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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, April 12th, 2024 - 7:00AM
Expires
Sat, April 13th, 2024 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 2500′. Strong winds yesterday formed fresh wind slabs 1-2′ deep that are still possible for a person to trigger today. Larger avalanches releasing on an icy crust about 1-2′ deep are also possible and we recommend evaluating how well the new snow from this week is bonding to the old snow surface before committing to steep terrain. If the sun comes out today wet loose avalanches on solar aspects will become possible.

Below 2500′ the avalanche danger is MODERATE. Smaller wind slabs and wet loose avalanches are most likely at these elevations.

PORTAGE/PLACER: Coastal areas received about 0.7″ of water in the past 24 hours which could be 6-10″ of new snow at upper elevations. Wind slabs are likely to be more reactive in these areas where more new snow is available for transport. Storm snow avalanches within the new snow from the past 24 hours are also possible.

Special Announcements

Avalanche Center End of Season Operations: This is the final week of daily 7am avalanche forecasts. Beginning April 15 we will forecast 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.

Fri, April 12th, 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
Sat, April 13th, 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
Sat, April 13th, 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
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 new avalanches reported yesterday, but with consistently strong winds at upper elevations some natural wind slabs likely occurred and were just not visible due to cloud cover.

Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

It’s been three days since the last major snowfall, which brought 6-10″ of new snow to Turnagain Pass and closer to 1.5-2′ to Girdwood and Portage/Placer. That storm snow fell on top of an icy crust which was covered in surface hoar or facets in some areas. Several human triggered avalanches occurred on Wednesday, including remotely triggered avalanches where a skier or rider triggers an avalanche on a steep slope from adjacent lower angle terrain. The 3-6″ of new snow that fell in the past 24 hours may be enough to make these older storm snow interfaces more reactive today.

Yesterday we found new breakable crusts on the surface on all aspects except north, which seemed to be adding some strength to the snowpack and we did not find any concerning results in our stability tests. However, the potential for a lingering layer of surface hoar or facets above the crust to cause a large avalanche 1-2′ deep still exists, and we recommend carefully evaluating how well the new snow from this week is bonding to the old snow surface before committing to steep terrain.

In addition, strong winds averaging 20 mph and gusting to 40 mph out of the NE yesterday built fresh wind slabs at upper elevations, which could still be possible for a person to trigger today. These wind slabs could be anywhere from 6″ to 2′ deep and are most likely to be located along ridgelines, gully features, and convex rolls above treeline. Keep an eye out for shooting cracks and try jumping on or riding across small, steep test slopes to check how reactive wind slabs are to the weight of a skier or rider. The 1-3″ of new snow that fell overnight might make these wind slabs harder to spot with the naked eye.

Snowpack structure on a N aspect near treeline, showing the interface with the old snow surface which is worth evaluating before committing to steep terrain. Photo 4.11.24

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

After a week of mostly unsettled and cloudy weather we are due for a transition towards sunny skies this weekend. Cloud cover is expected to decrease throughout the day today, which could give the sun an opportunity to melt the surface snow and cause wet loose avalanches. This late in the season the sun is very strong and can quickly change the snow surface conditions. Keep an eye out for wet snow on the surface or roller balls releasing from thin rocky areas of the snowpack as an indication that the sun is starting to weaken the snow surface. If you find yourself ankle deep in moist or wet snow the chances for large wet loose avalanches are rapidly increasing and it is best to transition to a shadier aspect.

Weather
Fri, April 12th, 2024

Yesterday: Light to moderate snowfall during the day with about 1-3″ of new snow in Turnagain and Girdwood, while Portage and Placer likely pickup up closer to 6-8″ of new snow at upper elevations. Winds were strong throughout the day with averages of 15-25 mph out of the east and gusts up to 45 mph at upper elevations. Temperatures stayed cold enough for snow to fall down to sea level, but it was not accumulating much at lower elevations due to the temperatures in the mid 30s F. At upper elevations temperatures stayed in the high teens to low 20s F throughout the day.

Today: Snowfall is expected to taper off this morning, with some light showers lingering throughout the day but only 1-2″ of new snow accumulation. Winds are also expected to shift to the SW this morning and drop to averages of 5-10 mph with gusts to 20 mph today. Temperatures should reach highs into the mid 30s F at low elevations and mid 20s F at upper elevations. Cloud cover might break up enough for the sun to poke through this afternoon.

Tomorrow: A clearing trend is expected on Saturday with partly to mostly sunny skies by the afternoon. Winds are expected to remain light at 5-10 mph out of the west or northwest. No new snow is expected over the weekend. Temperatures are expected to rise to highs in the low to mid 30s F at lower elevations and low to mid 20s F at upper elevations, but with clearing skies it will probably feel much warmer in the sun.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29 5 0.3 102
Summit Lake (1400′) 31 1 0.1 47
Alyeska Mid (1700′) 29 3 0.2 117
Bear Valley – Portage (132′) 36 trace 0.7 –
Grouse Ck (700′) 34 2 0.2 79

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19 ENE 20 46
Seattle Ridge (2400′) 23 SE 16 27
Observations
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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.
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