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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Tue, January 2nd, 2018 - 7:00AM
Expires
Wed, January 3rd, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

The  avalanche danger remains HIGH above 1000′  due to rain, heavy wet snowfall and strong winds.    Natural avalanches are likely and human triggered avalanches are very likely today.  As the new snow, rain and wind overloads a weak layer of buried surface hoar and facets, avalanches are becoming larger and more dangerous.  Travel in avalanche terrain is NOT recommended.  This includes areas that are in the runout from avalanche paths above.  

The avalanche danger is  CONSIDERABLE  below  1,000′ where debris from avalanches above may run.

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Tue, January 2nd, 2018
Alpine
Above 2,500'
High (4)
Avalanche risk
Treeline
1,000'-2,500'
High (4)
Avalanche risk
Below Treeline
Below 1,000'
Considerable (3)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
High (4)
Treeline
1,000'-2,500'
Avalanche risk
High (4)
Below Treeline
Below 1,000'
Avalanche risk
Considerable (3)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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
This graphic depicts how likely you are to trigger avalanches or encounter natural avalanches while traveling on avalanche prone slopes. Unlikely means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. The chance of triggering or observing avalanches increases as we move up the scale. Certain means that humans will be able to trigger avalanches on many slopes, and natural avalanches should be expected.

Size of Avalanches
This graphic depicts the potential size and destructive force of expected avalanches. 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, large enough to bury or destroy vehicles and break a few trees, and large enough to destroy railway cars, buildings, or a substantial amount of forest. Historic avalanches are massive events capable of destroying villages and gouging or altering the landscape.
More info at Avalanche.org

This series of storms continues to build storm slabs and overload the weak layer of surface hoar and near surface facets that formed last week. Yesterday folks continued to trigger small avalanches in the Tincan Trees and observed signs of instability including whumpfing, cracking and hand pits failing on isolation. There was low visibility which made it difficult to see into the alpine but strong winds were rapidly loading leeward slopes. Sunburst saw gusts as high as 102 mph. Temperatures rose and the snow became more and more upside down. Since the storms started on Saturday, Center Ridge Snotel has received 1.8″ of water and mid-elevation stations in Girdwood received 2.4″ of water.  This translates to 15-30″ of total snow since Saturday up high. Unfortunately at lower elevations some of this precipitation came as rain overnight as temperatures rose and rain fell to as high as 2300′. Today the recipe for avalanches is pretty simple. Weak snow has been overloaded by heavy snow, wind loading or rain. Slabs in upper elevation terrain could be 2-4 feet thick.  Travel in avalanche terrain (on slopes steeper than 30 degrees) is not recommended. Runout zones should also be avoided due to the potential for natural avalanches and as always steer clear of terrain traps. Even a small avalanche in the wrong spot could be very hazardous. 

Screen shot of Sunburst Weather Station this am. Note the wind profile since Saturday = rapid loading!

Layer of surface hoar that is buried below the storm snow. This is the weak layer of concern. 

Slopes triggered by skiers and boarders on Sunday were reloaded on Monday. 

 

Avalanche Problem 2
  • Wet Slab
    Wet Slab
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Wet Slab
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.

Likelihood of Avalanches
This graphic depicts how likely you are to trigger avalanches or encounter natural avalanches while traveling on avalanche prone slopes. Unlikely means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. The chance of triggering or observing avalanches increases as we move up the scale. Certain means that humans will be able to trigger avalanches on many slopes, and natural avalanches should be expected.

Size of Avalanches
This graphic depicts the potential size and destructive force of expected avalanches. 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, large enough to bury or destroy vehicles and break a few trees, and large enough to destroy railway cars, buildings, or a substantial amount of forest. Historic avalanches are massive events capable of destroying villages and gouging or altering the landscape.
More info at Avalanche.org

Rain fell overnight to as high as 2300′. It is adding weight to the already stressed snowpack and breaking bonds between snow grains. Water saturating new snow could cause natural wet loose or wet slab avalanches. Triggering wet avalanches in the treeline elevation band is also likely today in steep terrain. This is another reason travel in avalanche terrain is not recommended today. 

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

In the alpine, above 3,000’, rapidly loading slopes may awaken a large and dangerous deep slab avalanche. At these elevations, a hard slab, 3-5+ feet thick, is sitting on top of weak sugary snow (basal facets) near the ground. As new snow increases the load over this snowpack structure during the current storms, there will be the potential for large natural avalanches. Between storms, human triggered deep slab avalanches will be possible. This is a high consequence avalanche problem that is impossible to outsmart and can take a long time to heal. Keep this in mind as breaks between storms may allow for travel to the Alpine. 

 

Weather
Tue, January 2nd, 2018

Yesterday there were light rain and snow showers in the morning and the next storm system picked up mid-day with low visibility, gusty winds, and heavier snowfall and rain.  Approximately 10-15″ of  additional new snow fell in the upper elevations of Girdwood Valley and the Turnagain Pass area. 24-hour totals are below in the table. Lesser snow amounts were seen on the South end of Turnagain Pass and in the Summit Lake area. Ridgetop winds were Easterly averaging 35-45mph with gusts to 102mph. Temperatures were in the mid 20Fs at treeline and rose to mid 30Fs at 1,000′. Overnight rain fell to as high as 2300′.

Today, rain and snow showers continue with another 5-10″ of snow or .35 inches of rain possible.  Rain/snow line is forecasted to be around 2300′ today. Precipitation will taper off overnight. Temperatures start warm today in the upper 30Fs above 1000′ and 40Fs at sea level.  They will cool down this evening into the 20Fs. This will bring the rain/snow line down, with snow showers possible overnight.  Winds will be easterly 20-30 mph, gusting into the 50s. Winds speeds will decrease in the afternoon.

Wednesday into Friday looks to be a break in the storms with a chance of some sunshine, lighter winds and temperatures in the 20s. Stay tuned for the next storm details, as there is still a fair bit of uncertainty about what the weekend will bring.

*Seattle Ridge is anemometer is rimed and under reporting.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 34    9 .8    48
Summit Lake (1400′) 33 1 .2    14
Alyeska Mid (1700′) 32  6  1.35  41

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  23  ENE 42   102  
Seattle Ridge (2400′)  28  *n/a *n/a   *n/a  
Observations
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Riding Areas
Updated Wed, December 11th, 2019

Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff. Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations. Riding area questions contact: mailroom_r10_chugach@fs.fed.us

Area Status Weather & Riding Conditions
Glacier District
Johnson Pass
Closed
Closed.
Placer River
Closed
Closed.
Skookum Drainage
Closed
Closed.
Turnagain Pass
Closed
Closed.
Twentymile
Closed
Closed.
Seward District
Carter Lake
Closed
Closed.
Lost Lake Trail
Closed
Closed.
Primrose Trail
Closed
Closed.
Resurrection Pass Trail
Closed
Closed. Will be open for the 2019/20 season pending adequate snow cover.
Snug Harbor
Closed
Closed.
South Fork Snow River Corridor
Closed
Closed.
Summit Lake
Closed
Closed.

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