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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, February 9th, 2024 - 7:00AM
Expires
Sat, February 10th, 2024 - 7:00AM
Forecaster
John Sykes
Avalanche Warning
Issued: February 9, 2024 9:00 am
Travel in avalanche terrain is not recommended. Avoid being on or beneath all steep slopes.
Avalanche risk The Bottom Line

The avalanche danger is HIGH at all elevations. A strong storm started yesterday evening with winds gusting to 90 mph and up to a foot of new snow already fallen. Today the heavy snowfall will continue and we recommend avoiding avalanche terrain and runout zones of avalanche paths, which could produce avalanches running into valley bottoms. Widespread buried weak layers could cause unusually large avalanches in low elevation terrain and catch folks off guard in areas that are typically safe.

Roof avalanches are likely as the temperatures increase and precipitation continues. Be aware of where you park, enter buildings, and where kids or dogs are wandering.

Fri, February 9th, 2024
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
Sat, February 10th, 2024
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
Sat, February 10th, 2024
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. 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.
Recent Avalanches

We received an anonymous report of an avalanche triggered in the Girdwood Valley near Upper Winner Creek on Feb 6th. This avalanche surprised the skier who triggered it because it occurred at low elevation (1200′) on a small terrain feature, but produced a relatively large avalanche. It failed on a layer of facets or surface hoar about 18″ deep (45 cm) and was roughly 100′ wide (30 m). This is likely a precursor of the type of avalanche activity we can expect today as the widespread weak layers buried 1-2′ deep are loaded with new snow and wind by the current storm system.

Skier triggered avalanche at 1200′ in Upper Winner Creek in Girdwood. Photo from anonymous on 2.6.24

Avalanche Problem 1
  • Persistent Slabs
    Persistent Slabs
  • Aspect/Elevation
  • 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.

Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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 weak layers of facets and surface hoar that developed during the January cold and dry spell are now buried 2-3′ below the snow surface. Today’s storm will be the first time these weak layers are stressed by a significant snow load and it is very likely that we will see large natural avalanches occurring 2-4′ deep on these buried weak layers. Due to the temperature inversion that was in place while these weak layers formed, the buried surface hoar seems to be most widespread at elevations below 1500-2000′. That means large avalanches are possible on uncommon terrain features, like sheltered open meadows below treeline. Buried surface hoar can cause avalanches on low angle slopes, as low as 25-30 degrees, and can produce avalanches with very wide propagation.

We recommend avoiding avalanche terrain today and being very aware of the potential for natural avalanches to runout to valley bottoms.

Layer of buried surface hoar about 1′ deep at 1400′ in Crow Creek area. Photo from JT Fischer 2.8.24

Avalanche Problem 2
  • Storm Slabs
    Storm Slabs
  • Aspect/Elevation
  • 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.

Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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

In addition to the potential for larger avalanches on buried weak layers we also expect wind slabs and storm slabs to be releasing naturally and very likely for human triggering today. The winds have been averaging 30-40 mph with gusts of 60-90 mph for 12 hours now and there has been 6-12″ of snow accumulation so far, so natural avalanche activity has likely already started. Areas near Portage and Placer as well as Turnagain pass are expected to be favored by this storm. Avalanches 1-2′ deep in sheltered terrain where the new snow may not bond well to the old snow surface are most likely in these locations. Rain line is expected to move up to about 1300′ today, so there could be some funky conditions at low elevations with rain on dry snow. We strongly recommend keeping it simple today by avoiding avalanche terrain. 

NWS snowfall estimate from Friday at 3am to Saturday at 3am. Graphic from NWS Anchorage 2.9.24

Additional Concern
  • Glide Avalanches
    Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
More info at Avalanche.org

Glide avalanches are still a concern on any slope with glide cracks present. We have not seen or heard of one releasing into an avalanche for two weeks now, but that doesn’t mean a crack can’t release today. Continuing to avoid any time under glide cracks is prudent.

Weather
Fri, February 9th, 2024

Yesterday:  A storm system moved into the area yesterday, bringing mostly cloud skies, strong winds, increasing temperatures, and snowfall. Wind speeds at upper elevations increased from averages in the teens yesterday morning to 30-50 mph out of the east overnight with gusts to 90 mph. At upper elevations temperatures have remained in the teens to low 20s F, while closer to sea level temperatures started out in the low 30s F yesterday morning and have risen to the mid 30s F. Snowfall began around 4pm on Thursday with roughly 6-10 inches of new snow falling overnight. As usual Portage and Placer have the highest totals so far, with Turnagain Pass coming in second, and Girdwood currently trailing.

Today: Heavy snowfall is expected to continue throughout the day on Friday, with another 6-10″ of snowfall expected in Girdwood and Turnagain Pass and 15-25″ in Portage and Placer. Rain line is expected to rise from roughly 300′ this morning to 1300′ this afternoon. Wind speeds should remain strong with averages of 40-50 mph and gusts of 75+ mph out of the east.

Tomorrow: On Saturday light snowfall is expected to continue with a few more inches of accumulation. Rain line should drop back down to 200-300′ by Saturday morning. Winds should shift to the south and are expected to decrease to averages of 20-30 mph and gusts of 45 mph. Temperatures are expected to remain in the mid 20s F at upper elevations and low 30s F at lower elevations. Cloud cover may break up slightly in the afternoon, but low visibility conditions are expected for most of the day.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28 10 0.9 85
Summit Lake (1400′) 29 1 0.1 n/a
Alyeska Mid (1700′) 28 7 0.57 85
Bear Valley – Portage (132′) 36 rain/snow 0.9
Grouse Ck (700′) 33 2 0.2 58

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20 E 31 90
Seattle Ridge (2400′) 24 SE 14 36
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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.