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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, January 16th, 2020 - 7:00AM
Expires
Fri, January 17th, 2020 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

Above  2500′ the avalanche danger is MODERATE. Triggering a large avalanche on a buried weak layer remains possible in the Alpine.  Be on the lookout for wind effected snow and watch your sluff  in steep protected terrain.

Below 2500′ the avalanche danger is LOW and normal caution is advised. As always use safe travel protocol.

SUMMIT LAKE: This region is out of our advisory area. The overall snowpack is shallower, the weak layers are more developed and the wind effect is from the recent winds is more pronounced. Extra caution is advised.

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Thu, January 16th, 2020
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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
  • 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

As we move farther and farther away from the New Year’s storm and the wind events of last week, the likelihood of triggering an avalanche is decreasing. The last human triggered avalanche was Friday evening(1.10.20) on the Crow Pass trail. However, our current persistent slab issue still warrants caution in the Alpine. Weak snow buried in the snowpack has the potential to be triggered especially where there is a hard slab sitting on top of weak facets “sugar snow”. Overall signs are pointing to improving stability but as your choose where to travel keep the lingering possibility of triggering an avalanche in mind. Pay attention to potential consequences. If the slope were to slide with you on it, where would the debris pile up? Watch for areas with very hard snow over soft snow and use safe travel protocols. Regions on the periphery of our forecast zone like Crow Pass or just outside the forecast zone like Summit Lake saw more wind effect last week and extra caution is advised.

Wind effect on Seattle Ridge, 1.15.20.

Weak faceted snow found below wind hardened surface snow in a snow pit on Fresno at 3000′, 1.15.20. 

 

Additional Concern
  • Dry Loose
    Dry Loose
Dry Loose
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.
More info at Avalanche.org

Loose snow avalanches (sluffs):  On slopes out of the wind expect sluffing in steep terrain.  The surface snow is becoming looser and looser by the day with the cold temperatures.

Cornices:  Give cornices plenty of space and limit your exposure when passing beneath them.

Glide avalanches: Due to the unpredictable potential to release, limit your time spent under glide cracks.

Cornices on the ridgeline and glide cracks opening just behind Manitoba, 1.15.20.

Weather
Thu, January 16th, 2020

Yesterday: Skies were mostly clear. Winds were light and variable and temperatures were inverted. Upper elevations were in the single digits and valley bottoms were a few degrees below 0°. Overnight the inversion remained in place and winds were calm.

Today: Clear skies continue to day with temperatures in the valleys ranging from just below  0° to the low single digits and upper elevations reaching highs in the teens. Winds are forecast to be mostly calm. Temperatures dip down again tonight and winds will be light and easterly.

Tomorrow: Another day of clear and cold with inverted temperatures and light east winds. Saturday a few clouds move in and build overnight into Sunday. Temperatures are forecast to rise into the 20°Fs and there looks to be the potential for some snow in the forecast.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 3 0 0 37
Summit Lake (1400′) -6 0 0 15
Alyeska Mid (1700′) 3 0 0 36

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 4 E 7 21
Seattle Ridge (2400′) 6 variable 2 11
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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.