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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, April 4th, 2024 - 7:00AM
Expires
Fri, April 5th, 2024 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is MODERATE at all elevations. Small wind slabs could form during the day with 1-4″ of new snow expected to fall with moderate east winds. Otherwise, we are still concerned about a person triggering a larger slab that fails on a slick crust under the storm snow from last weekend.

UPPER GIRDWOOD and PORTAGE VALLEY:  These areas might see more than 4″ of new snow today. In this case the danger could rise toward CONSIDERABLE for fresh wind slabs that may release naturally.

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Thu, April 4th, 2024
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Fri, April 5th, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Fri, April 5th, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
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 known avalanches yesterday. The last avalanches were two days ago on April 2nd when several large slabs were triggered remotely by snowmachiners in Main Bowl on Seattle Ridge and a skier triggered slab in the Notch Mtn area of Girdwood Valley.

We investigated the avalanches in Main Bowl yesterday and were taken with how large and scary they were. They all ran on a slick bed surface under last weekend’s storm snow with signs showing they could still be triggered, especially on a hot afternoon when the surface crusts melt.

This is a screenshot of a video taken by Dillon Vought of a snowmachiner triggering the smallest of the slabs in Main Bowl while climbing up the slope. The rider was not caught. 4.2.2024.

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

After two quiet weather days, a weak front is passing through bringing light snowfall and moderate ridgetop winds. Between 1-4″ of new snow is expected with ridgetop winds blowing 10-20mph with gusts in the 30s. This is not much snow or blow, but probably just enough to form some small touchy wind slabs on leeward slopes (6-10″ thick). A surface crust exists on most east, south, and west aspects that could act as a slippery bed surface for new wind slabs. Areas with more snow and wind such as Portage Valley could see larger wind slabs.

Keep an eye out for areas with active wind loading and before getting onto anything steep understand there are more issues than just shallow wind slabs out there. That storm snow from last weekend is still a concern. More on this in Problem 2 below.

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

Under this new round of snow and wind is 1 to 2 feet of settled storm snow from last weekend. That snow fell on slick crusts on east, south, and west aspects and harder snow on northerly aspects. It did not bond well and is still showing signs that a person could trigger a slab, similar to those on Tuesday, April 2nd.

We were able to get into the crown of the slab on Widowmaker in Main Bowl yesterday. We confirmed the weak layer was a very thin layer of small buried surface hoar with some facets mixed in over a slick crust as a bed surface. We also did some snowpit tests and found the layer is still reactive, but getting tougher to trigger. This is a scary setup because it’s not as touchy yet still able to produce a large avalanche.

As mentioned above, during the heat of the day when the surface crusts melt, is the most likely time a slab could be triggered. On shaded slopes without surface crusts it’s a bit trickier as they may not have this setup with the crust/weak layer – but this has not been confirmed and something we’ll be looking for. For those hunting for the soft surface conditions in avalanche terrain, having a cautious mindset and looking for any signs this weak interface exists will be prudent. Digging into the snow and seeing if there is a crust in the top few feet is a good first step. If wishing to avoid these issues, sticking to lower angle slopes is always an answer.

 

Photo of the avalanche crown on Widowmaker in flat light. 4.3.2024.

 

Weather
Thu, April 4th, 2024

Yesterday:  Overcast skies were over the region with some sun poking through in the afternoon. No precipitation. Ridgetop winds were light from a south and east direction (5-10mph). Daytime temperatures warmed to the upper 20s F along ridgelines and 40F at sea level.

Today:  Cloudy skies and light snow is expected today with light rain below 500′ at times. Only 1-4″ is expected with higher amounts near the coast (Portage Valley). Ridgetop winds are increasing this morning and should average 10-20mph with gusts in the 30s. Temperatures should remain in the low 20s F along ridgelines and mid 30s at sea level.

Tomorrow:  Cloud cover should break up a bit tomorrow with a shift in winds to the northwest. These winds look to increase into the 15-25mph range along ridgetops and possibly stronger in favored areas. A few snow flurries could be seen in the morning. Temperatures should be near 32F at sea level and 20F along ridgelines. As of now, the weekend looks like active weather will continue with a chance for a few inches of snow.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 26 0 0 98
Summit Lake (1400′) 27 0 0 46
Alyeska Mid (1700′) 28 0 0 114
Bear Valley – Portage (132′) 32 0 0
Grouse Ck (700′) 32 0 0 77

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22 E 5 30
Seattle Ridge (2400′) 23 SE 6 17
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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.