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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, April 21st, 2024 - 7:00AM
Expires
Mon, April 22nd, 2024 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE at all elevations today. With four days of above freezing temperatures and continued warm weather with light rain possible up to 2000′ by this afternoon, the chances for large wet snow avalanches are increasing. These may be natural or human-triggered. Wet loose avalanches are likely, and the chances for wet slab avalanches will be on the rise. Be especially cautious if you are sinking up to your boot tops in wet and sloppy snow, and look for drier snow at high elevations and on northerly aspects.

MONDAY – AVALANCHE OUTLOOK: Avalanche conditions are expected to remain similar tomorrow as mostly cloudy skies and light precipitation continue into Monday night. The onset of a wet slab avalanche cycle can be very uncertain, but as long as we are seeing continued warm temperatures with little or no freezing overnight we will be on the lookout for bigger wet snow avalanches.

Special Announcements

Avalanche Center End of Season Operations: We are continuing to issue forecasts on Tuesday, Thursday, Saturday, and Sunday mornings. For the non-forecast days we will give an avalanche outlook on the day prior. The final forecast for the season will be April 28.

Sun, April 21st, 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
Mon, April 22nd, 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
Mon, April 22nd, 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

There were multiple glide avalanches observed yesterday, including activity on Raggedtop, Penguin Ridge, the Placer Valley, and Seattle Ridge. All of these were observed from a distance and did not involve people.

New glide avalanche on Seattle Ridge, just north of the motorized uptrack. 04.20.2024

 

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

It’s been four days since we saw a solid overnight refreeze, and the chances for wet snow avalanches are increasing. The most likely avalanches we will see today will be wet loose avalanches involving the wet snow on the surface of the snowpack. With partly to mostly cloudy skies and light scattered showers today, the chances of natural avalanches are probably not as high as the chances of a person triggering a loose wet avalanche. These are easily manageable by avoiding steep terrain where there is wet, sloppy snow on the surface, but they can be hard to escape if you are caught in moving snow.

The chances for Wet Slab Avalanches are also on the rise. The timing of the onset of a wet slab cycle can be hard to nail down, so there is always a larger degree of uncertainty trying to predict when we will start to see these avalanches. Yesterday we were noticing wet conditions on all aspects up to around 2500′, and that snow has not had a good chance to lock up overnight for the fourth day in a row. Whenever we have a lot of uncertainty with big avalanches, the best bet is to reduce your exposure by avoiding steep terrain. Unlike the wet loose avalanches mentioned above, if we do start to see wet slab avalanches it is likely they will be natural. Keep this in mind as you travel under steep terrain, and be sure to move quickly and travel one at a time if you need to traverse under steep slopes.

Glide Avalanches: We’re continuing to see glide activity picking up throughout all of our forecast zones from Girdwood to Seward. This includes many glide cracks opening up and several glide avalanches releasing over the past few days. These avalanches are large and unpredictable, so it’s important to avoid spending any time under open glide cracks.

The snowpack is heating up, and we are seeing continued glide activity. Multiple glide cracks with several recent glide avalanches in the Skookum Valley. 04.20.2024

Additional Concern
  • Persistent Slabs
    Persistent Slabs
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.
More info at Avalanche.org

Although it is not a widespread problem, there are isolated higher elevation, north-facing slopes that harbor a weak layer of buried surface hoar in the upper snowpack. A group found this layer in Spokane Creek last Saturday, and when we went back there yesterday to see how it was behaving we were surprised to find it still producing really unstable results in our pit (details here). We don’t think this layer exists everywhere, but it is important to dig and assess for this layer before getting into big terrain.

This layer of surface hoar was still very reactive in the snowpit yesterday on a high north-facing slope in the Spokane Creek drainage. 04.20.2024

 

Weather
Sun, April 21st, 2024

Yesterday: Skies were partly to mostly cloudy, with easterly winds backing down from around 30 mph in the morning to 5-10 mph overnight with gusts around 10-20 mph. High temperatures reached the upper 30s to mid 50s F, with lows ranging from the low 30s F at upper elevations to around 40 F at lower elevations. No precipitation was recorded.

Today: Cloud cover is expected to increase through the day, with chances for precipitation picking up this afternoon into tonight. Most areas will likely only see a trace of precipitation, with light rain possible up to 1500-2000’. Winds are expected increase this afternoon into tonight, blowing out of the east at 10 to 20 mph with gusts of 20 to 35 mph. High temperatures will be in the low 30s to low 40s F, with lows in the mid 20s to low 30s F.

Tomorrow: We are looking at another day of mostly cloudy skies with scattered showers bringing a trace to 2” snow at upper elevations and rain below 1000-1500’. Winds should remain out of the east at 10 to 20 mph with gusts around 20 to 25 mph. High temperatures will be in the low 30s to low 40s F with lows in the mid 20s to low 30s F.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 45 0 0 90
Summit Lake (1400′) 41 0 0 40
Alyeska Mid (1700′) 42 0 0 105
Bear Valley – Portage (132′) 44 0 0
Grouse Ck (700′) 43 0 0 67

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 34 ENE 18 56
Seattle Ridge (2400′) 39 SE 9 21
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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.