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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, April 11th, 2018 - 7:00AM
Expires
Thu, April 12th, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

Above freezing temperatures and rain showers will keep the danger at  CONSIDERABLE today. Human triggered wet snow avalanches are likely and naturals are possible. In the Alpine triggering a slab avalanche 1-2′ thick remains possible on Northerly aspects where drier snow exists. Give cornices a wide berth.

Wed, April 11th, 2018
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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
  • Wet Loose
    Wet Loose
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.
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

Yesterday saw heavier rain in the early morning then rain showers on and off throughout the day. Portage picked up over an inch of rain. Temperatures were above freezing to around 3500′. Observers reported snow becoming progressively more saturated especially below 2000′. Skies cleared briefly last night and there was a superficial freeze. However, today a bit more rain is in the forecast, we may see some afternoon clearing with direct sun and the temperatures remain warm. All these factors are continuing to make triggering a wet loose avalanche likely and naturals remain possible. There were a number of natural wet loose avalanches observed yesterday with the largest occurring on the west face of Pyramid mid morning. The snowpack could continue to get more saturated throughout the day. If you do decide to recreate today pay attention to the depth you are sinking in and get off the slope if the snow is over your boot tops or your machine is trenching in. Watch for roller balls.  Expect loose snow moving to entrain more as moves downhill. Any fresh snow in the Alpine may be especially reactive on solar aspects if the sun comes out today. 

Wet slab avalanches:  We have not seen any wet slab avalanche activity yet, but it’s not out of the question that a wet loose slide (or the weight of a skier or snowachiner) could trigger a wet slab today as water penetrates to lower layers. 

Natural wet loose avalanche on Pyramid

Wet loose avalanches on Magnum

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

Above the rain line triggering a slab avalanche 1-2′ thick on Northerly shaded aspects is still possible. Overall we have seen that the snow from last week has been bonding but haven’t forgotten that there is a weak layer under the old storm slab.  The slab/facet combo remains suspect and may be more tender in the Alpine with the additional few inches of heavy snow from yesterday. Wind loading may also add stress, as the ridgetop winds remained strong yesterday and continue today. 

Additional Concern
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

It’s that time of year where the snowpack is warming up and cornices will begin to break. Give cornices a wide berth.

Weather
Wed, April 11th, 2018

Yesterday was partly to mostly cloudy with heavier rain falling in the early morning and localized rain showers on and off throughout the day. Over an inch of rain fell in Portage Valley. Temperatures were in the 30Fs to 40Fs. Winds were Easterly 15-25 mph with gusts into the 40s. Overnight there was brief period of clearing. Temperatures remained above freezing to around 3500′. Sunburst (3812′) hit 32F this morning at 5 am. Easterly winds decreased overnight and are picking up again this morning.  

Today is forecast to be mostly to partly cloudy with rain showers. Winds will be easterly 15-25 mph with gusts into the 40s. Temperatures will be in the 30Fs and 40Fs. Tonight temperatures will cool down a bit as colder air moves into the region.  

The weather for the next couple of day looks to be a mix of sun and clouds, rain and snow showers and variable temperatures. If the cooler air lines up with some clearer skies at night we could get more of a freeze. Sunshine is in the forecast for the weekend.    

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  38  0 0.2 73  
Summit Lake (1400′)  36   0    0.1  28    
Alyeska Mid (1700′)  37  0  0.1  69    

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  28  ENE  20 45  
Seattle Ridge (2400′)  33  ESE  13  27    
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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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