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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, February 3rd, 2023 - 7:00AM
Expires
Sat, February 4th, 2023 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 1000′. It still may be possible for a person to trigger a large avalanche on a layer of surface hoar buried 2-3′ deep. We know this layer to be widespread throughout the forecast area and as such it is worth testing and identifying any features of concern before committing to big terrain.  Shallow areas that are wind scoured or near rock outcroppings are more like spots to find and potentially trigger this weak layer with your skis or snowmachine.

Additionally, some steep alpine slopes are harboring enough new or weak surface snow where loose snow avalanches/ sluffs could be a concern if folks push into steeper terrain today.  Below 1,000’ the danger is LOW.

SUMMIT LAKE: The snowpack in the Summit Lake area has a similar structure to Turnagain Pass, but it is thinner and weaker. This makes human-triggered avalanches a little more likely, which means we should be a little more cautious around steep terrain.

Special Announcements

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Fri, February 3rd, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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.
Recent Avalanches

The last human-triggered avalanches were 13 days ago now, when multiple people were caught, carried, and some partially buried in avalanches failing on the layer of buried surface hoar we are still concerned with. You can find details from some of these avalanches here and here.

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

Today looks to be another insignificant day of weather for contributing to any heightened avalanche activity.  We are however, still dealing with a lurking layer of buried surface hoar 2-3’ under the surface that was responsible for a very active human-triggered avalanche cycle two weekends ago.  Our benign weather this week has further allowed this layer to gain strength and it is proving more stubborn to trigger but we can’t forget about it just yet.

Buried surface hoar is a notoriously tricky weak layer that tends to persist (ie: persistent slab) and has potential to cause problems weeks after it is buried.  Hence why a cautious mindset is still warranted even two weeks after it was last active in our area.  It’s still worth taking the time to test this layer on your snowmachine or skis before committing to any bigger, more consequential terrain.  A quick snowpit or sled cut on a small no-consequence slope is a great way to feel out this weak layer 2-3’ down.  If you get any propagating results from a snowpit or other red flags (cracking or collapsing in the snowpack), that should be evidence enough to keep your terrain mellow.  You can still avoid this problem entirely by sticking to slopes less than 30 degrees.

Loose snow/ sluffs: There have been reports of just enough loose snow at the surface (4-8″ of new snow this week combined with surface faceting) to justify mentioning the possibility of loose snow avalanches/ sluffs.  Keep your sluff management in mind if you find yourself pushing onto steep, high alpine slopes today.


This avalanche ran on the layer of buried surface hoar that is widespread throughout or region, 2-3′ below the surface.  Though becoming less likely to trigger,  it’s important to keep consequences in mind.  If the slope does slide, am I above a terrain trap or is there a clean runout?  Photo: Goat Mountain (Girdwood Valley), 2.2.2023

 

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Additional Concern
  • Deep Persistent Slabs
    Deep Persistent Slabs
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.
More info at Avalanche.org

We’ve catalogued the Thanksgiving crust/ facet layer as an ‘additional concern’.  It still exists toward the bottom of our snowpack (4-8’ deep) but is becoming very unlikely that a person could trigger an avalanche that deep.  With a major loading event or significant warm up this may pop back up as a more prominent concern in the forecast but for now its very unlikely to see an avalanche breaking this deep in our snowpack.

Weather
Fri, February 3rd, 2023

Yesterday: Another day of eerily calm winds at ridgetop locations where gusts at Sunburst (3800’) never broke single digits. A bit of sunshine over Turnagain Pass in the morning gave way to partly cloudy skies by the afternoon.  Temperatures were mild and fluctuated very little throughout the day/ night.  Low 20s F at ridgetops and right around 30 F at lower elevations.

Today: Low clouds/ fog may persist in Girdwood and eastern Turnagain Arm this morning as there isn’t much wind expected to mix the atmosphere.  Temperatures have dropped a bit from yesterday and are expected to be in the high teens F at ridgetops and mid to high 20s near sea level.  We may see periods of light snow with the passing shortwaves moving through but accumulation is expected to be in the 1-2″ range through the forecast period.

Tomorrow: Another shortwave trough moves north tomorrow and we may see another trace to an inch of snow, mostly cloudy skies and light winds from the East.  Tomorrow night there appears to be a lull before a larger, more potent winter storm moves out of the Gulf of Alaska and into our area.  Details are still being refined but Sunday/ Monday looks to be our next good chance for a reset.  Stay tuned!

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25 0 0 62
Summit Lake (1400′) 23 0 0 34
Alyeska Mid (1700′) 25 2 .15 63
Bear Valley – Portage (132′) 28 trace .01

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20 var 3 13
Seattle Ridge (2400′) 23 var 0 2
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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.