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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, March 2nd, 2021 - 7:00AM
Expires
Wed, March 3rd, 2021 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is MODERATE today, and it is still possible a person could trigger an avalanche on layers of near-surface facets and surface hoar buried 1-3′ deep. These weak layers are present at all elevations throughout most of our advisory area. As they become more stubborn to trigger, they are less likely to present clear warning signs prior to avalanching. We need to be patient dealing with these persistent weak layers, avoiding dangerous terrain where an avalanche could have serious consequences. In addition to these persistent weak layers, there may still be isolated wind slabs that formed in the past few days and are still reactive today.

Seward/Lost Lake/Snug Harbor/Portage Valley: These areas are expected to see more wind today than the core advisory area. Be aware of increased likelihood of triggering wind slab avalanches in these areas.

Tue, March 2nd, 2021
Alpine
Above 2,500'
2 - Moderate
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.
Avalanche Problem 1
  • 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 fairly quiet weather on tap for today, our main concern is triggering an avalanche 1-3’ deep on persistent weak layers that were buried in late January and early February. For anyone who has been following the advisory, these layers are old news by now. But that doesn’t mean we can ignore them. We have seen avalanches fail on these weak layers as recently as Sunday– clear evidence that they are still a problem. With incremental loading during the month of February, we have slowly buried these layers deeper and deeper. They are slowly gaining strength, and although we do not expect to see a widespread cycle without a major loading event, it is still possible to trigger avalanches big enough to bury a person.

These persistent weak layers are a real test of patience, with potentially severe consequences if we don’t treat them with caution. The only way to manage a problem like this is to realize that most steep slopes have the potential to avalanche, and to treat them accordingly. If you are trying to access steep terrain, avoid slopes with terrain traps like cliffs, trees, rocks, and gullies. Now is not the time to push it into big terrain. Be smart with safe travel protocol, only exposing one person at a time to steep slopes, and watching your partners from safe spots out of the avalanche path. If you are interested in digging in a little deeper, this post from the Utah Avalanche Center shares good insight regarding safe travel protocol, and how being better with group management can reduce fatal accident rates.

Wind slabs: Light winds today and yesterday are not likely to form large or widespread wind slabs, but it will still be attention to pay attention to signs of recent wind loading– especially in the most suspect terrain below ridgetops, convexities, and in cross-loaded gullies. It may still be possible to find small reactive wind slabs on isolated features.

Sluffs: With 2-6” new snow in the past 24 hours, we can expect to see dry loose avalanches (sluffs) in most steep terrain. It is unlikely these will be large enough to bury a person, but they can have serious consequences if they carry you over terrain traps.

Small wind slab releasing on a test slope in the Tincan trees yesterday. 03.01.2021

Two persistent weak layers in the upper snowpack on Tincan. 03.01.2021

Weather
Tue, March 2nd, 2021

Yesterday: Light snow showers through the day brought 2-4” snow under cloudy skies. High temperatures were in the mid to upper 20s F, with lows in the single digit to mid teens F, dipping just below 0 F at Summit Lake. Winds were light out of the east at 5-10 mph for most of the day, and shifted to the west yesterday evening.

Today: We should see quiet weather today, with partly cloudy skies and light westerly breezes around 5 mph. High temperatures are expected in the mid teens to upper 20s F, with lows in the single digits to mid teens F. We might see a few snowflakes during the day, but no accumulation is expected.

Tomorrow: Expect another day of quiet weather tomorrow, with mostly sunny skies and calm to light northwesterly winds around 5 mph. High temperatures should be in the mid teens to mid 20s F, with no precipitation expected. Our chances for snow are looking better towards the end of the week.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 16 2 0.3 114
Summit Lake (1400′) 14 3 0.2 47
Alyeska Mid (1700′) 19 3 0.4 119

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 11 E-W* 8 30
Seattle Ridge (2400′) 12 SE-N* 5 21

*Wind direction changed around 4 p.m. yesterday

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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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