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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, February 17th, 2024 - 7:00AM
Expires
Sun, February 18th, 2024 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 1000′. Strong winds continue to form wind slabs at treeline and above which remain likely for human triggering today. A weak layer buried 1-3′ deep could also cause very large human triggered avalanches. We recommend sticking to low angle terrain to avoid this widespread weak layer.

Below 1000′ the avalanche danger is MODERATE. Warm temperatures and light rain up to 1500′ today could cause wet avalanches on the snow surface on steep terrain features.

Special Announcements

Summit Lake Avalanche Accident:  An avalanche was triggered on Tuesday February 13th by a group of three backcountry skiers on John Mtn. Two of the three people involved sustained injuries and the third did not survive. Our deepest condolences go out to the friends and family of the deceased. A preliminary accident report is available here and we will publish a full report by the end of next week.

Sat, February 17th, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Sun, February 18th, 2024
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Sun, February 18th, 2024
Alpine
Above 2,500'
2 - Moderate
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. 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

We have not received any reports of new avalanche activity.

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

It’s been an unusual week of weather, with mostly clear skies, unseasonably warm temperatures, and very strong sustained east winds for 5 days. The winds are expected to gradually decrease today as the weather pattern shifts and some light precipitation and cloud cover move into the area. The most likely avalanche to encounter today is a wind slab, which you could find anywhere from treeline up to alpine elevations. Due to the sustained strong winds, it is possible that wind slabs could be firmer and larger than typical. Human triggered and natural wind slab avalanches are likely and could be anywhere from 1-2+’ deep. Keep an eye out for wind slabs in areas with active wind loading – like along ridgelines, gullies, and convex features. You can identify wind slabs by looking for hollow feeling snow or shooting cracks and using small test slopes to see if they are reactive to the weight of a skier.

Wet loose avalanches are also possible today, especially below 1500′ where the warm temperatures and rain showers expected today could melt the icy surface crust. These tend to occur on steep slopes and are typically not large enough to bury a person but can grow in size if they run far down slope and entrain more snow on the way down.

Sustained strong east winds with clear skies are not a typical weather pattern for us. Graphic 2.17.24

Wind transport down to treeline elevations on Sunburst, with lots of snow blowing off ridgelines. Photo 2.15.24

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 buried 1-3′ deep in the snowpack which formed during the January cold spell has been causing a lot of large avalanches this week, including the avalanche fatality on John Mtn in the Summit Lake area. This persistent weak layer is a little bit less concerning in Turnagain Pass compared to Summit Lake, but human triggered avalanches are still possible. This type of weak layer is concerning because it can cause very large avalanches and release on uncommon terrain features like in sheltered areas at low elevations. Avalanches can also be triggered remotely with this type of weak layer, which means you can trigger an avalanche from low angle terrain which releases on adjacent steeper slopes. In parts of the Turnagain forecast area with a typically shallower snowpack, like Silvertip, Lynx Creek, or Crow Creek, this layer may be weaker and act more similarly to Summit Lake.

The best way to stay safe during periods with an active persistent weak layer is to stick to lower angle terrain and be aware of runout areas from overhead avalanche paths. Persistent weak layers can be tricky to assess in the field because they often do not provide any obvious signs of instability, like red flags, before causing an avalanche. Digging a snowpit to identify the weak layer in the snowpack and test it using a stability test is a good way to track the weak layer in the area you are travelling. Often snowpit tests can give inconclusive or false stable results on persistent weak layers, so we recommend using them as a tool to get more information about the snowpack structure but not making a decision to go into avalanche terrain based on the results of snow pit tests. It is important to be patient during periods of persistent avalanche problems and give the snowpack time to heal.

Weather
Sat, February 17th, 2024

Yesterday: Another day of clear skies, warm temperatures, and strong winds. Temperatures were in the mid 20s F at upper elevations and low to mid 30s F at low and mid elevations. Winds averaged 25-40 mph out of the east with gusts up to 66 mph at upper elevations.

Today: The saga of clear, warm, and windy days looks like it is coming to a close today. Cloud cover is expected to build up overnight leading to mostly cloudy to overcast conditions on Saturday. Snow showers should start in coastal areas in the morning and could gradually push inland towards Turnagain Pass and Girdwood. Only a trace to 1″ of snow accumulation is expected today, with the precipitation switching to rain up to about 1500′. Temperatures should remain in the low to mid 30s F at lower elevations and mid 20s F at upper elevations. Winds are expected to taper off throughout the day, but will still be averaging 20-35 mph out of the east with gusts possible up to 50 mph.

Tomorrow: Sunday looks similar to Saturday but winds are expected to continue to die down. Warm temperatures in the 30s F at low elevations and mid 20s F at upper elevations should persist. Snow and rain showers are expected throughout the day with rain line up to 2000′ and only about 1″ of accumulation at upper elevations. Skies should remain mostly cloudy to overcast. Winds will shift slightly to the southeast and drop to averages of 5-15 mph and gusts to 25 mph. Monday afternoon looks like our next chance for significant new snowfall, which could be a welcome refresh for snow conditions!

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 36 0 0 84
Summit Lake (1400′) 29 0 0 43
Alyeska Mid (1700′) 36 0 0 94
Bear Valley – Portage (132′) 44 0 0
Grouse Ck (700′) 32 0 0 61

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 26 ENE 28 66
Seattle Ridge (2400′) 32 SE 12 25
Observations
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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.