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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, March 16th, 2018 - 7:00AM
Expires
Sat, March 17th, 2018 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

Today spring like conditions and above freezing temperatures have elevated the avalanche danger to HIGH across our forecast zone. Natural avalanches are anticipated and human triggered avalanches are likely on all aspects and elevations. Triggering a dangerous slab 2-5′ thick is very likely in avalanche terrain and natural wet loose and large slab avalanches are also likely today. The only way to manage this avalanche problem is by avoiding avalanche terrain, slopes steeper than 30 degrees and their runout zones.

A skier remotely triggered an avalanche on the skin track in Summit Lake Monday – see Saturday’s  Summit snowpack and avalanche summary  and recent observations  HERE.

Special Announcements

*Dangerous avalanche conditions are being seen region-wide. Recent human triggered and natural avalanches have been observed as far South as Lost Lake and North in Eagle River and Hatcher Pass.

**Roof avalanches are a serious concern today with warm temperatures, particularly around Girdwood where there is still a lot of snow on roofs. Be mindful of where children and pets are playing, vehicles parked and entrances to buildings in relation to loaded roofs.

 

Fri, March 16th, 2018
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
4 - High
Avalanche risk
Below Treeline
Below 1,000'
4 - High
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
  • 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

Spring time avalanche danger is upon us! Overnight temperatures remained above freezing below 1500’ and light rain has been falling in the lower elevation band. This is the first night without a freeze in the mid and lower elevations and today’s warming is expected to increase the danger. In the Alpine Easterly winds 15-45mph have been loading leeward slopes and adding stress to a very stressed out snowpack. Weak faceted snow is sitting 2-5’ below the surface and has proven to be very reactive this week. Everyday since Monday we’ve had reports of large human triggered avalanches across the region including Girdwood, Placer, Turnagain Pass, Lost Lake, Summit Lake and Grandview. Some natural avalanches have also released including an avalanche on a NE aspect of Skookum Valley with a crown almost a mile wide on Tuesday. Most of these avalanches have occurred below 3000’ and have been large enough to bury and kill a person. Some of these avalanches have been remotely triggered while others have released after the skier or snowmachiner was well onto the slope. So far we have not received any reports of anyone fully buried or injured, but several people have taken rides and deployed airbags this week. All avalanche terrain including smaller unsupported features and steep terrain in the trees are suspect. 

Three snowmachine triggered avalanches on Southerly aspects in Lost Lake on Wednesday that filled up a terrain trap with 20′ of debris.  A big thanks to Leif Hagen for letting us share your photos and posting these on social media.

 

On Wednesday a skier-triggered avalanche caught and carried 3 skiers in small terrain on Silvertip (southern Turnagain Pass) on a NE aspect around 1500’. 

 

Yesterday a handful of skier triggered and remote triggered avalanches occurred in the Girdwood Valley of Notch Mountain. Its unknown if anyone was caught or carried. If you have any photos or info consider submitting an observation HERE. Reporting a near miss could save the life of others.

 

Avalanche Problem 2
  • 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

Wet loose avalanches or roller balls today are indicative of solar radiation and/ or daily warming temperatures affecting change on the snowpack.  Wet loose avalanches could be small or large and have the potential to trigger larger, more dangerous slabs, particularly on southerly slopes greater than 30 degrees. We have been seeing this throughout the week and today it will be more likely if the sun pokes through the clouds.

 

Weather
Fri, March 16th, 2018

Yesterday was overcast becoming obscured in the afternoon by rain and snow showers. Temperatures remained above freezing at 1000′ and light rain fell as high as 1500′ overnight. Two to four inches (.2 € SWE) is estimated in the upper elevations. Easterly ridge top winds increased in the afternoon and evening to 15-45mph.    

Today skies are expected to remain overcast, but clouds may be thin allowing some sun through in the afternoon. Ridgetop winds will decrease this morning (5-20mph), but remain in the 30’s mph in the gap areas of Portage Valley. Scattered snow showers are possible in the morning with up to 1-2 € of snow in the upper elevations. Rain/snow line is expected to remain around 1700′. Daily temperatures could rise into the low 40F’s by mid-day at sea level. Temperature in the upper elevations may reach the mid 30F’s this afternoon.  

Temperatures are expected to remain warm, but we may see freezing tonight if skies clear. Clear skies and sun are expected tomorrow and day time temps will be in the mid 30s and could increase into the 40F’s at sea level. Winds will be light and variable. Snow and rain showers may resume again on Sunday with an active pattern still in the forecast. There is talk of cooler temperatures returning to our area next week.    

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28   1   .3   87  
Summit Lake (1400′) 30   0   0   33  
Alyeska Mid (1700′) 30    2 .26   81  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22   ENE   15    44
Seattle Ridge (2400′) 26   SE   20   52  
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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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