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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Tue, April 6th, 2021 - 7:00AM
Expires
Wed, April 7th, 2021 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

Today’s avalanche danger is MODERATE. Strong winds yesterday into last night have built wind slabs on top of a snowpack with multiple weak layers near the surface, making it possible to trigger an avalanche a foot deep or deeper. It will be important to recognize and avoid steep, wind-loaded terrain today. The combination of stiff wind slabs on top of weak snow makes it possible that a person could make it out into the middle of a slope before triggering an avalanche, or for multiple skiers or riders to put tracks on a slope before somebody triggers an avalanche.

SUMMIT/SNUG HARBOR/LOST LAKE/SEWARD: Strong winds continue today in these areas, which means the likelihood of human-triggered avalanches remains high and natural avalanches will be possible today. Be extra cautious if you are getting out in these areas.

Special Announcements

Hatcher Pass:  Dangerous avalanche conditions remain in this area. Large avalanches have been occurring with one covering the road on Sunday. Please keep tabs on the Hatcher Pass Avalanche Center’s website and Facebook page.

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Tue, April 6th, 2021
Alpine
Above 2,500'
Moderate (2)
Avalanche risk
Treeline
1,000'-2,500'
Moderate (2)
Avalanche risk
Below Treeline
Below 1,000'
Moderate (2)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Moderate (2)
Treeline
1,000'-2,500'
Avalanche risk
Moderate (2)
Below Treeline
Below 1,000'
Avalanche risk
Moderate (2)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Recent Avalanches

We received reports of several natural and human-triggered avalanches during yesterday’s wind event:

Seattle Ridge: A snowmachiner triggered a large wind slab on a steep, rocky convexity near the Seattle Headwall (photo below). Nobody was caught in the slide.

Seward Highway: Large natural wind slabs were observed along the Seward highway near Girdwood and in the Summit Lakes area.

Snowmachine-triggered wind slab avalanche near the Seattle Headwall yesterday. Nobody was caught in the slide. Photo: Clint Kyffin. 04.05.2021.

Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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.

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

Strong winds yesterday into last night have formed fresh wind slabs on top of weak snow, making it possible to trigger an avalanche large enough to bury a person today. Although we do not expect to see the same natural activity as we did yesterday, these fresh wind slabs have not had much time to bond to old snow surfaces, and in many places are just waiting for a person to come along and trigger an avalanche. Although the likelihood of triggering an avalanche today is slightly lower than it was yesterday, there are a few factors that might actually make it more difficult to manage this problem today:

  • Lighter winds today will mean the signals for wind loading are going to be more challenging to identify.
  • The hard wind slabs that formed yesterday and overnight have had some time to stiffen, which will make it possible for a person to get out into the middle of a slab before triggering an avalanche. This increases the consequences of triggering a slide, since it would be more difficult to get off the slab if you do trigger something.
  • Calm winds and clear skies in the afternoon will make it easy to get lulled into a sense of complacency, and tempting to convince yourself it is a good idea to get on steeper terrain.

Navigating in the backcountry today will require identifying and avoiding recently wind-loaded terrain. Be extra cautious near ridgelines, on the downslope side of steep convexities, and in cross-loaded gullies. Cracks shooting out from your snowmachine or skis, or the ‘whumpf’ of a weak layer collapsing are clear signs that the snowpack is unstable. Wind slab problems tend to be short-lived, but we are still close enough to the latest wind event that a little extra caution is warranted today.

 

Natural avalanche on a wind-loaded slope in the Summit Lake area yesterday. Photo: Alex McLain. 04.05.2021

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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.

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

In addition to the wind slab problem mentioned above, we are still concerned with multiple weak layers that are buried in the upper 3′ of the snowpack. We have been talking about these weak layers for weeks now, and have seen avalanche activity associated with them as recently as last Thursday. Persistent weak layers may have also played a role in the activity in the Summit Lake area yesterday. Although it is becoming less likely to trigger something on one of these layers, they are still capable of producing large avalanches. These weak layers may be a little more sensitive today where they were loaded by wind-transported snow during yesterday’s wind event. This persistent slab problem still requires dialing your terrain use back a notch, avoiding steep and unsupported slopes, or avalanche terrain with traps like rocks, cliffs, trees, gullies, or abrupt transitions in the path or runout zones.

Weather
Tue, April 6th, 2021

Yesterday: The big story yesterday was the northwest wind event, with sustained speeds of 10-20 mph into last night and gusts at 35-40 mph. Skies were sunny but temperatures remained cool, with daytime highs in the low teens at upper elevations and upper 20s in the valleys. Overnight lows were in the single digits to low teens F.

Today: Northwest winds have died down near Girdwood and Turnagain Pass since yesterday, although they are expected to remain around 15 mph in the Summit Lake area and as high as 20-30 mph near Seward. Temperatures are expected to reach the low teens to low 20s F during the day before dropping back into the single digits F tonight. A few scattered clouds this morning should break up by this afternoon.

Tomorrow: North winds around 5-15 mph are expected to shift to the southeast tomorrow evening into tomorrow night as more active weather enters the area. Cloud cover is expected to increase during the day, and highs are expected in the low teens to low 20s F. Chances for precipitation increase tomorrow night, with a couple inches anticipated by Thursday morning.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 20 0 0 109
Summit Lake (1400′) 17 0 0 48
Alyeska Mid (1700′) 21 0 0 120

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 9 WNW 11 38
Seattle Ridge (2400′) 16 NW 10 22
Observations
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Riding Areas
Updated Thu, April 01st, 2021

Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff. Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations. Riding area questions contact: mailroom_r10_chugach@fs.fed.us

Area Status Weather & Riding Conditions
Glacier District
Johnson Pass
Open
No parking in turnaround at end of the road near the outhouse.
Placer River
Open
Please do not ride along Railroad tracks. Cross tracks at 90 degree angle and clear the right of way.
Skookum Drainage
Closed
The Skookum Valley is closed to snowmachines. This closure occurs annually on April 1 as per the CNF Forest Plan.
Turnagain Pass
Open
Twentymile
Open
Please do not ride along Railroad tracks. Cross tracks at 90 degree angle and clear the right of way.
Seward District
Carter Lake
Open
Lost Lake Trail
Open
Primrose Trail
Open
Resurrection Pass Trail
Closed
Closed for the 2020/21 winter season.
Snug Harbor
Open
South Fork Snow River Corridor
Open
Summit Lake
Open

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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.