/LOST LAKE:  Strong northwest outflow winds are expected through today and into the remainder of the week. Natural wind slab avalanches remain likely in this area.

Tue, February 27th, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
Wed, February 28th, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
Wed, February 28th, 2024
Alpine
Above 2,500'
3 - Considerable
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 known avalanches were reported yesterday, however yesterday was cloudy with little visibility. Chances are there were small wind slabs occurring last night with the strong easterly ridgetop winds and few inches of new snow. The last known avalanches were natural wind slabs caused by strong westerly winds in the higher elevations from over the weekend.

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

Aw shucks…. The storm that was supposed to bring heavy snowfall to Turnagain Pass last night was a bust. Snow lovers can be grateful for 4-6″ of new snow and possibly 8″ in favored locations, but it was not the foot+ some were hoping for. Girdwood Valley faired a bit worse with only 2-4″ and Summit and the central Kenai 1-3″.  The winds on the other hand did produce, they were easterly 15-25 mph with gusts in the 40’s during the snowfall.

This morning skies are clearing and the ridgetop winds are turning northwesterly, ushering in cold temperatures for the week. It looks like Turnagain Pass will be spared the majority of the NW winds today, but could see them tomorrow. Areas surrounding Turnagain could see stronger winds, especially areas to the south near Seward. That said, wind slabs from last night or new ones today is our main concern.

With varying degrees of northwest wind forecast today, it’ll be key to watch for areas with active wind loading. If the wind is calm, look for clues where last night’s easterly winds may have built slabs. Stiff snow over softer snow, smooth rounded surfaces, and cracks that shoot out from you will hopefully be easy to see. Wind slabs are likely to be around a foot deep and composed of soft or stiffer snow.

Cornices:  Other things to watch for as skies clear and allow for much easier travel to the high elevations are cornices. Be sure to give these features an extra wide berth. Cornices may just need a bit of weight to break off and could trigger a slab below (such as seen in the photo).

SUN EFFECT:  Steep and rocky south facing terrain could see enough warming by the sun that wet loose avalanches may release. These have the potential to trigger wind slabs below. Keep a close eye out for warming conditions, it’s that time of year that the sun can definitely effect the snow. Warming can also make wind slabs easier to trigger if we are headed to those slopes facing south.

 

Older natural cornice fall that triggered a slab avalanche below near Kickstep at Turnagain Pass. This photo was taken on Sunday and the avalanche likely occurred during stormy weather last week, or even as recent as Saturday 2/24. Thanks to Scott Johnson for the image, 2.25.24.

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 layer of faceted snow sits between 1.5-3′ deep in many areas. This layer has been showing signs of gaining strength in snowpits and in the Turnagain forecast zone has not been producing avalanches. However, due to facets being notoriously tricky we are still keeping it on our radar. The most concerning elevations are 2,000′ to 3,000′. The most concerning locations are those south of the forecast zone around Summit Pass and in the central Kenai mountains. As skies clear and folks get out to more areas, be sure to know this layer exists and though unlikely, could produce a surprise avalanche without any warning signs in advance.

To avoid to issue, sticking to lower angle terrain, especially in areas south of Turnagain Pass, is a good plan.

 

Weather
Tue, February 27th, 2024

Yesterday:  Overcast skies were over the region with strong easterly winds along the ridgetops (15-25 mph gusting 30-50 mph). A heavy snowfall event forecast for the overnight hours did not produce from what weather stations are reporting. Only 4-8″ of snow for Turnagain Pass and Placer/Portage Valleys and 2-4″ for Girdwood Valley.

Today:  Clearing skies, valley fog, and decreasing winds are expected today as the weather system moves out. No additional precipitation is expected. Ridgetop winds are turning northerly this morning and should blow 5-10 mph from the northwest, gusts near 20 mph. Temperatures look to sit in the teens to 20s F before falling to near 0 F tonight.

Tomorrow:  Mostly clear skies with patchy valley fog is expected tomorrow, Wednesday. The northwest winds look to pick up into the 10-15 mph range with gusts in the 20s around Turnagain and double these numbers for the Seward area south of Turnagain. Temperatures should be cold, -10 to +10 F at all elevations.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 20 5 0.3 93
Summit Lake (1400′) 20 1 47
Alyeska Mid (1700′) 20 2 0.1 94
Bear Valley – Portage (132′) 26 3 0.25
Grouse Ck (700′) 24 0 0 66

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 10 ENE 20 46
Seattle Ridge (2400′) 13 SE 11 24
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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.