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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, January 24th, 2019 - 7:00AM
Expires
Fri, January 25th, 2019 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

The avalanche danger is HIGH above 1500′ due to strong winds and heavy snow this morning. In Girdwood and Portage Valley storm slabs 2-3+’ could release naturally. In Turnagain Pass where less snow has fallen triggering a storm slab up to 1.5′ deep is likely today on slopes greater than 30 degrees. Active wind loading, heavy precipitation and natural avalanche activity are redflag warnings to avoid avalanche terrain. The avalanche danger is CONSIDERABLE below 1500′ where wet loose snow avalanches are possible.

GIRDWOOD:  Warming temperatures and rain on old snow = ROOF AVALANCHES. Pay attention to children and pets and where you park your car.  

PORTAGE and PLACER VALLEY:    Heavier rain and snowfall rates have occurred and large avalanches above treeline may send debris to sea level and over summer hiking trails such as Byron Glacier Trail.

SUMMIT LAKE / JOHNSON PASS:    A poor snowpack structure exists in this area, which is very different than Turnagain Pass. New snow/rain and wind may overload weak layers in the snowpack and triggering a larger avalanche is possible. Look for signs of instability and pay attention to changing conditions.  

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Thu, January 24th, 2019
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
4 - High
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
  • Storm Slabs
    Storm Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.
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

Over the last two days periods of heavy snow and strong winds have created storm slabs in the mid and upper elevations of our forecast zone. The size of these slabs depends on precipitation totals and proximity to coastal areas. Portage Valley and Girdwood have received the highest amounts of precipitation and storm slabs could range from 2-3+’ in these zones. In Turnagain Pass where less snow has fallen storm slabs are expected to range from 8-16” thick. Strong Easterly ridgetop winds have been loading leeward features including a gust to 98 mph at Sunburst Wx station at 6am. Strong Easterly winds are expected to decrease to Moderate and precipitation is also expected to back off. The avalanche danger in Turnagain may decrease to CONSIDERABLE as winds decrease. If you head to an area that received less snowfall please keep in mind that triggering slab today could still be large enough to bury or kill a person. This new snow has fallen on a widespread weak layer of surface hoar  that is sitting on a hard bed surface in many places. Signs of avalanche activity, shooting cracks and collapsing are expected today and will be reminders to avoid avalanche terrain. Be aware –  another storm will impact our region overnight with up to another 1.0” of snow water equivalent (~12” of snow) expected for Turnagian and higher amounts in Girdwood and Portage.

Shooting cracks and small skier triggered storms were observed yesterday on Tincan. 

 

 

A crust bed-surface exists to 2500′ and buried surface hoar to ridgetops is also sitting under all of this new snow. This is a bad set-up for new snow to be falling on.  

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

Below 2000’ where rain and wet snow have been falling the last two days natural wet loose avalanche are possible today. This avalanche hazard is most concerning in Portage Valley where a natural avalanches from above could run to valley bottoms in steep channeled terrain. Avoiding areas like Byron Glacier trail is recommended.

Additional Concern
  • 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

South of Turnagain – Johnson Pass/Summit Lake zone: A poor snowpack structure exists in these areas.  Multiple mid-pack weak layers of facets and buried surface hoar have been found as well as a facet/crust combination in the bottom of the snowpack. Strong winds, rising temperatures and a few inches of snow over the last two days has added stress to older layers in this zone. The possibility of triggering a slab deeper in the snowpack is the primary concern in this area and a person’s weight may be enough to tip the balance. Look for signs of instability and snow and terrain carefully. 

Weather
Thu, January 24th, 2019

Yesterday: Above freezing temperatures caused precipitation to fall as rain below 1000′. A mix of rain and snow was observed below 1500′ in Turnagian Pass. Girdwood and Portage received the highest precipitation totals with Girdwood at 1.16 € SWE and Portage at 2.37 € SWE in 24 hours. Turnagain Pass DOT lot recorded 0.6 € slightly higher than the Turnagain Pass Snotel at 0.4 € SWE. Strong Easterly ridgetop winds in the yesterday morning decreased to Moderate for most of the daylight hours. Overnight Easterly ridgetop winds increased to strong averaging 30-60mph’s with a few gusts in the 90mph’s. Temperatures at sea level reached 40F’s overnight. Rain/snow line may have reached 2000′.

Today: Expect temperatures to remain around 40F at sea level and mid 30F’s at 1000′. Rain/snow line may reach 2500′. Strong winds are expected to decrease this morning to Moderate for most of the day. Tonight another storm is lined up to impact our region. Expect another round of warm temps, heavy rain and strong winds overnight.

Tomorrow: Another low-pressure system will move through our region with heavy rain expected through the morning at lower elevations. Rain/snowline is expected to be around 2500′ which means heavy snow in the upper elevations. This storm will see another round of strong Easterly winds, but winds are expected to decrease in the afternoon as the storm front passes.

 *Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed. We are currently working to replace it.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 34   3   0.4   53
Summit Lake (1400′) 36   rain   0.1   20  
Alyeska Mid (1700′) 33   3″ wet snow   1.16   40  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 25 ENE   33   98  
Seattle Ridge (2400′) 30   *N/A    *N/A *N/A  
Observations
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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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