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Issued
Tue, January 28th, 2020 - 7:00AM
Expires
Wed, January 29th, 2020 - 7:00AM
Forecaster
Ryan Van Luit
Avalanche risk The Bottom Line

Today the avalanche danger is CONSIDERABLE in the Alpine and MODERATE below.  Recent wind and 8-14″ of new snow have created the conditions to make it likely for a human to trigger a wind slab up to 2′ thick.  Storm snow could be forming slab character and easier to trigger in steeper terrain.  Cornices are actively developing and likely to be touchy to human trigger.  Extra caution and conservative decision making are advised.

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Tue, January 28th, 2020
Alpine
Above 2,500'
3 - Considerable
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. Dangerous avalanche conditions. 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

Our storm yesterday dropped more snow with more intensely than expected.  Snow accumulations totaled 8-14″ at sea level and estimates of up to 18″ in the Alpine.  We have more snow in the forecast over the next two days, potentially adding another 4 – 8″.  Along with this new snow, we experienced winds in the 20mph range for several hours, easily capable of transporting the new snow.  On top of our new snow and increase in wind, we saw a temperature spike of 25°F at some mid elevation stations.

The good news is that our concerns lay within the top 2′ of the snowpack.

In the Alpine and exposed areas near Treeline, we likely have wind slabs forming in the lee of ridges and cross loaded gullies.  These wind slabs are relatively fresh and laying atop a faceted surface so could be touchy and likely for a human to trigger today as they adjust.  Stay alert if you see freshly pillowed areas, areas with wind effected surfaces, and where the snow feels firmer or hollow.

Cornices:  Cornices which have seen recent wind will likely be larger and more touchy than in previous days as they adjust to the additional load.  Give them a wide margin.

Avalanche Problem 2
  • Storm Slabs
    Storm Slabs
  • 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.

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

At least 8-14″ of snow accumulated at all elevations.  This storm snow sits atop a base of varying layers and is unlikely to have had the chance to bond well at the old snow/new snow interface.  Additionally, with the temperature increase during the storm, there’s a good chance the new snow is consolidating enough to form slab character.  Where this is the case, the snow could feel a bit heavier, or you could see cracks shooting out from ahead of you.  Give extra caution on steeper slopes where you feel this distinct difference in density at the old/new interface.

The MLK Jr. storm (1/20) is now consolidated to about 5″ and has formed the first dense layer below our new snow.  We should also keep our eye on the weak layer of facets immediately below this density as this could be a slab/weak layer combination of concern. Either way, our primary concerns lay within the top 2′ of the snowpack.

Weather
Tue, January 28th, 2020

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

Yesterday: Skies were cloudy with low visibility and intense snow until mid-day. Accumulation totals of 8″ – 12″ . Winds were easterly 10-15 mph with gusts into the 40’s. Temperatures rose into the single digits to low twenties.

Today:  Cloudy skies today with snow flurries this afternoon accumulating to 1″ during the day and 4″ – 8″ overnight.  Temperatures will range in the teens °F with winds 10-20 mph from the east, then switching from the north by this evening.

Tomorrow:  More snow likely in the forecast for Wednesday. Temperatures increasing to range from the teens to low twenties °F. Winds will be out of the east from 5 to 10 mph.
Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 12 12 0.7 52
Summit Lake (1400′) 2 1 0.1 17
Alyeska Mid (1700′) 11 9 0.4 48

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 8 E 17 41
Seattle Ridge (2400′) 9 VAR 8 16
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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.