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Issued
Thu, April 27th, 2023 - 7:00AM
Expires
Fri, April 28th, 2023 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 2500′. Avalanches failing at the interface of the new snow and old snow surface up to 2′ deep are likely for human triggering today. On northern aspects at upper elevations these new snow avalanches could be larger due to a buried weak layer of facets. Deeper buried weak layers 3-6′ deep are still a concern on northern aspects. Below 2500′ the avalanche danger is MODERATE. Lighter winds and moist snow at lower elevations will make wind slabs and storm slabs less likely but wet avalanches releasing in the new snow as the temperatures warm up in the afternoon are possible.

PORTAGE/PLACER: Be aware of avalanches occurring overhead during warm afternoons. This area can see large wet slides that can run close to commonly traveled areas.

*FRIDAY AVALANCHE OUTLOOK: Continued light snowfall and moderate winds are expected through Friday, which will keep the avalanche danger similar to today. The next forecast will be issued on Saturday April 29.

Special Announcements

End of Season Operations:  We will be issuing forecasts 4 days/week (Tuesday, Thursday, Saturday, Sunday) until April 30th. If conditions warrant, updates will be posted in early May.

Thu, April 27th, 2023
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
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.
Recent Avalanches

The 12″ of light and cold new snow that fell overnight Tuesday was gradually heating up with the sun and warming temperatures which caused widespread roller balls and small wet loose avalanches on steep terrain on E, S, and W aspects. We did not see any recent slab avalanches within the new snow but did see some areas where dry loose debris had run down slope far enough to make sizable debris piles.

Widespread roller balls on an SE aspect caused by the sun warming up the dry new snow on steep slopes. Photo 4.26.23

Dry loose avalanche debris at the base of elevator shaft of the N face of Sunburst, we saw a similar but larger debris pile at the base of Todd’s Run on the N side of Tincan as well. Photo 4.26.23

Avalanche Problem 1
  • 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

With about a foot of new snow on the surface after Tuesday night’s storm plus 3-5″ falling overnight Wednesday there is plenty of new snow to be create storm slabs or be blown around into fresh wind slabs. These avalanches within the new snow could be up to 2′ deep and you will be most likely to find them at upper elevations where colder temperatures will help keep the new snow dry and make it easier for the wind to redistribute. On northern aspects above roughly 2000′ there is a layer of facets underneath the new snow that could cause avalanches to propagate more widely and potentially be remote triggered from lower angle slopes below, above, or to the sides of steeper terrain. Keep an eye out for hollow feeling wind stiffened snow by stepping off the beaten path and feeling the snow surface around you. Small test slopes can be a good way to check for shooting cracks and see how wind loaded features are acting under the weight of a skier or snowmachine.

On southern aspects where the new snow from this week sits on top of supportable crusts, hand pits are an effective way to check how well the new snow is bonded onto the old surface. Yesterday we found the the new snow was sticking onto the old crust for the most part but that could change as temperatures increase today and melt water could weaken the interface with the new snow. On northern aspects it is worth taking the time to test out the snowpack using a snow pit because there is a weak layer of facets buried about 1-1.5′ deep that failed twice in our extended column test yesterday. High elevation northern aspects still have a cold and dry mid-winter snowpack and the potential for triggering a larger and more connected avalanche still exists due to the buried facet layer.

Cornice fall and glide avalanches are also a concern as we continue to experience warm temperatures. We recommend avoiding areas underneath these features because they can release spontaneously.

Wind texture on lots of features at and above treeline on Tincan yesterday, but the snow remained soft and did not show signs of shooting cracks on test rolls. Photo 4.26.23

Snowpack structure on N aspects above 2000′ with a weak layer of facets buried about a foot deep. Photo 4.26.23

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Aspect/Elevation
  • 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.
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

Lingering deeply buried weak layers still exist 3-6′ deep in our snowpack. At this point it is unlikely to trigger an avalanche on this weak layer, but the consequences could be severe. The last avalanche on this layer was on a NW aspect on Big League in Girdwood last Wednesday, which resulted in a skier being carried 1000′ down slope over multiple cliff bands. The snowpack on northern aspects above 2000′ is still cold and dry compared to a typical late April, which means seeking out bigger objectives is more risky than normal. The best way to truly manage this problem is by avoiding big, consequential terrain. Consider the consequences of getting taken for a ride before trying to get out into steeper slopes.

Weather
Thu, April 27th, 2023

Yesterday: Partly sunny skies in the morning that shifted towards overcast or obscured cloud cover in the afternoon. Temperatures were quite warm, in the low 30s at the road level in the morning rising to close to 40 F in the afternoon. Temperatures remained in the low 20s F at upper elevations throughout the day. Winds were light until the afternoon when averages picked up to roughly 15 mph with gusts to 30 mph along upper elevations ridgelines. About 12″ of new snow fell overnight Tuesday and into Wednesday morning, with another pulse of precipitation falling Wednesday night into Thursday that brought 3-5″ of additional snow to Turnagain pass.

Today: Lingering snow showers are expected today with up to another 1-2″ of snow throughout the day. Temperatures are expected to increase today, reaching up to 40 F at lower elevations and bringing the snow line up to roughly 1400′ in the afternoon. At upper elevations daily high temperatures will be closer to 32 F today, which is a significant increase over yesterday. Winds should decrease this morning with averages of 5-15 mph and gusts up to 25 mph during the day today.

Tomorrow: Friday looks very similar to Thursday, with lingering snow showers possibly adding another few inches of snowfall at upper elevations. Snow line is expected to be around 1100′ in the afternoon as temperatures reach daily highs in the upper 30s F at lower elevations and low 30s F at upper elevations. Wind speeds should shift slightly to the SE and increase to 10-20 mph with gusts to 30 mph.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 33 4 0.4 90
Summit Lake (1400′) 36 1 0.1 37
Alyeska Mid (1700′) 34 1 0.1 75
Bear Valley – Portage (132′) 38 0 0.55 –

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22 ENE 11 33
Seattle Ridge (2400′) 26 SE 8 18
Observations
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05/07/24 Turnagain Observation: Turnagain Pass Wet Slabs A S
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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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