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Sat, April 11th, 2015 - 7:00AM
Sun, April 12th, 2015 - 7:00AM
John Fitzgerald
Avalanche risk The Bottom Line

The danger is MODERATE in the Alpine.   Areas of CONSIDERABLE danger exist on steep North facing slopes where slabs up to 3-5′ thick, while difficult to trigger, have the potential to be large and carry severe consequences.   Elsewhere it will be possible for slabs 1-2′ thick to be triggered by skiers and riders on steep slopes during the heat of the day.   Wet loose avalanches and cornices are additional hazards to avoid today.

The danger is also MODERATE in the Treeline elevations where wet slabs 1′ thick could release on steep previously wind loaded slopes.   These slabs have the potential to pull out deeper layers of weak wet snow and produce enough volume to injure or bury a person.

Special Announcements

Beginning next week, we will be issuing advisories 5 days a week.   Advisories will be posted at 7 am on all days of the week except Mondays and Wednesdays.   The final advisory of the season will be posted on Thursday April 30th.

Sat, April 11th, 2015
Above 2,500'
2 - Moderate
Avalanche risk
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
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
  • Persistent Slabs
    Persistent Slabs
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.
More info at Avalanche.org

A storm that deposited 2 feet of snow in the higher elevations 2 days ago has left slabs that are generally well bonded to underlying surfaces.  While this bonding has been good in recent tests, we saw natural slab activity after the storm had subsided.  Yesterday this occurred mainly on steep slopes that were receiving direct sun.  More of the same can be anticipated today.  Factors that will increase the likelihood of triggering will be slopes over 35º and approaching 40º, heating from direct sunlight and large triggers such as groups.  

We have limited data from North facing terrain.  As such it will be important keep slope angles 35 or lower if you find yourself on terrain facing the North half of the compass.

Natural slab activity on Seattle Ridge on steep sunlit slopes observed yesterday, April 10th.

2′ Slab with a close up of the crown.  Starting zone ~2,300′ SE aspect

Slab post storm Seattle 4-10-15

A view of the debris of the avalanche pictured above.  Avalanche ran ~1,000′ vertical.


Avalanche Problem 2
  • Wet Loose
    Wet Loose
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
More info at Avalanche.org

Both wet loose and wet slab activity can be expected on steep sunlit slopes.  In the higher elevations, sun will impact slopes and “grease” the interfaces between the slab and bed surfaces.  Those bed surfaces are crusts on East, South and West aspects.
In the lower elevations, a warm and weak snowpack will allow for the chance of medium to high volume wet loose avalanches during the heat of the day.  Volume will be dictated by terrain; more sustained steep slopes will produce more debris.  
Pay attention to the temps and sun today.  Dial back your slope angles if and when you notice rollerballs, wet loose avalanches or the snow surface becomes damp and sloppy…

…or when you sink into your waist after stepping out of your skis or board:

isothermal sloppiness

Additional Concern
  • Deep Persistent Slabs
    Deep Persistent Slabs
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.
More info at Avalanche.org

Weak layers of faceted snow buried as deep as 5’ sit on some slopes on North facing terrain in the Alpine.  This set up will not be found everywhere.  Because of this variability, it is especially important to treat this terrain with suspicion.  Assessment of this issue is very difficult.  The best strategy for “managing” this problem is to avoid this terrain for the time being and give those underlying weak layers ample time to adjust to the large loads (3’+ snow/3”+ H20 over the last week) that have been placed on them.  This is a low likelihood/high consequence scenario that requires patience and time.

Sat, April 11th, 2015

The last 24 hours has seen the beginning of a slow and gradual exit of a large low pressure system from our area.   That system is in fact still circulating around the Gulf of Alaska.   Precipitation, winds, and temps have all diminished.   Just a trace of snow fell in the morning hours yesterday.   Winds have slowed and temps have cooled slightly overnight.

Today expect showery conditions, with occasional clouds and mostly sunny skies as that system continues to weaken and spin slowly around the Gulf.   Temperatures at 1,000′ will climb into the mid 30s F.   Winds will be calm and just a trace of precipitation will fall.   Snow showers could produce greater amounts in some areas.

The extended outlook is showing a continuation of this pattern over the next several days, as we remain under the influence of broad and weak circulation around the Gulf.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 30 trace .1 72
Summit Lake (1400′) 29 0 0 12
Alyeska Mid (1700′) 32 trace 0 42

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22 ENE 7 24
Seattle Ridge (2400′) 26 sensor is rimed
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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.