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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, February 23rd, 2024 - 7:00AM
Expires
Sat, February 24th, 2024 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 1000′. The new snow that fell on Wednesday has been blown around by strong westerly winds making wind slabs 1-2′ deep possible for people to trigger today. Larger avalanches failing on a buried weak layer 1.5-3′ deep are possible on isolated slopes above 2000′. We are uncertain about how widespread these buried weak layers are and recommend a cautious approach to steep terrain. Below 1000′ the avalanche danger is LOW.

Special Announcements

Friday night in Seward!
Forecaster Chat, 5-6 pm at the Community Library and Museum room
. Come chat with us about the new “Weekend Avalanche Outlook” product for Seward and Summit Lake. We’ll also be talking about the state of the snowpack in Summit and Seward, and any other questions you have. More info Here.

Fri, February 23rd, 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
Sat, February 24th, 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
Sat, February 24th, 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

Several natural and human triggered avalanches were observed yesterday in the aftermath of the latest round of snowfall. In the Seattle Ridge area there were a handful of small human triggered avalanches that released within the new snow about 1-2′ deep. A larger natural avalanche occurred in Lynx Creek which likely failed sometime yesterday during the storm. This avalanche connected across several gully features and was several hundred feet wide, which is a strong indication that it released on a persistent weak layer.

Snowmachine triggered storm slab from Main Bowl on Seattle Ridge. Photo 2.22.24

Large natural avalanche on a W aspect at 3000′ in Lynx Creek. 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

Strong winds continued to move the new snow into fresh wind slabs yesterday. Overnight the winds started to back off, but it is still possible for a person to trigger a lingering wind slab 1-2′ deep at upper elevations. This type of avalanche typically occurs along ridgelines, gullies, and convex rollovers where snow tends to accumulate from wind transport. Keep an eye out for shooting cracks or hollow feeling snow on the surface to identify areas with wind slabs. You can also use small test slopes or hand pits to check how reactive the new snow is to the weight of a skier or rider.

Storm slab avalanches are also possible in areas that received a foot or more of new snow on Wednesday. These can occur in sheltered areas and fail at the interface with the old snow surface. You can use the same techniques mentioned above to test how well the new snow is bonding to the old snow surface and determine if storm slabs are an issue in the area you are travelling.

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

The warm and rainy whether over the weekend seems to have put a damper on the persistent slab problem at lower elevations. The combination of a strong icy crust and moist snow capping the January facets should make triggering a persistent slab very unlikely below 2000′. Above the rain line from the weekend storm it is still possible that persistent avalanches could occur on isolated wind sheltered slopes where the weak layer is preserved. We recommend carefully evaluating the snowpack conditions in the area you are travelling before committing to steeper terrain. The weak layer of facets is now buried 1.5-3′ deep and can be evaluated using a snow pit to identity and test the weak layer. To avoid this problem entirely we recommend sticking to low angle slopes and being aware of runout zones of overhead avalanche paths.

A natural avalanche in Lynx Creek from Wednesday’s storm highlights the potential for this type of avalanche to connect multiple terrain features and create a very large avalanche. We have a lot of uncertainty about how widespread these conditions are at upper elevations, and the Lynx Creek avalanche seems to be a bit of an outlier compared to the rest of the observations we have heard about in the Girdwood and Turnagain Pass area. The Lynx Creek area often has a thinner snowpack, similar to Silvertip and Johnson Pass areas, which can act more like the Summit Lake snowpack than the rest of Turnagain Pass.

Close up of the crown, connecting three gully features to create a large avalanche with two distinct lobes of debris. Photo 2.22.24

Weather
Fri, February 23rd, 2024

Yesterday: Snowfall tapered off in the early morning and cloudy skies slowly broke up to let some sunshine through. Winds shifted to the west around 8 am but stayed elevated at upper elevations, with averages of 15 mph and gusts up to 50 mph. Temperatures started to drop as the cloud cover cleared, with temps in the teens to low 20s F at upper elevations and mid 20s to low thirties at lower elevations.

Today: Skies should start off mostly clear with cloud cover expected to build up throughout the day. Winds should be lighter than yesterday, with averages of 5-10 mph out of the west in the morning and then shifting to the southeast in the afternoon. Temperatures should start out in the teens to 20s F and rise to highs in the mid 20s F today. No new snowfall is expected today.

 Tomorrow: Overnight on Friday and during the first half of Saturday snow showers are expected. Weather models disagree about how much accumulation there will be, with a range of 1-4″ between the different models for Turnagain Pass and Girdwood and closer to 4-8″ for Portage and Placer. Winds are expected to stay light at 5-10 mph out of the SE during the snowfall and then shift to NW around midday Saturday and increase to averages of 25-35 mph. Skies look to clear again on Sunday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 24 0 0 91
Summit Lake (1400′) 22 5 0.3 47
Alyeska Mid (1700′) 26 1 0.1 93
Bear Valley – Portage (132′) 32 0 0.1
Grouse Ck (700′) 26 0 0.0 73

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 15 W 14 48
Seattle Ridge (2400′) 19 W 4 17
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.