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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, February 13th, 2023 - 7:00AM
Expires
Tue, February 14th, 2023 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 2500′. It is likely that a person will be able to trigger an avalanche 1-2′ deep on fresh wind slabs that formed with last night’s northwest outflow winds. It is also still possible to trigger a bigger avalanche on lingering weak layers buried in the upper 2-4′ of the snowpack. The danger is MODERATE below 2500′, where wind slabs will be more isolated, but the same weak layers still make human-triggered avalanches possible.

 

PORTAGE/PLACER: This zone picked up over an inch of precipitation in the past 24 hours, which could amount to over a foot of snow at higher elevations. Combined with the strong winds, this will make for dangerous avalanche conditions.

SUMMIT LAKE: These northwest winds tend to hit the mountains around Summit Lake harder than Turnagain Pass. This weather, combined with the thinner and weaker snowpack in the Summit zone will make large human triggered avalanches likely today. Extra caution is warranted around Summit Lake.

SEWARD/LOST LAKE/SNUG HARBOR: Last week’s storms favored these zones, and today’s winds are looking to howl through them. Expect to see very dangerous avalanche conditions, with large human-triggered avalanches very likely and natural activity likely. Travel in avalanche terrain is not recommended in these southern mountains.

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Mon, February 13th, 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

No new avalanches were reported yesterday. The last known avalanche was a large skier-triggered avalanche on a steep southwesterly rollover on Eddie’s on Saturday. The avalanche likely failed at the 2/5 storm interface, which would have had a crust on the bed surface at that elevation.

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

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

It’s looking like we’ve made it through the worst of this outflow event with minimal damage in Girdwood and Turnagain Pass. That said, with sustained wind speeds of 10-15 mph and gusts near 30 mph for the past 12 hours, we can expect to find reactive wind slabs at the upper elevations today. The mountains hit hardest by these winds tend to be close to the Turnagain Arm, Summit Lake, and especially Seward and Lost Lake with this round of weather. It is looking like visibility will be good enough to get up high today, so pay attention to increasing avalanche danger as you gain elevation. You can expect to find unstable snow near ridgelines, below convex rolls, and in steep cross-loaded gullies. The northwesterly winds will load terrain differently than our more common easterlies, so be on the lookout for dangerous wind slabs in unusual places.

Fresh wind slabs are easy to recognize as you travel if you are paying attention. Watch out for stiffer snow on top of softer snow, and take special note of cracks shooting out from your skis or snowmachine. It can be incredibly informative just taking a second to hop off your snowmachine or take a few steps off the skin track to look for those stiffer wind slabs. Remember, the wind has been loading a snowpack with weak layers near the surface, which makes larger avalanches possible. More on this in problem 2.

Cornices: Along with these strong winds come fresh and sensitive cornices. They may be forming in weird places with the northwesterly winds, so keep your eyes open while you are traveling. Be sure to give them plenty of space as you travel along ridgelines, and minimize time spent traveling below them.

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

There is still a lot of uncertainty with the interface below the 2/5-2/6 storm, now buried 1-2′ deep. At elevations below around 2000′, there is a thin crust at this interface that was likely a contributing factor in the most recent avalanche on Eddie’s. At upper elevations, we saw storm slab-type activity lasting just a little longer than usual following the storm, so we are still treating this as a layer of concern. The layer is a tricky one to assess, since it has now been affected by two different wind events so the depth can vary greatly. The best way to see how the interface is behaving is to dig down and look, but the easiest way to manage the problem is to step back your terrain use for now until that layer has healed.

In addition to the 2/5 interface, that deeper layer of buried surface hoar from January 10 is still lurking down there around 3-4′ deep. It is becoming really unlikely that a person will trigger an avalanche on this layer, but we are still keeping it on the radar for now. With fresh wind loading today and another round of snow and wind expected tomorrow, extra weight is adding stress to the snowpack. This will nudge any buried weak layer closer to its breaking point. One more reason to be a little more conservative with terrain today.

In the past 24 hours we’ve added another fresh layer of wind slabs on top of this sandwich. There is plenty to look out for in the upper snowpack right now. 02.09.2023

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

The weak snow near that Thanksgiving crust is still sitting there near the bottom of the snowpack. For most of the advisory area, a human-triggered avalanche on this layer is very unlikely. There may still be some thin spots where the weight of a person could trigger an avalanche, especially at the south end of our advisory area around Silvertip and down towards Summit.

Weather
Mon, February 13th, 2023

Yesterday:  Skies were mostly cloudy with winds around 5 mph for most of the day, picking up in the afternoon. Winds were out of the east in Portage and closer to the coast, and northerly to westerly for most of the forecast area. Temperatures reached the mid 20’s F in the middle of the day, and have been dropping since then. Currently temperatures range from single digits to mid teens F. The only weather stations that measured any precipitation were in Portage, which measured 1.32” precip in the past 24 hours.

Today: Winds have been out of the northwest at 10-15 mph since yesterday afternoon, and are expected to slowly back off through the day, switching to southeasterly by this evening. Temperatures will struggle to make it above the single digits F today, with lows dropping to single digits below 0 F tonight. Skies will be partly cloudy.

Tomorrow: The next storm system is moving in starting tonight. Unfortunately it’s looking like the biggest factor will be the southeasterly wind, with sustained speeds between 20-30 mph and gusts of 40-50 mph. It is looking like we will only see 2-4” snow during the day tomorrow and another 1-3” tomorrow night in Girdwood and Turnagain Pass, with roughly double those totals in Portage and Placer. Temperatures will bump back up to the mid 20’s F during the day, with overnight lows in the mid teens to 20 F.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 21 0 0 69
Summit Lake (1400′) 19 0 0 38
Alyeska Mid (1700′) 19 0 0 70
Bear Valley – Portage (132′) 27 1.32

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 11 NW 5 16
Seattle Ridge (2400′) 14 NW 7 16
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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.