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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, February 16th, 2024 - 7:00AM
Expires
Sat, February 17th, 2024 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 1000′. Human triggered and natural avalanches in areas being actively wind loaded are likely. A weak layer buried 1-3′ deep could also produce very large avalanches. We recommend sticking to low angle terrain and being aware of overhead runout zones to avoid avalanches today.

Below 1000′ the avalanche danger is MODERATE. A thick icy crust exists on the surface at these elevations which could melt due to warm temperatures an sunshine today leading to wet avalanches on the surface.

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.

Fri, February 16th, 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
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
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
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

There were no new avalanches that we are aware of yesterday. The combination of warm temperatures and the sun gaining power this time of year has led to reports of some wet loose avalanches over the past couple days. So far these seem to mostly be on steep south facing slopes.

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 are expected to continue today, forming fresh wind slabs at treeline and above. Despite days of strong winds this week, there was still lots of snow being transported along the surface yesterday. Wind slabs will be forming along ridgelines, gullies, and convex features from treeline elevations and up. Natural and human triggered avalanches are likely with the constant wind loading that has taken place all week. To identify areas with wind slabs look for active wind loading, shooting cracks, and hollow feeling snow. Keep in mind that wind slabs could be firmer than typical due to the steady strong winds and may cause larger than normal avalanches. In addition to the surface wind slabs we also have a persistent weak layer buried about 1-3′ deep in the snowpack that could cause larger avalanches (more details in problem 2).

Wet loose avalanches have also been observed on steep south facing terrain. With temperatures in the 30s F for the past several days combined with the sun gaining strength, it feels like a springtime snowpack out there. We have observed roller balls and wet loose avalanches on steep south facing terrain. These avalanches tend to start out small from a single point near a tree or rock, but can grow to be quite large if they run down a slope and entrain more loose snow on the way down.

Cornices are also a concern today due to the combination of constant loading from the winds this week and warm temperatures and sun weakening their bond to the ridgelines. Try to avoid travelling underneath large cornices if possible, especially if they are being actively wind loaded and receiving sun on the windward side.

Winds are still transporting enough snow to form large plumes along ridgelines, despite the prior 3 days of strong winds. 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 weak layer of faceted snow buried about 1-3′ deep that formed during the cold and clear weather in January remains a concern. We are still uncertain about how widespread this weak layer is at upper elevations, but we have consistently found the weak layer from roughly 1000′ to 2500′ in elevation. This persistent weak layer has the potential to cause very large avalanches and could be remote triggered from above, below, or to the sides of steeper terrain. The best way to avoid triggering an avalanche on this type of weak layer is to stick to low angle terrain and try to avoid runout areas of overhead avalanche paths. One of the tricky parts about persistent weak layers is that you need to dig into the snowpack to assess them. Performing stability tests like an extended column test is a great way to get some information, but if you get a stable test result the layer could still be a problem. It takes time for this type of weak layer to heal, being patient and sticking to safe terrain is the only way to be confident you won’t be surprised by a persistent weak layer.

Snowpack structure from 1500′ on Sunburst. Photo 2.15.24

Weather
Fri, February 16th, 2024

Yesterday: Warm and windy with mostly sunny to partly cloudy skies. Wind speeds averaged 30 mph out of the east at upper elevations with gusts to 66 mph. Near treeline the winds were still blowing roughly 15 mph from the east and still capable of transporting new snow. Temperatures were in the mid 30s F at the road and mid 20s F at upper elevations.

Today: Weather conditions look very similar to yesterday, with strong winds being the defining factor. Temperatures should be in the mid 30s F at lower elevation and mid 20s F at upper elevations. Winds will remain in the 30-45 mph range out of the east with gusts up to 65 mph. Clear skies are expected throughout the day. No precipitation is expected.

Tomorrow: Some cloud cover is expected to move into the area overnight on Friday. Temperatures are expected to remain warm with highs in the mid 30s F a low elevations. Winds will remain strong with averages of 30-45 mph out of the east overnight and through the first half of the day on Saturday, but should start to decrease slightly during the day on Saturday to averages of 20-30 mph out of the east on Saturday afternoon. We could see some light snowfall on Saturday afternoon as well, with up to 1-2″ of snow accumulation. Rain line is expected to be at 1500′ on Saturday afternoon and rise to 2000′ on Sunday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 35 0 0 85
Summit Lake (1400′) 46 0 0 43
Alyeska Mid (1700′) 34 0 0 94
Bear Valley – Portage (132′) 42 0 0
Grouse Ck (700′) 34 0 0 61

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 26 ENE 31 66
Seattle Ridge (2400′) 30 SE 15 33
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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.