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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, April 9th, 2021 - 7:00AM
Expires
Sat, April 10th, 2021 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains CONSIDERABLE above 1,000′ as strong northwest winds continue over the region. Natural wind slab avalanches and cornice falls are possible. These may step down to buried weak layers, creating a larger avalanche. Watch for blowing snow, shooting cracks and recent avalanches. Cautious route-finding and conservative decision-making are essential.

PORTAGE VALLEY and PLACER VALLEY: There is potential for avalanche debris, from a slide occurring above, to run to low elevation terrain. Avoiding summer trails that run through avalanche runout zones, such as Byron Glacier trail, is recommended.

SUMMIT LAKE/SNUG HARBOR/LOST LAKE/SEWARD: The northwest winds continue to affect these areas with significant amounts of blowing snow. Natural avalanches remain likely.

A High Wind Warning issued by the National Weather Service remains in effect till 1pm today.

Special Announcements

Hatcher Pass:  The road to Hatcher Pass remains closed at MP10. For updates go to AK 511 and follow AKDOT&PF on social media. Click HERE for an ADN article about the closure. For the Hatcher Pass’s Thursday Conditions Summary go to hpavalanche.org.

Fri, April 9th, 2021
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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
  • Wind Slabs
    Wind Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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

“Honking”… This was my favorite descriptor of the strong northwest winds pummeling the region yesterday. Well, those winds continued overnight and will keep honking through most of today. Ridgetops have been seeing averages from 20-40mph and gusts twice this. To add insult to injury for the second week of April, cold arctic air is being ushered in and the Sunburst weather station just hit -6F at 6am! When the winds come from the northwest they can sometimes split around Turnagain Pass, but unfortunately this isn’t the case this time. It’s likely going to be quite difficult to find any soft snow in anything but the most sheltered treed areas today.

Skies were a bit obscured yesterday with all the blowing snow. However, we can have a pretty safe bet there were several naturally triggered wind slab avalanches and maybe even some cornice falls over the past 24 hours. This possibility remains today with the continuation of the winds. Despite a limited amount of soft snow available for transport, the winds seem to be eroding harder old snow and finding a way to keep loading slopes.

If you do head out today, pay attention to active wind loading and slopes with recent wind deposited snow. Human triggered wind slabs will be likely. Slabs could be anywhere from a few inches thick to a few feet depending on the amount of loading. Watch for cracks that shoot out from you and stiff snow over softer snow. Winds may have impacted mid elevations treed zones or other areas not commonly affected. Remember, wind slabs that release could step down into buried weak layers and create a larger avalanche (more on that in Problem #2).

Winds blowing snow off the west face of Pyramid and loading the easterly face. 4.8.21.

 


Blowing snow in the Summit Lake region. 4.8.21. Paul Wunnicke.


Strong winds south of Turnagain Pass. 4.8.21. Paul Wunnicke.


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

With new wind loading occurring, we are suspect this might overload buried weak layers and cause them to become more reactive. There are a few layers of facets and buried surface hoar in the top 3′ of the snowpack. These layers have been showing varying degrees of strength and just enough weakness in places that we are still concerned a person could trigger a persistent slab avalanche. Shaded aspects are the most suspect places to trigger these now due to southerly slopes having various sun crusts. Knowing these layers are there, listening/feeling for whumpfing and watching for any other signs of instability are good things to keep in mind. As always, use good travel protocol, and consider the consequences if an avalanche does occur.

Weather
Fri, April 9th, 2021

Yesterday:  Cold and windy. Partly cloudy skies with haze from all the wind blown snow suspended in the air. Northwest winds were strong at the mid and high elevations, averaging ~15-25mph with gusts to 40mph. Temperatures were 20-15F along ridgelines midday before dropping to the single digits overnight.

Today:  Cold, clear and windy. The northwest winds will remain strong through the afternoon with averages ~25-40mph and stronger gusts. Single digit temperatures are being brought in with the wind and daytime highs should only be ~20F at the lower elevations and near 10F in the mountains. The winds should die off late tonight.

Tomorrow:  The next weather system moves in tomorrow. Skies will begin clouding over and winds will be light (5-10mph) from a south and easterly direction. A few snow flurries may fall but no accumulation is expected at this point. Temperatures will slowly rise to the 20’sF at mid elevations and the teens in high elevations. A pattern shift to more active weather and precipitation is slated for this coming week, starting Sunday. Stay tuned.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 14 0 0 108
Summit Lake (1400′) 13 0 0 48
Alyeska Mid (1700′) 14 0 0 117

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 4 W 12 41
Seattle Ridge (2400′) 10 NW 14 38
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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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