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Issued
Sat, March 30th, 2024 - 7:00AM
Expires
Sun, March 31st, 2024 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is HIGH above 2500′. Strong winds and heavy snowfall will make natural and human triggered avalanches 1-2+’ deep very likely today. The potential size of avalanches will be larger in areas that receive more new snowfall, like Portage and Girdwood. We recommend avoiding avalanche terrain and the runout zones of overhead avalanche paths in areas receiving heavy snowfall today.

Below 2500′ the avalanche danger is CONSIDERABLE. Natural and human triggered avalanches are likely within the new snow. We recommend evaluating how well the new snow is bonding to the old snow surface before entering avalanche terrain. Below 1000′ the snowfall is expected to switch to rain, which will make wet loose avalanches possible on steep terrain.

PORTAGE AND PLACER: This area is expected to receive up to 2-3′ of snowfall today, with rain falling below 1000′. Natural avalanches are very likely and we recommend avoiding avalanche terrain and runout zones of avalanche paths. This includes many popular hiking trails in near Portage Lake, such as the Byron Glacier Trail.

Special Announcements

NWS Winter Weather Advisory for Turnagain Pass, Whittier, Moose Pass, Seward

Motorized Area Closure for Skookum Valley:  Per the National Forest Plan the Skookum Valley is closed to motorized use beginning April 1. Placer Valley remains open along with all other areas on the Forest.

Avalanche Forecast Survey: Simon Fraser University is collaborating with US avalanche centers to better understand how useful avalanche forecast information is for trip planning. This research will help drive development of future avalanche forecast products. Click here if you are interested in participating in a 20 minute survey.

Sat, March 30th, 2024
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
Avalanche risk
Sun, March 31st, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Sun, March 31st, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
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 observations that came in yesterday. On Thursday there were several observations of small to large wet loose avalanches and a small wet slab that released on Penguin ridge during the warm and sunny afternoon.

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

A potent storm system will impact the forecast area today, bringing strong winds and snowfall across the region. Coastal areas, like Portage and Placer, are expected to see the heaviest precipitation with weather models predicting up to 2.5″ of water which could be 2-3′ of new snow at upper elevations. Slightly more inland locations, like Girdwood and Turnagain Pass, are expected to see less precipitation with 0.5-1.0″ of water or roughly 6-12″ of new snowfall expected above 1000′. Strong winds are expected throughout the day, with wind speeds peaking this afternoon at averages of 40-60 mph and gusts possible up to 75 mph out of the east. Rain line will rise to roughly 800′ this afternoon.

If the weather forecast is correct and coastal areas actually see 2-3′ of new snow today very large natural and human triggered avalanches will be very likely. We recommend avoiding all avalanche terrain in these areas, including runout zones from overhead avalanche paths. 

For the more inland portions of the forecast zone natural and human triggered avalanches will still be likely and the potential size of avalanches will depend on the amount of new snow we get today. If there is 10-12″ or more of new snow on the surface then storm slab avalanches within the new snow will be possible even in wind sheltered areas. Wind slabs 1-2′ deep will also be forming and are likely to cause natural or human triggered avalanches at upper elevations, especially along ridgelines, gullies, and near convex rolls. To assess how sensitive storm slab or wind slab avalanches are to human triggering you can use hand pits and small test slopes to evaluate how well the new snow is bonding to the old snow surface. Below 2500′ the new snow is likely to be falling on a melt freeze crust that formed on Thursday night, which may mean that it does not bond well with the old snow surface. We recommend a cautious approach avalanche terrain today and carefully evaluating the new snow before entering steep slopes.

Snowfall forecast for Saturday at 4am through Sunday at 4am, with coastal areas strongly favored for higher snow totals. Graphic courtesy of NWS Anchorage 3.30.24

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

Below 1000′ we are expecting the precipitation to switch to rain by this afternoon, which will make wet avalanches possible on steep slopes. Warm temperatures and rain at low elevation over the past week have formed a thick layer of wet snow on the surface, which could cause wet loose avalanches to be larger than typical today. This type of avalanche tends to start out small, but they can grow quickly as they run down slope and create surprisingly large and destructive avalanches. Common hiking areas like the Byron glacier trail in Portage can easily be struck by this type of avalanche and we recommend avoiding these hiking areas with overhead avalanche paths today.

Large wet loose avalanche from earlier in the week in the Seattle Ridge area. Photo from Jay Liska 3.28.24

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

Buried about 1-3′ deep in the snowpack there are a few different weak layers that we are still monitoring which formed in early March. These weak layers caused a widespread cycle of human triggered avalanches two weeks ago, and the new snow and wind today will add stress to these layers and make them more likely to cause larger avalanches. Based on our recent field observations the weak layers have gained a lot of strength since their active period two weeks ago, but we want to be aware of the outside chance for a very large avalanche to occur on one of these persistent weak layers. The best way to mitigate this problem is to give the weak layers some time to adjust to the new snow load and keep a conservative mindset today by sticking to low angle terrain.

Weather
Sat, March 30th, 2024

Yesterday: Strong winds averaging 20-35 mph from the east with gusts to 65 mph at upper elevations. Girdwood and Portage received 0.5-0.6″ of water or about 5-7″ of new snow while Turnagain Pass only picked up 0.2″ of water for about 1-3″ of snow. Temperatures stayed cold compared to the trend over the past week, with highs in the high 20s to low 30s F at lower elevations and teens to low 20s F at upper elevations.

Today: Another stronger pulse of precipitation is expected to impact the forecast area today, with 0.5-1.0″ of additional water or 6-12″ of snowfall in Turnagain Pass and Girdwood. Portage and Placer could see up to 2.5″ of additional water or 20-30″ of new snow at upper elevations. Rain line is expected to rise to about 800′ this afternoon. Winds should remain strong with averages of 25-35 mph from the east in the morning with gusts to 50 mph, then increasing to averages of 40-60 mph out of the east with gusts possible to 75 mph in the afternoon and evening during the most intense period of snowfall. Temperatures are expected to reach the low to mid 30s F today at low and mid elevations and the upper 20s F at upper elevations.

Tomorrow: The most intense snowfall is expected to taper off overnight as the winds shift to the southwest around midnight. A few more inches of snow are expected throughout the day on Sunday with snow line expected to drop back down to sea level. Southwest winds look to remain elevated with averages of 20-35 mph during the day and gusts up to 50 mph possible, in the evening wind speeds should switch to the south and start to decrease to averages of 10-15 mph.  Temperatures look to reach highs in the low to mid 30s F at lower elevations and low 20s F at upper elevations.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29 1 0.2 95
Summit Lake (1400′) 33 0 0 41
Alyeska Mid (1700′) 29 6 0.56 102
Bear Valley – Portage (132′) 36 0 0.63
Grouse Ck (700′) 34 3 0.6 64

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19 ENE 24 67
Seattle Ridge (2400′) 23 SE 16 32
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.