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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, December 18th, 2023 - 7:00AM
Expires
Tue, December 19th, 2023 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

Avalanche danger is MODERATE today. There is a lingering chance a person can trigger an avalanche 1-2′ deep failing within the new snow from the past few days, and a smaller chance of triggering a very big avalanche 3-5′ deep on weak snow buried deeper in the snowpack. These layers will be hard to assess, so the safest bet is to be a bit more cautious with terrain choices than you would normally be two days after a storm.

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Mon, December 18th, 2023
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
Tue, December 19th, 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
Tue, December 19th, 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

Yesterday while folks were out and about we saw evidence of a widespread natural cycle from Saturday night’s storm from Girdwood to Seward. Most of this activity failed within the new storm snow, but there were some avalanches that appeared to fail on deeper weak layers in the Girdwood area.

We saw a lot of storm slabs like this one on the steeper terrain features in the Notch area yesterday. 12.17.2023

Skiers in Seward noticed similar activity. Photo of a natural avalanche on Mt. Eva, shared anonymously. 12.17..2023

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

With another day of quiet weather on the way before things pick up again tomorrow, the main avalanche concerns come from layers that are buried in the snowpack. There are two very different kinds of avalanches that should be on your radar today. The first is the lingering chance of triggering an avalanche failing within low-density storm layers from the snow within the past week. While it is unusual for a storm or wind slab problem to remain reactive for much longer than a day or two after a storm, we saw multiple storm events that left very low-density, cold dry snow buried under layers of warmer, wetter snow that will still be a concern today. These storm layers take a little longer to settle out, and can behave like persistent weak layers for several days following a storm. Although we’re not expecting any major weather today, these lingering instabilities will be something to pay attention to while you’re out. They can be assessed using quick travel tests like hand pits and test slopes, as well as formal stability tests. Warning signs like shooting cracks or collapsing are a clear indicator of unstable conditions, but the absence of these signs should not automatically rule out the problem.

The second thing to be aware of is the potential for triggering a much bigger avalanche failing on weak snow buried near the Thanksgiving crust, now 3-5′ deep in most places. We’ve seen problematic structure in almost every part of our advisory area as well as the surrounding zones. We continue to get mixed test results in our snowpits, but we’ve started to see some avalanches failing on this layer over the past two weeks (see our observation from Girdwood yesterday for an example). There is a lot of uncertainty with this layer. It’s a stubborn weak layer that hasn’t yet produced a widespread cycle, but it is the kind of problem that can lie dormant for weeks before it wakes up to make a big avalanche.

Neither of these problems are straightforward, and both will leave a lot of uncertainty even with a thorough snowpack assessment. It’s the kind of setup where most people probably won’t trigger an avalanche today, but if you were the unlucky one to find the slope that is primed to avalanche, you could find yourself in a really bad situation. Keep that combination of high uncertainty and high consequences in mind while you travel today, and consider staying a bit more conservative with terrain choices than usual given the challenging setup.

Lingering slab avalanches failing within the new snow from the past few days are becoming less likely, but still worth keeping on your radar today. Photo from Notch Mtn., 12.17.2023

This photo is probably old news to those of you following the advisory regularly. The Thanksgiving crust is getting buried deeper, and the problem is not going away. 12.17.2023

Weather
Mon, December 18th, 2023

Yesterday: Skies were partly to mostly cloudy with decreasing cloud cover through the day. Winds were out of the west at 10-15 mph with gusts of 25-35 mph for most of the day before switching to the southeast and slowing to 5-10 mph overnight. High temperatures were in the 20’s F with overnight lows in the teens to low 20’s F. Summit is the coldest this morning with temperatures in the single digits F. No precipitation was recorded.

Today: We should see another day of quiet weather today, with spotty snow showers bringing an inch or two of snow to some areas through the day. Skies should be partly cloudy with light winds out of variable directions. High temperatures will be in the high teens to low 20’s F, with overnight lows in the mid teens F tonight.

Tomorrow: Active weather returns tomorrow, with the chances for snow picking up during the afternoon and into tomorrow night. We could see 2-5” snow during the day tomorrow, with snow lines staying down around 100’. Seward will likely see closer to a foot of snow during the day. Winds will pick up with precipitation, blowing 15-25 mph out of the east with gusts of 20-30 mph. Snowfall will continue into tomorrow night, with another 6-8” possible by Wednesday morning.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 23 0 0 72
Summit Lake (1400′) 17 0 0 37
Alyeska Mid (1700′) 25 0 0 66
Bear Valley – Portage (132′) 24 0 0
Grouse Ck – Seward (700′) 25 0 0 36

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17 W-E 8 30
Seattle Ridge (2400′) 20 SW-SE 5 12
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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.