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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, April 14th, 2023 - 7:00AM
Expires
Sat, April 15th, 2023 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 2500′. Snowfall and wind throughout the day will build fresh wind slabs up to 1′ deep. The new snow is falling onto an icy snow surface and wind slabs forming along upper elevation ridgelines will be likely for human triggering and possible for natural avalanches. There is some uncertainty about whether lingering deeper weak layers could become reactive again with this added snow load.

From 1000′ to 2500′ the avalanche danger is MODERATE. Wind slabs are less likely in this elevation band and there is potential for wet loose avalanches associated with daytime warming. Below 1000′ the avalanche danger is LOW. A springtime melt freeze cycle is in place at lower elevations and the snow surface will likely soften throughout the day and make wet loose avalanches likely in the late afternoon and evening.

Special Announcements

Avalanche Center End of Season Operations:  This is our last week of 7 day/week forecasting. Beginning April 17 we will forecast 4 days/week (Tuesday, Thursday, Saturday, Sunday). The final forecast is scheduled for April 30th.

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Fri, April 14th, 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
Sat, April 15th, 2023
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
Sat, April 15th, 2023
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
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. Dangerous avalanche conditions. 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

We saw lots of evidence of loose snow avalanches from the past few days in Turnagain Pass, but did not see any recent slab avalanches. The light was fairly flat throughout the day so it was not the best conditions for avalanche spectating. We saw many avalanches from the last few days that released naturally in steep rocky areas and caused large loose snow avalanches that entrained a fair bit of snow on their way down the mountain. This type of avalanche was observed in the majority of areas with sustained steep terrain, regardless of aspect.

Example of a long running wet loose avalanche from earlier this week that entrained enough new snow on the way down to create a large debris pile. Photo 4.13.22

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

A little bit of new snow overnight coupled with sustained winds at 10-15 mph with gusts to 25 mph are forming fresh wind slabs up to a foot deep that will be likely for human triggering today. In areas closer to Prince William Sound, like Portage and Placer, the new snow could add up to 6-12″ today which will create larger wind slabs up to 2′ deep. The most likely place to find these fresh wind slabs will be along upper elevation ridgelines and cross loaded gullies. Looking for signs of active wind loading, stepping off the beaten path to check for hollow feeling wind transported snow, and using test slopes to check for shooting cracks are all good ways to identify areas with sensitive wind slabs. The new snow fell onto a melt freeze crust in most locations so wind slabs might not bond well with the old snow surface and avalanches could run further than normal thanks to the firm bed surface.

In areas that saw more new snow earlier this week, like Portage and Placer, today’s new snow and wind could cause larger avalanches to release back down to the old snow interface 1-3′ deep. This seems unlikely due to the melt freeze crusts in the upper snowpack, but we have a fair bit of uncertainty right now due to sparse field observations and the fact that the recent storms heavily favored parts of the forecast zone near Portage and Placer. In addition there is an older weak layer from mid-March that we are still tracking (see additional concern).

Despite the cloudy skies and temperatures staying close to freezing yesterday there was still enough energy from the sun to melt surface crusts up to roughly 2000′. This made for much improved skiing and riding conditions at lower elevations in the afternoon and could also make wet loose avalanches possible later in the day. In addition the warm temps and wind could cause cornices and glide cracks to be more likely to fail.

1-2″ thick supportable crust on the surface in the morning that, surprisingly, melted up to 2000′ in the afternoon despite cloudy skies and temperatures barely above freezing. Photo 4.13.23

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 weak layer of facets buried about 3-6′ deep in the snowpack is still on our radar due to widespread very large avalanche activity on this layer in the second half of March. It has been two weeks since we have seen any avalanche activity on this layer and we think it is very unlikely for a human triggered or natural avalanche. However, due to the consequences of being involved with an avalanche this size we are still keeping track of it.

Snowpack structure showing upper ‘crust sandwich’ and deeper buried weak layer from mid-March. Photo 4.13.23

Weather
Fri, April 14th, 2023

Yesterday: Broken sky cover to start out the day shifting to overcast by around noon. Light snow started around noon with little accumulation during the day. Winds moderate at 5-15 mph with gusts up to 25 mph. Temperatures stayed fairly warm at lower elevations, hovering around 32 F up to 2000′ throughout the day. At upper elevations temperatures stayed closer to 20 F throughout the day.

Today: Light snowfall and cloudy skies are expected to continue today, with around 2-3″ of snow accumulation in Turnagain and Girdwood and up to 6-12″ in areas closer to Prince William Sound like Portage and Placer. Moderate E winds in the 10-20 mph range will continue throughout the day with stronger gusts at upper elevations. Temperatures are expected to warm up again today with snowline reaching up to 700′ in the afternoon.

Tomorrow: The snow is expected to taper off Friday evening and the winds will gradually decrease overnight. Cloudy skies are still expected during the day Saturday, but no new snow and decreased wind speeds in the 0-10 mph range. Temperatures will climb a little bit higher Saturday afternoon with highs in the 30s F above 3000′.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 31 2 0.1 94
Summit Lake (1400′) 31 0 0 46
Alyeska Mid (1700′) 30 2 0.1 89
Bear Valley – Portage (132′) 33 5 0.35

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19 NE 10 26
Seattle Ridge (2400′) 24 SE 7 14
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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.