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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, March 25th, 2019 - 7:00AM
Expires
Tue, March 26th, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE this morning but will increase to HIGH  by this afternoon with direct sunlight and very warm air temperatures making natural large wet slab avalanches likely. This is the first day of sunshine after many inches of rain and feet of snow at upper elevations. The snowpack needs time to adjust, drain and have a substantial overnight freeze. Avalanches could still  gouge to the ground, run to valley bottoms and be very destructive. Above 3,000′ deep slab avalanches and cornice falls are likely.  Once again, travel in avalanche terrain is NOT recommended.

PORTAGE VALLEY:  Very large avalanches could continue to release naturally. Avoid summer hiking trails that travel through avalanche paths, such as Crow Pass, Byron Glacier and Johnson Pass Trail.

SUMMIT LAKE (& INTERIOR EASTERN KENAI MTS):   Despite less precipitation, natural and human triggered avalanches remain likely. The snowpack has many weak layers and the direct sunshine and warm temperatures will keep the hazard elevated in this region.

SEWARD/LOST LAKE:  Similar to Turnagain – the hazard will remain elevated as the potential for natural avalanches will continue today with warm temperatures and direct sunshine.  Travel in avalanche terrain is not recommended.

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Mon, March 25th, 2019
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. Dangerous avalanche conditions. 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
  • Wet Slab
    Wet Slab
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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
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

After weeks of rain and HIGH danger the avalanche advisory may sound like a bit of a broken record. The sunshine and blue skies in the forecast today may be very enticing but the snowpack needs more time.  The rain has essentially made a giant slurpee on the slopes that has been avalanching, gouging to the ground and running to sea level. There was a superficial freeze last night that will quickly deteriorate. Today the slurpee is going to get heated by sunshine and unseasonably warm air temperatures with highs in the 40Fs. Free water running through the snow could stress the snowpack out and trigger large wet slab and wet loose avalanches, especially on solar aspects. Rocky areas and steep slopes in the direct sun will heat up first. Travel in avalanche terrain is not recommended today, this includes runout zones. The snowpack needs a solid freeze and time to drain and adjust.

Glide avalanches are a totally unpredictable hazard that are also a concern today. There have been a number of glide avalanches throughout the advisory area over that past few days and there are cracks in popular terrain. This is another reason to steer clear of avalanche terrain. Don’t mess with the brown frown!

Lipps, 3-21-19.

Orca, 3-22-19.

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.
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

Above 3000′ and the wet snow hazards the Alpine terrain has received feet of snow with strong winds. There is now a potential deep slab issue. Underneath all the snow the interface with the March 8th old snow is a concern. Buried facets and surface hoar at this interface was the suspected weak layer in the impressive avalanche cycle that occurred before this past week of feet of snow. Today the sunshine and warm upper elevation temperatures could stress the cold snow slabs as well. Large, deep slab avalanches are likely in the Alpine and could occur naturally or be triggered from thin spots by humans on skis or snowmachines. Don’t be lured into avalanche terrain today on a quest for spring powder. These slabs could be very dangerous and destructive. Again, the snowpack needs time to adjust. Be patient and remember the travel advice for the day is travel in avalanche terrain is not recommended.

Tincan North side avalanche on 3-18-19, Photo 3-19-19: Travis Smith. There is the potential for slab avalanches like this in the Alpine today. 

Additional Concern
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Cornices are very large and warm air temperatures and direct sunshine could destabilize them today. They have the potential to trigger very large and destructive avalanches on the slopes below and break way farther back than expected. Give them a wide berth and recognize that they could release naturally today.

Weather
Mon, March 25th, 2019

Yesterday: There was mostly cloudy skies with some pockets of clearing and light rain/snow showers. Temperatures were in the 20Fs in the Alpine and the 40Fs at sea level. Winds were easterly 15-25 mph with gusts into the 50s. Overnight skies became broken and temperatures cooled slightly. Easterly winds were in  teens with gusts into the 20s.

Today: Partly sunny skies with temperatures in the 30Fs and 40Fs. Winds will be very light and southerly. Overnight temperatures will be in the high 20Fs to 30Fs with partly cloudy skies. Valley fog is in the forecast as an inversion sets in.

Tomorrow: Valley fog in the morning with  mostly clear and sunny skies with highs in the 40Fs and light north winds. The week ahead looks to very similar as the ridge of high pressure is forecast to sit over the area.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 35   0 0.1 72
Summit Lake (1400′) 34  0 0 24
Alyeska Mid (1700′) 35  0 0.3 67

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  25  NE  16 55
Seattle Ridge (2400′)  30   SE  12 32
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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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