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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, March 7th, 2020 - 7:00AM
Expires
Sun, March 8th, 2020 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains MODERATE. Finding and triggering a wind slab avalanche, around a foot thick, remains possible. These could be older wind slabs or new ones in areas seeing enough wind to transport snow today. They also may break wider than anticipated as some could be sitting on buried surface hoar. Additionally, there is still a chance a person could trigger a large slab avalanche that breaks in weak snow 3-6′ deep.

SUMMIT LAKE TO SEWARD REGION:  Strong wind last week impacted Summit Lake and Lost Lake, which resulted in several natural avalanches. Triggering a lingering wind slab is still possible and continued caution is advised on wind affected slopes.

Special Announcements

Heading to Hatcher Pass or Chugach State Park? New snow is headed this way over the weekend and be sure to check out the recent avalanche observations. Also don’t forget to check the Hatcher Pass Avalanche Center Saturday forecast!

Sat, March 7th, 2020
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.
Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • 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.
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

Cloudy skies have moved in over the region, brought temperatures up to a more habitable 20-25°F and even a few snow flurries have fallen this morning. With a relatively weak weather system moving through this weekend, it looks as though our forecast zone will see an inch or so of snow today, 2″ tonight and up to an additional 4″ by tomorrow. This is a pretty meager ‘re-fresh’ and that said, it will be old and new wind slabs that will be our main avalanche concerns.

Lingering wind slabs from last week’s outflow wind event may still be possible to trigger. These wind slabs could propagate further than expected and could also step down into buried weak layers. This was the case with the Lynx Creek avalanche that occurred Thursday. In case you missed it, those details are HERE. These older wind slabs could be sitting on buried surface hoar, making them take longer to bond and why they could break out wider than one might think.

Fresh wind slabs are also something to look for today in upper elevation terrain. Ridgetop winds may reach averages of 20mph from the east. This will be enough to transport loose snow and load slopes, if the loose snow is available. If you are headed out today, watch for wind textured surfaces, hollow feeling snow and stiffer snow over softer snow. These are all signs of a wind slab. These slabs are likely to be around a foot thick. Quick hand pits are also good ways to suss out a wind slab. If you find one in your hand pit, look close, you may see some buried surface hoar feathers under it.

 

Wind textured surfaces along the Southeast face of Seattle Ridge at Turnagain Pass on Thursday. 3.5.20.

 

Wind loaded slopes on Colorado Peak in the Summit Lake area along with a very wide propagating natural slab avalanche lower on the slope. This also occurred also on Thursday. 3.5.20. Photo: Alex McLain

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.
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 is still important to remember that weak faceted snow from January sits 3 to 6 feet deep in the snowpack. The wind slab triggered in Lynx Creek two days ago was a larger avalanche because it ‘stepped down’ into these old weak layers. Not only can this happen again, but it is possible a person triggers the deep layer from the start. As we know, with time the likelihood of triggering a deep slab is decreasing, but we have too much proof to write this problem off. Keeping our terrain choices in line with the ‘what if I trigger a large slab’ question remains prudent.

Cornices:  As always, avoid travel on or underneath cornices.

Loose snow avalanches:  In areas that were protected from the wind, sluffs are possible in steep terrain.

Weather
Sat, March 7th, 2020

Yesterday:  Partly cloudy to overcast skies filled the region. Ridgetop winds were light to moderate from the east (5-15mph) with Sunburst gusting to 29mph yesterday morning. Temperatures rose into the 20’s°F at most locations from sea level to ridgetops.

Today:  A weather system is moving in and brining cloudy skies and light snow showers to the forecast area. Only a trace to an inch of accumulation is expected today with 2″ of new snow possible tonight. Ridgetop winds are forecast to remain easterly and blowing in the 10-20mph range. Temperatures will remain in the 20°’F at all elevations.

Tomorrow:  Snowfall will continue tomorrow with an additional 2-4″ and up to 6″ or more in favored area likely near the coast. Ridgetop winds are slated to increase to the 15-25mph range along with the peak of snowfall looking to occur mid-day. Temperatures look to say in the mid to upper 20’s°F with snow to sea level.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 21 0 0 71
Summit Lake (1400′) 18 0 0 31
Alyeska Mid (1700′) 20 trace trace 80

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 13 NE 9 29
Seattle Ridge (2400′) 17 SE 7 17
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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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