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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, March 19th, 2024 - 7:00AM
Expires
Wed, March 20th, 2024 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger will start out MODERATE and rise to CONSIDERABLE in the afternoon, as the sun and warm temperatures melt the snow surface. Natural wet loose avalanches are likely on steep southern aspects later in the day which can become large on bigger terrain features. Larger avalanches on buried weak layers 2-3′ deep are also possible for human triggering today. To avoid the potential to trigger a large avalanche on a buried weak layer we recommend sticking to lower angle slopes.

Special Announcements

Turnagain Pass Avalanche Awareness Day – Saturday, March 23!
Swing by the Turnagain motorized parking lot between noon and 4pm to grab a hotdog, practice your beacons skills, chat with the forecast team, and possibly test out a demo snowmachine provided by local dealers.

Tue, March 19th, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
Avalanche risk
Wed, March 20th, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
Avalanche risk
Wed, March 20th, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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 avalanche activity was reported yesterday.

Avalanche Problem 1
  • Wet Loose
    Wet Loose
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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

Today is the first in a string of very springlike days coming this week, with partly sunny skies and temperatures expected to rise to the freezing point (32 F) even at upper elevations. The most likely type of avalanche to encounter today is a wet loose avalanche caused by melt on the snow surface on solar aspects (east, south, west). Overnight the temperatures dropped into the mid 20s F, which should have allowed the wet snow and rain that fell yesterday to freeze into a crust on the surface. The name of the game today is tracking the melt of the surface crust on solar aspects to know when to switch to a shadier aspect to avoid wet loose avalanches. A helpful rule of thumb is if your boot sinks into wet surface snow above ankle depth it is time to switch to a shadier aspect. 

Wet loose avalanches are typically small, but on large slopes they can pick up a lot of additional snow and knock you off balance or force into terrain you might rather avoid. Small wet loose avalanches can also be a trigger for larger avalanches on buried weak layers (see problem 2 for more details). Cornices tend to follow a similar pattern to wet avalanches, becoming weaker when they are warmed up by the sun and having a much higher chance of failing naturally. We recommend trying to avoid spending time underneath cornices that are receiving direct sunlight and giving them a wide berth if you are traveling along ridgelines.

In addition to sun triggered avalanches, there are a few other surface type avalanches to look out for today. Lingering wind slabs that formed on Monday could be possible for a person to trigger along ridgelines, gullies, or convex features at upper elevations. Look for shooting cracks or small avalanches releasing on small test slopes to check for reactive wind slabs. Dry loose avalanches are also likely at upper elevations on steep north facing terrain that was sheltered from the latest round of winds.

Digging up a photo from the CNFAC archives with large wet loose avalanches releasing naturally on south facing slopes, similar to what we expect this afternoon. Photo 3.30.22

 

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 larger concern right now is the buried weak layers which are now about 2-3′ deep. The snowpack setup has become more complex as we head into spring, with differences in the snowpack structure and type of weak layer based on aspect and elevation. On southern aspects there is a buried sun crust about 2-3′ deep with either facets or surface hoar on top that caused a lot of human triggered avalanches last week. On shadier norther aspects the sun crust doesn’t exist, but we still saw several human triggered avalanches last week which likely failed on buried surface hoar about 3′ deep at the interface with the new snow from last week. Regardless of the exact weak layer structure, we think it is still possible for a person to trigger a large avalanche 2-3′ deep.

The big picture is that the snowpack is complex right now, but you can simply stick to lower angle terrain to avoid the potential to trigger a large avalanche on a buried weak layer. It is difficult to assess buried persistent weak layers in the field because snow pits are the only tool that is effective to evaluate them. While snow pits provide useful information for tracking weak layers, even correctly executed snow pit tests can provide false stable or inconsistent results and we don’t recommend relying entirely on snow pit information to make ‘go’ or ‘no-go’ decisions. If you decide to travel in steeper terrain we recommend giving yourself a wider margin for error, carefully evaluating the snowpack, and using safe travel practices to minimize your group’s exposure.

Weather
Tue, March 19th, 2024

Yesterday: Cloudy skies and warm temperatures with 0.1-0.3″ of precipitation, which fell as rain up to about 1500′ and 1-3″ of snow above that. Temperatures reached into the mid to upper 30s F at low elevations and stayed in the mid 20s F at upper elevations. Winds started out moderate with averages of 5-15 mph and gusts to 30 mph out of the east until about noon, and then winds backed off to averages of 0-10 mph with gusts to 10-15 mph for the rest of the day.

Today: Quiet weather is expected today, with light winds out of the east and southeast averaging 0-10 mph. Cloud cover is expected to break up today, letting some sunshine through and warming temperatures up into the mid to upper 30s F at low elevations and low 30s F at upper elevations. No new snowfall is expected.

Tomorrow: Wednesday looks very similar to today, with a continued clearing trend bringing mostly sunny skies. Winds will remain light at 0-10 mph out of the west. Temperatures are expected to fall into the mid 20s F overnight and rise back to the mid to upper 30s F at low elevations by the afternoon.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 32 1 0.1 101
Summit Lake (1400′) 32 1 0.1 45
Alyeska Mid (1700′) 31 1 0.1 104
Bear Valley – Portage (132′) 34 0 0.3
Grouse Ck (700′) 35 0 0.2 69

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 26 ENE 6 29
Seattle Ridge (2400′) 29 SE 5 15
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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.