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Issued
Fri, March 13th, 2020 - 7:00AM
Expires
Sat, March 14th, 2020 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

A MODERATE avalanche danger exists at all elevations. It remains possible for a person to trigger a slab avalanche around 1-2 feet thick. Steep wind loaded terrain is the most likely place to trigger a slab. Slabs may be sitting on buried surface hoar, making them break out larger than expected. Additionally, watch for surface warming on solar aspects. This can help destabilize slabs and also create moist sluffs on steep sunny slopes.

SUMMIT LAKE to LOST LAKE and SEWARDTriggering a large slab avalanche is possible in these regions. Strong outflow winds mid-week induced several natural avalanches. The Summit Lake area harbors a shallow snowpack and many weak layers. Extra caution is advised.

Special Announcements

Hatcher Pass area:  Many large avalanches occurred in this region yesterday. Please check the reports for Hatcher Pass along with the Hatcher Pass Avalanche Center Facebook Page.

Region-wide:  Looking ahead, the warming trend for the weekend could cause wet loose avalanches in steep rocky terrain that may send debris near or over summer trails, such as the Byron Glacier trail near the ice caves. Heads up if looking to get out on hikes or other outings not necessarily related to backcountry skiing/snowboarding or snowmachining.

Fri, March 13th, 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.
Recent Avalanches

There was a very close call Wednesday on the Raven Headwall. A solo skier, on the final leg of the Eklutna traverse, was caught, carried around 800′ and buried to their shoulders in a large avalanche. They were able to self rescue and are OK. The skier triggered the slab on the upper mid-slope while descending. The slab fractured around 30-50′ above the skier. This appears to be a large wind slab formed by the strong northerly outflow winds on Tuesday and Wednesday. The wind slab may have been sitting on weak snow, allowing it to propagate across the entire headwall. We would like to thank the skier for telling their story and please take a moment to read their account HERE.

Large skier triggered slab on the Raven Headwall. 3.11.20. Photo: Anonymous

 

Close up of the crown face. This appears to be a wind slab that formed from the strong outflow winds Tuesday and Wednesday. 3.11.20. Photo: Anonymous

Avalanche Problem 1
  • Persistent Slabs
    Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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

And…. it’s springtime. Light winds, warm temperatures and long sunny days are here. Starting today, we are on a warming trend that should last through the weekend and even into Monday. Not only does this weather invite trips into the far reaches of the mountains, it also invites a new factor for avalanches; sunshine and warming. We can’t let the hospitable weather lower our guard.

As we’ve been mentioning, there is a thin layer of buried surface hoar around 1-2 feet below the surface in many areas. Even deeper in places that saw significant wind loading mid-week by the northwest outflow winds, such as the Raven Headwall avalanche mentioned above. This layer has not been found everywhere, but has been prevalent enough that it’s keeping us concerned. It also may be sitting on a sun crust on steep southerly slopes, making it that much more reactive. That said, triggering a slab large enough to ruin our day is still very much possible. Keep a close eye out for stiff snow over softer snow and cracking in the snow around you. Watch for slopes with wind loading as these are the most suspect places to trigger a slab.

Sun Effect:  With light winds, direct sunshine and warm air moving in aloft we can expect solar aspects to warm dramatically over the next couple days. This change is a shock to our snowpack. It not only moistens the surface, creating easily triggered moist sluffs (and natural ones as well), but it can increase the likelihood of triggering a slab avalanche. It can also help to re-active older weak layers that sit very deep in the snowpack. In our case the January facets that are 3-6 feet deep. Will the sunshine this weekend be enough to do this? Probably not, but something to keep in mind as we move forward.

Cornices:  Avoid travel on cornices and move quickly underneath them. Remember, warming temperatures and direct sun are classic mechanisms for destabilizing these monsters.

Large cornice that sits on the north side of Superbowl (head of Cornbiscuit and Magnum). 3.12.20. Photo: Heather Johnson

 

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

Weather
Fri, March 13th, 2020

Yesterday:  Sunny skies and warming temperatures were over the region yesterday. Lower elevations rose from the single digits to the mid 20’s°F and upper elevations rose to the mid-teens. Ridgetop winds were light with a few moderate gusts from the NW.

Today:  Another sunny day is on tap with a stout inversion in place this morning. High pressure is building over Southcentral and bringing in warm air aloft, which is warming the higher terrain (Sunburst is reporting 17°F at 5am) but valley bottoms remain in the single digits. Daytime highs should reach into 20’s°F at all elevations today. Ridgetop winds are expected to be light and westerly.

Tomorrow:  Sunny skies, light winds and warming temperatures are forecast for the weekend. Springtime weather is here. Ridgetops could see temps hit 30°F Saturday and even higher on Sunday. Winds are slated to remain light and variable.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 13 0 0 68
Summit Lake (1400′) 10 0 0 31
Alyeska Mid (1700′) 13 0 0 80

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 13 W 8 18
Seattle Ridge (2400′) 12 N 3 7
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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.