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

The avalanche danger is CONSIDERABLE at all elevations today. Wet avalanches are likely due to the warm temperatures and cloud cover over the past several days. Wet loose avalanches will be very easy to trigger and can grow quite large when running down long slopes. Wet slab avalanches are also likely today, which are highly destructive and unpredictable. We recommend avoiding steep slopes where there is wet snow on the surface and melt water running through the snowpack. On northern aspects there are some lingering buried weak layers that could cause a surprise avalanche in steep terrain.

WEDNESDAY – AVALANCHE OUTLOOK: Clouds are expected to clear this evening which should finally give the snow surface a chance to freeze overnight. This should lead to typical spring conditions tomorrow, with a solid crust in the morning adding lots of strength to the snowpack and generally safe conditions. In the afternoon when the crust melts and water starts moving through the snowpack again conditions quickly become dangerous and wet avalanches become likely. It is important to monitor snow surface condtions and move to a shadier aspect once the wet snow on the surface is more than ankle deep.

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.

Tue, April 23rd, 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, April 24th, 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, April 24th, 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

Warm temperatures and cloudy skies have brought the onset of the spring shed cycle to our region, with the first known observation of a large wet slab avalanche coming out of Portage along with many glide avalanches across the forecast area. Coastal areas, like Portage, have seen about 1.5″ of water over the past few days which fell as rain below roughly 2000′. This rapid additional of water to the snowpack creates prime conditions for wet slab avalanches to start releasing. The large wet slab below was observed on a NW aspect at 1400′ near Explorer Glacier in Portage and released naturally sometime on Sunday night or Monday morning.

First proper wet slab of the spring shed cycle, with the crown up to 6′ deep. Photo 4.22.24

A small section of this glide avalanche released naturally on the S face of Cornbiscuit with much more snow waiting to come down. Photo from Anonymous 4.21.24

Avalanche Problem 1
  • Wet Slab
    Wet Slab
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Wet Slab
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.

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

Cloudy skies and warm temperatures last night likely prevented a solid refreeze of the wet snow on the surface. After multiple days of warm and cloudy weather the snowpack has rapidly transitioned to springtime conditions and large wet slab avalanches are likely, especially near the coast where an additional 1.5″ of rain over the past few days has weakened the snowpack even more. Wet slab avalanches are very difficult to predict in terms of location, but the timing for wet slabs is prime right now after several days without a solid freeze. These avalanches can be very large and destructive and tend to release naturally once the melt water moving through the snowpack causes existing weak layers to loose strength.

Near the Crow Pass Trail yesterday we found a concerning weak layer about 1′ above the ground where the melt water from the surface was pooling on top of a layer of basal facets from early in the season. The exact depth and weak layer that wet slabs release on is likely to vary across the forecast area depending on the existing snowpack structure. We recommend carefully evaluating the snow surface conditions and avoiding steep slopes where the surface snow has melted and there is water actively moving through the snowpack. To avoid this problem you could try to access higher elevation northern aspects where the snowpack still consists of mostly dry snow.

Wet loose avalanches are also likely today, and can be a precursor to wet slab activity. These are easy to trigger on any steep slope once the snow surface has melted. The size of wet loose avalanches depends on the depth of the wet snow on the surface and the size of slope that the avalanche runs down. These are typically small avalanches but can grow very large under the right conditions.

Glide Avalanches are also releasing naturally and can be very large and destructive. If you can see a glide crack on the slope it is best to avoid spending time underneath because they can release randomly.

Cornice Fall is also likely as the sun weakens the snowpack’s connection with the underlying ridgelines. When cornices are receiving direct sun it is best to avoid being underneath them and give them a wide berth if you are travelling along a ridgeline.

Wet loose avalanche debris along the road to the Crow Pass Trailhead containing some very large chunks of wet snow. Photo 4.22.24

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

There are some lingering persistent weak layers in the snowpack from earlier in April and late March which could cause an outlier avalanche, especially in steep terrain on northern aspects at upper elevations. These layers do not seem to be very widespread but it would be easy to be caught off guard by a persistent weak layer if you are in the springtime mentality and expecting stable snow conditions. If you are trying to access this kind of terrain we recommend digging a pit to evaluate the snowpack structure and look for suspicious weak layers with stability tests before committing to steep terrain.

Weather
Tue, April 23rd, 2024

Yesterday: A mix of partly cloudy and overcast sky cover across the region, with denser clouds near the coast. Precipitation was isolated to areas near the coast, with 0.2″ of water falling as rain below approximately 2000′ in Portage. Winds were light to moderate at upper elevations with averages around 10 mph out of the east and gusts up to 30 mph. Temperatures reached the mid to upper 40s F at low elevations but stayed in the mid 20s F at upper elevations.

Today: Mostly cloudy skies are expected to start the day, which should give way to some breaks of sunshine in the afternoon and evening. Temperatures are expected to reach highs in the 40s F up to about 2000′ with highs in the low 30s F at upper elevations. Winds should be light with averages of 0-10 mph out of the east. Some rain showers are possible along the coast but are not expected to produce much water.

Tomorrow: A clearing trend is expected overnight with mostly sunny skies throughout the day tomorrow. No new precipitation is expected. Temperatures should reach into the 50s F at low elevations and mid to upper 30s F at upper elevations. Winds look to remain light at 5-10 mph out of the east.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 35 0 0 88
Summit Lake (1400′) 36 0 0 38
Alyeska Mid (1700′) 36 0 0 101
Bear Valley – Portage (132′) 40 0 0.2
Grouse Ck (700′) 39 0 0 65

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24 ENE 10 28
Seattle Ridge (2400′) 28 SE 7 16
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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.