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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sun, April 21st, 2019 - 7:00AM
Expires
Mon, April 22nd, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

The avalanche danger is CONSIDERABLE  above 1000′  due to strong winds and a mix of rain and snow impacting the area today. Natural avalanches are possible. Human triggered slab avalanches 1-3′ will be likely above 2000′. Below 2000′ where rain is falling, triggering  wet loose avalanches will be likely.  Extra caution is advised. 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.

WHITTIER:  Between 1-3 feet of new snow has likely fallen at the upper elevations in the Whittier Glacier region. Gale force winds and more snow and rain are in the forecast today. Heads up as natural avalanches are possible and large human triggered avalanches are likely.

LOST LAKE / SEWARD:  Extra caution is also advised in this region with new snow and rain in the forecast.

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

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Sun, April 21st, 2019
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
Avalanche risk
Below Treeline
Below 1,000'
No Rating (0)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Considerable (3)
Treeline
1,000'-2,500'
Avalanche risk
Considerable (3)
Below Treeline
Below 1,000'
Avalanche risk
No Rating (0)
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
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

The mountains are forecast to get another foot of snow today with easterly winds gusting into the 50s. Since Friday morning upper elevations have received between 10-30″ of snow favoring Girdwood. Natural storm slabs 1-3′ will be possible in the Alpine and human triggering will be likely on slopes 35 degrees and steeper. High elevation northerly aspects are the most suspect for poor bonding as the new snow fell onto weak older snow and possibly surface hoar. Slabs will be the deepest on wind-loaded slopes. Visibility will be limited so avoid runout areas where natural avalanches could fall from above.  Look for signs of recent avalanches, shooting cracks and drifted snow, and listen for whumphing.

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 last weekend failing on this layer. There is a chance that this layer could be overloaded, resulting in an avalanche stepping down deeper in the snowpack and is another good reason to avoid steep northerly slopes in the Alpine today!

Choose terrain wisely, visibility will be limited and natural avalanches will be possible. Tincan yesterday 4-20-19.

Avalanche Problem 2
  • Wet Loose
    Wet Loose
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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

Rain falling on snow today below 2000′ could initiate natural wet loose avalanches. These will be larger in areas with the more snow prior to the rain. A skier or a machine on a steep slope may also trigger these. As the snowpack becomes saturated wet sluffs may gouge deeper. Watch for roller balls and sinking into punchy wet snow.

CORNICES: There are still large cornices along ridge tops. Wind and snow today will make them even larger. Give them lots of space as they can break farther back than expected an could release naturally.

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

Watch for roller balls as snow becomes a saturated! 

Weather
Sun, April 21st, 2019

Yesterday:  Skies were broken in the morning and became mostly cloudy. There were light snow and rain showers throughout the day. The rain/snowline was around 700′. Temperatures were in the low 40Fs at sea level, 30Fs in the mid elevations and 20Fs at ridgetops. Winds were easterly gusting into the 20s during the day, increasing to gusts in the 40s overnight. 0.5-1.0″ SWE fell in the early morning hours again favoring Girdwood with temperatures in the high 30Fs to mid 20Fs.

Today: Rain/snow  could be heavy at times today. An inch of water is forecast to fall. This could translate to over a foot of new snow at upper elevations. Rain/snowline is forecast to be 1600′ but may rise to 2000′. Easterly winds will be 20-40 mph gusting into the 50s. Precipitation and winds will decrease overnight.

Tomorrow: Mostly cloudy skies with light rain/snow showers. Winds will be northerly 5-15 mph gusting into the 20s. Temperatures will be in the 30Fs dropping to the 20Fs later in the day. Snow may fall to sea level overnight into Tuesday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 32 8 0.6 72
Summit Lake (1400′) 34 1 0.1 22
Alyeska Mid (1700′)  34 6 0.5  66

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  28 NE 15 42
Seattle Ridge (2400′)   27 SE  8 18
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Riding Areas
Updated Mon, October 26th, 2020

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

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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.