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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, April 23rd, 2019 - 7:00AM
Expires
Wed, April 24th, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is MODERATE  above 1000′.  Human triggered slab avalanches 1-3′ are possible on slopes 35 degrees and steeper. Watch for warming and wet loose activity on solar aspects if the sun makes an appearance. Evaluate snow and terrain carefully, ease into steep terrain and look for signs of instability. As always, give cornices a wide berth and limit travel under glide cracks.

PORTAGE VALLEY:  Cornice fall and/or avalanches from above have the potential to send debris to valley bottoms. Traveling along summer hiking trails, such as the Byron Glacier Trail with steep slopes overhead is not recommended on rainy/snowy days or on sunny afternoons. Portage Valley received over 4′ of snow at upper elevations and cornices are looming large.

WHITTIER:  Between 2-4 feet of new snow has likely fallen at the upper elevations in the Whittier Glacier region.   Heads up as there is limited snowpack information for this area. Large human triggered avalanches are possible and extra caution is advised.

LOST LAKE / SEWARD:  There is also limited snowpack information for this region as well. Ease into steeper terrain and look for signs of instability.

WEDNESDAY AVALANCHE OUTLOOK:
No avalanche forecast will be issued tomorrow.  Avalanche danger is expected to be similar to today. Pay attention to changing conditions and look for signs of instability.    

Special Announcements
  • Advisories:  Our last avalanche advisory will be posted Thursday April 25th. The avalanche center will close for the season on Saturday, April 27th when we will post our springtime tips. Thank you everyone for tuning in! Big thanks to all of our observers and the local avalanche professionals that share information with the CNFAIC.
Tue, April 23rd, 2019
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
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. 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

#hellowinterwherewereyouinmarch. An active weather pattern starting Thursday April 17th brought snow and rain to the area and total water amounts ranging from 3.6″ at the Alyeska top station, 4+” in Portage, 2.1″ at Center Ridge on Turnagain Pass to 1.1″ in Summit Lake. This equates to upper elevations receiving between 10-40″ of snow favoring Girdwood and Portage. Temperatures cooled Sunday afternoon into Monday and brought light snow to sea level. Sunday there were strong sustained easterly winds that gusted as high as 78 mph on Sunburst. These eased off that evening and were mostly light yesterday. Triggering a 1-3′ slab will be possible on slopes 35 degrees and steeper today. Slabs will be the deepest on wind-loaded slopes. High elevation northerly aspects are the most suspect for poor bonding as the new snow fell onto weak older snow and possibly surface hoar. Warm temperatures for much of the weekend followed by cooling will have helped stabilize the new snow. However, the snowpack is untested and it will be important to look for recent avalanches, shooting cracks and drifted snow, and listen for whumphing. Ease carefully into steeper terrain and use safe travel protocols. There is some uncertainty today about cloud cover and direct sunshine. Pay attention to changing surface conditions and watch for roller balls. On solar aspects warming may increase instability and likelihood of triggering.

PERSISTENT SLABS: Steep northerly slopes above 3000′ also harbor buried surface hoar and near surface facets that were buried 1-2′ deep on April 5th. Two human triggered avalanches occurred just over a week ago failing on this layer. There is a chance that triggering an avalanche in the storm snow on a north aspect above 3000′ could step down resulting in a large avalanche breaking deeper in the snowpack.

Sunburst wind profile over the weekend. Pay attention to loading patterns and watch for cracking. 

Wintery weather on Tincan, Sunday April 21st. Photo: Ray Koleser

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

There is a chance that the sun could come out today or tomorrow and warm up the new snow. If this happens watch for signs that the surface is becoming wet. Roller balls may build into loose wet avalanches. Due to the amount of new snow these could become large as they entrain snow. Change aspects if the snow starts to show signs of warming and be especially suspect of being on or under steep, rocky, southerly terrain.

CORNICES: There are still large cornices along ridge tops. They will have grown during the storm and may be tender. Give them lots of space as they can break farther back than expected.

GLIDE AVALANCHES: It’s been over two weeks since our last known glide avalanche, but keep in mind glide cracks are continuing to creep downhill. As always, limit traveling under their runout. They are unpredictable and can avalanche at any time.

Weather
Tue, April 23rd, 2019

Yesterday:  Skies were mostly cloudy with pockets of good visibility. There were snow showers on and off throughout the day with snow falling to sea level and a few inches of accumulation. Winds were variable gusting into the 20s. Temperatures ranged from in the high teens and 20Fs at upper elevations to right around 30F at sea level. Overnight temperatures cooled slightly and winds remained light.

Today:  Mostly to partly cloudy skies. There is a chance of snow showers. Winds will be light and easterly. Temperatures will be in the 30Fs to 40F at sea level and in the 20Fs at upper elevations.   Overnight snow showers are likely, temperatures will cool slightly and winds remain light.

Tomorrow:  Mostly to partly cloudy skies. Slight chance of snow showers. Winds continue to be light and temperatures will be similar to Tuesday. Looking ahead there is sunshine and warmer temperatures on tap for Thursday and Friday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27  1 0.1 74
Summit Lake (1400′)  27  2 0.2 21
Alyeska Mid (1700′) 27  2 0.19 69

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  18 NE 4 24
Seattle Ridge (2400′)  23 SW  3 8
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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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