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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, February 24th, 2021 - 7:00AM
Expires
Thu, February 25th, 2021 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

A MODERATE avalanche danger remains at all elevations. An increase in east winds forecast today should blow a few inches of new snow and any old soft snow into small shallow wind slabs. These will be possible to trigger on any slope with new wind deposited snow. In areas out of the winds at all elevations, triggering a slab avalanche 1-2′ deep is possible due to buried weak layers.

PORTAGE/PLACER VALLEYS:  Small natural wind slab avalanches and cornice falls will be possible, as this is where most the new snow is falling.

Wed, February 24th, 2021
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

The winds will be at it once again. If the weather models are right, by noon today ridgetops will be seeing east winds averaging 30-40mph with stronger gusts. That’s pretty good wind for the small weather system skirting us by to the south… Unfortunately, the wind will only be accompanied by 2-4″ of new snow, at best. If we add this to the 1-3″ of snow from last night, the wind has a total of 3-7″ to work with. Add to that limited amounts of old snow still available to blow around, and we can expect any fresh slab to be on the shallower side, 6-10″ thick or so. Areas that often see more snow, such as Portage and Placer Valley, could have larger slabs up to a foot thick form today.

With poor visibility, travel into the higher elevations where most the action is will likely be a bit difficult. However, we could see some wind at the mid elevations as well. So keeping a close eye out for active wind loading will be key. And as always, watch for cracks that shoot out from you and stiff hollow feeling snow over softer snow. There could be some linger old wind slabs lurking in areas out of the winds as well.

Avalanche Problem 2
  • 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

Buried in the top two feet of the snowpack are a couple layers of buried surface hoar and facets. The most recent avalanches on these layers were three days ago on the steep SW face of Eddies Ridge. At elevations below 2,000′ a crust sits under the weak layers and was producing avalanches last week. We know these layers are present in most areas, but they are not reactive everywhere, which makes it tricky. The fact we are still seeing avalanches however is not something to ignore.

A few things to consider:

  • Low traffic zones are the most suspect for triggering an avalanche in these layers.
  • As well is steep terrain that has not been completely blasted by the February winds and still harbors soft inviting snow.
  • Understanding an avalanche could be triggered, how great the consequences might be if it does, are good to think about before getting on slopes steeper than 35 deg.

 

Andrew found this layer of buried surface hoar (the thin gray line) on Magnum two days ago. It sat just over a foot below the surface. 2.22.21.

Weather
Wed, February 24th, 2021

Yesterday:  Mostly overcast skies and an east breeze were over the region yesterday. Ridgetop winds averaged 10-20mph with gusts in the 20’s. Temperatures climbed from the single digits to the 20’s yesterday morning where they have remained. Light snowfall began overnight with a few inches accumulating in upper Girdwood and Portage Valleys while only an inch or two has fallen at Turnagain Pass.

Today:  Cloudy skies with light snowfall should continue through today as a low pressure skirts just to our south. Only 1-3″ of accumulation is expected of very low density snow. Snow should taper off tonight. However, ridgetop winds are forecast to increase into the 30-40mph range before also calming down tonight. Temperatures will stay civil, in the 20’s in the mountains and ~32F at sea level.

Tomorrow:  Mostly clear skies are expected for tomorrow as the low exists and we see a return to the northwest winds. Ridgetop averages look to average in the 15-25mph range at this time, which isn’t too bad. Temperatures should cool to the teens. A chance for more snow is slated for the weekend. Stay tuned.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 22 1 0.1 113
Summit Lake (1400′) 20 1 0.1 44
Alyeska Mid (1700′) 20 2 0.1 113

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 14 E 14 35
Seattle Ridge (2400′) 16 SE 12 22

 

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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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