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Wed, March 25th, 2015 - 7:00AM
Thu, March 26th, 2015 - 7:00AM
Heather Thamm
Avalanche risk The Bottom Line

Today the avalanche danger will start off LOW in the Alpine, but could increase to MODERATE later in the evening as 20-30 mph winds combined with new snow will increase the size of wind slabs. Wind slabs up to 8 € are possible today on steep leeward aspects and if triggered could knock you off of your feet. This avalanche problem is unlikely to bury a person, but could cause trauma if triggered above rocky terrain.  

LOW avalanche danger exists at Treeline.

LOW danger does not mean NO danger. Triggering a slab up to 2′ thick is unlikely, but is worth considering if riding/sking on steep Northern and Western shaded slopes. Cornices will also be a terrain feature to avoid today.  

Wed, March 25th, 2015
Above 2,500'
1 - Low
Avalanche risk
1 - Low
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. 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
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.
More info at Avalanche.org

Yesterday ridgetop winds 20-30mph were observed from the Northeast and will continue throughout the day. Isolated wind slabs up to 8” thick are possible on leeward aspects of ridges and terrain features. Due to the limited amount of snow available for transport these slabs will be small and are unlikely to bury a person. However if triggered in the wrong place it will be possible to take a ride over a cliff or through rocks. Use caution if traveling along ridgetops and avoid hollow sounding snow and pillow-like features on steep complex terrain. 

Wind slabs are likely to become thicker later in the evening with snowfall, up to 3″, forecasted for the upper elevations.    


Avalanche Problem 2
  • 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

It’s been just over a week since a layer of faceted snow was covered by a 2-3′ slab (post St. Patty’s Day storms 3/16 and 3/19) and widespread avalanche activity ensued. We have been talking about this problem extensively and the variable nature of where it exists and where it does not. In short, triggering a persistent slab is becoming more and more unlikely because 1) many slopes have either already avalanched, 2) the facets were blown away before the storm or 3) the slab/weak layer has adjusted. 

That said: there is still a remote chance a person could trigger a slab up to 2′ thick that fails in weak faceted snow. Shaded aspects with no surface crust are the most suspect. Safe travel practices are encouraged – expose only one person at a time on a slope and have an escape route planned.

Additional Concern
  • Normal Caution
    Normal Caution
Normal Caution
Normal Caution means triggering an avalanche is unlikely but not impossible.
More info at Avalanche.org


Cornices have been growing over the last few weeks and today’s moderate Eastern ridgetop winds will continue their growth. If traveling near a ridgeline, be aware of cornice formations and give them lots of space. Steady moderate winds could increase the likelihood of cornice fall – thus avoid travel below cornices as well. 

Cornices like this one on Seattle Ridge have been getting larger over the last few weeks.


Wet Avalanches:

Overnight temperatures have remained above freezing (F) at mid and lower elevations. Rain will start later this afternoon and small wet loose avalanches on Southern and Eastern aspects below 2000’ are possible, yet should remain a low hazard.


Wed, March 25th, 2015

Yesterday skies were mostly clear. Temperatures spiked into the mid 40’s F below 2000′ and reached the mid 20’s F at upper elevations. Ridgetop winds were 20-30 mph. No precipitation was recorded.

Overnight winds remained the same. Temperatures were warm, mid 20’s F along ridgetops and mid 30’s F at lower elevations.

Today moderate ridgetops winds 20-30 mph from the Northeast will continue throughout the day. Temperatures will remain above freezing (F) at sea level and light precipitation is expected to start this afternoon. Up to 3″ of snow could fall this evening in the upper elevations. Expect rain along the coast and rain/snow line could be as high as 2000.’

Rain is expected overnight through tomorrow and is likely to be heaviest tomorrow morning and into the early afternoon. Temperatures will stay warm and rain/snow line is likely to be between 1000′-2000′. Ridgetop winds will remain moderate from East.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 35   0   0   54  
Summit Lake (1400′) 30   0   0   12  
Alyeska Mid (1700′) 36   0   0   31  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 25   ENE   19   48  
Seattle Ridge (2400′) 28   NNW   20   45  
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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.