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Issued
Sun, April 30th, 2023 - 7:00AM
Expires
Mon, May 1st, 2023 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger will start out MODERATE and rise to CONSIDERABLE above 2500′ as strong winds pick up this afternoon. On northerly aspects there is a buried weak layer of facets about 1-2′ deep that could become more sensitive to human triggers in areas being actively wind loaded. From 2500′ to 1000′ the avalanche danger is MODERATE. Temperatures reaching into the 40s F today combined with potential for rain up to 1300′ this evening will make wet loose avalanches likely. Below 1000′ the avalanche danger is LOW. 

PORTAGE VALLEY: Avalanches failing at mid and upper elevations often impact the low-elevation hiking trails in the spring. Be aware of this overhead hazard even if you are not trying to get up high into the mountains.

Special Announcements

End of season operations: This is the final advisory for the 2022/23 winter season. We will continue to issue periodic snowpack updates for the following week as active weather continues, and will post our final springtime tips at the end of the week.

Sun, April 30th, 2023
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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 recent avalanches have been observed since the two small wind slabs that Andrew reported yesterday (ob here)

Avalanche Problem 1
  • 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 primary concern for today is a layer of facets that exists at upper elevations on northern aspects which is buried underneath 1-2′ of storm snow from the past week. This layer has shown signs that it could produce avalanches with wider propagation based on stability tests in the past week (details here and here). Winds increasing to 30-40 mph throughout the day will start to load this weak layer again which will make triggering a persistent slab avalanche more likely in the afternoon or evening. Shallower wind slabs 6-12″ deep are also possible today in areas with dry snow on the surface that can be transported onto leeward slopes. To identify problematic areas keep an eye out for northern aspects above 2000′ that are being actively wind loaded.

Since this persistent weak layer is in the upper snowpack it is pretty accessible to test with either travelling tests, like hand pits and jumping on small test slopes, or by digging a snowpit to test the weak layer using stability tests like a compression test or extended column test. We have not seen any notable avalanche activity on this layer yet, but it could still be problematic if more cold storm systems arrive this week and add additional snow load. The combination of this weak layer 1-2′ deep and the deep persistent weak layer 3-6′ deep from mid-March are making the potential consequences of triggering an avalanche on high elevation north facing slopes much higher than a typical spring (see additional concern for more info).

The weak layer of facets is buried about 1-2′ deep on northern aspects and in this extended column test it failed both at the top and middle of the 4″ thick layer of facets. Photo 4.26.23 

Avalanche Problem 2
  • 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

The sunshine this morning combined with temperatures reaching into the 40s F at low and mid elevations and the potential for rain up to 1300′ this evening will make wet loose avalanches likely. Yesterday the surface melt freeze crust was starting to soften to the point of being unsupportable around 1-2 pm on east and south aspects and at lower elevations. Pay attention to changing conditions and increasing danger as the snow surface heats up later in the day. If you start to notice wet and sloppy snow near the surface it is time to move to shady aspects or head back to the parking lot. There is a small chance that a wet snow avalanche failing near the surface may pull out a bigger slab on its way down.

Glide Avalanches have been starting to release along south and southeast aspects of Seattle ridge as well as on the south faces of Eddies and Wolverine. Yesterday we saw a lot more glide cracks starting to open up at mid elevations on Seattle Ridge, which could be an indication that the spring shed cycle is around the corner. These can be very large and destructive avalanches when they release, so we recommend trying to avoid spending time in the runout of an open glide crack.

A handful of open glide cracks are visible mid slope on Seattle Ridge. Photo 4.29.23

Additional Concern
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
More info at Avalanche.org

A layer of facets buried 3-6′ deep that caused a series of very large avalanches over that past month and a half (details of most recent near miss here) is still a concern for isolated north facing aspects at upper elevations. This weak layer has been responsible for many close calls so far this season and the uncertainty around where it could still be possible to trigger an avalanche like this makes the consequences of seeking out steep north facing terrain much higher than a typical spring. Unfortunately, the layer is too deep to assess anymore and the only way to mitigate it is to avoid steep north facing terrain. Eventually when the winter weather yields to warmer temperatures we might see this layer become active again as a potential weak layer for wet slab avalanches.

Weather
Sun, April 30th, 2023

Yesterday: Partly sunny to start then day then shifting to overcast cloud cover in the afternoon. Light winds averaging less than 5 mph. High temperatures reached the mid to upper 40s at lower elevations and stayed in the high 20s to low 30s at upper elevations. No new snowfall or rain.

Today: Sky cover is expected to start out mostly clear with clouds moving into the area in the late morning. Winds start out light before ramping up throughout the day, with averages reaching 30-40 mph by late Sunday evening. Light snowfall will accompany the increasing winds this evening and overnight into Monday, with 1-2″ expected to fall in Turnagain pass and Girdwood. Snowline will increase to 1300′ during the warmest part of the day.

Tomorrow: Winds will peak overnight Sunday then drop of slightly with sustained wind speeds of 20 mph throughout the day on Monday. An additional 1-3″ of new snow is expected from Monday morning to Tuesday morning. Snowline is expected be 700-900′ on Monday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 37 0 0 82
Summit Lake (1400′) 37 0 0 35
Alyeska Mid (1700′) 37 0 0 73
Bear Valley – Portage (132′) 38 0 0

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 27 WNW 2 8
Seattle Ridge (2400′) 31 NW 1 5
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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.