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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, March 17th, 2024 - 7:00AM
Expires
Mon, March 18th, 2024 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The danger is CONSIDERABLE above 1000′. The most likely avalanches you will encounter today will be up to a foot deep on wind-loaded slopes. These will be easy to trigger and may occur naturally as winds pick up through the day. The scarier avalanche problem is lurking 2-3 feet deep or deeper, where last week’s storm snow is sitting on a weak interface that may still produce larger avalanches today. The danger is MODERATE below 1000′, where wind slab avalanches are unlikely but those deeper avalanches are still possible. With continued rain on snow at lower elevations, wet snow avalanches will also be a concern up to around 1500′.

ROOF AVALANCHES: Be on the lookout for roof avalanches as light rain continues today. Keep an eye on children and pets, be mindful of where you park your vehicles, and keep your eyes up as you enter and exit buildings.

PORTAGE/PLACER: These areas are once again expected to receive much more precipitation than the rest of the advisory area. Unfortunately this will be falling as rain up to around 2000′, but could amount to close to a foot of snow by the end of the day at upper elevations. Be aware of increasing potential for natural and human-triggered avalanches at all elevations as the precipitation continues today.

Special Announcements

Turnagain Pass Avalanche Awareness Day – Saturday, March 23!
Swing by the Turnagain motorized parking lot between noon and 4pm to grab a hotdog, practice your beacons skills, chat with the forecast team, and possibly test out a demo snowmachine provided by local dealers.

Sun, March 17th, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Mon, March 18th, 2024
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
Mon, March 18th, 2024
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.
Recent Avalanches

Yesterday was the first day since Tuesday that we did not receive any reports of human-triggered avalanches. Since the snow started earlier this week we have seen many skier and snowmachine-triggered avalanches, including multiple avalanches that were triggered remotely. These have been 2-3’ deep on average and range from 50-200’ wide.

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

Today’s weather is looking to be very similar to yesterday’s, and the avalanche concerns are very similar. There are two main concerns. Wind slabs (Problem 1 for today) will be the most likely avalanches to encounter. Avalanches failing on deeper weak layers (Problem 2) are a little less likely, but arguably scarier.

Winds have backed off slightly overnight, but are expected to quickly ramp up during the day  with average speeds of 15 to 35 mph and gusts of 25 to 45 mph. This will be accompanied by a few inches of new snow, with plenty of soft snow already on the ground ready to build fresh wind slabs. Watch for unstable snow on steep wind loaded terrain – places like steep slopes below ridgelines, convex rolls, and steep gullies. This type of avalanche problem should give warning signs like shooting cracks, and is likely to provide feedback in quick travel tests like hand pits and test slopes. Be suspect of any slopes with stiff snow sitting on top of weaker snow, and look for the best surface conditions and the most stable snow on slopes that are sheltered from the winds. Keep in mind, the wind slabs are not the only problem and there are deeper weak layers below the surface that may produce much larger avalanches. Read on to Problem 2 below for more on this.

Wet Snow Avalanches are a concern at lower elevations that are expected to see continued rain on snow today and tonight. Keep an eye out for loose wet avalanches in steep terrain overhead, and be on the lookout for unusual avalanches in areas where the upper snowpack is getting saturated.

Stormy weather moving in to the Girdwood Valley yesterday afternoon. Photo: John Pearce, 03.16.2024

Avalanche Problem 2
  • 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

Our other big concern for today is the weak interface between last week’s impressive storm and the older snow under it. Prior to last week’s storm, we observed a well-connected layer of surface hoar and near-surface facets from the road to ridgetops on both sides of the highway at Turnagain Pass. For solar aspects on the southern half of the compass, this was forming on top of a melt/freeze crust that formed during several days of clear weather. As we piled up several feet of snow on top of that weak interface, we saw four days in a row with human-triggered avalanches. This includes several avalanches that were triggered remotely from lower-angle terrain connected to steeper slopes. We did not receive any reports of avalanches yesterday, but based on what we saw in snow pits on Seattle Ridge (details here and here), it still looks like there is a good chance we could trigger an avalanche on that weak layer today.

These persistent problems are tricky. Unlike the wind slabs mentioned in Problem 1, these deeper problems do not give easy feedback. However, based on the widespread activity during the storm, the flurry of human-triggered avalanches following the storm, and the concerning pits yesterday, this layer is showing us plenty of reasons to be concerned. The best way to manage it is to avoid big steep terrain for now, and give the snow some time to heal.

This photo was taken just before it started snowing last week. Those tiny surface hoar grains are a big concern now. 03.09.2024

Weather
Sun, March 17th, 2024

Yesterday: Light precipitation came in with easterly winds averaging 10 to 25 mph gusting 35 to 40 mph. Stations are showing 0.1 to 0.3” snow water equivalent (SWE), with rain up to around 1000 to 1500’. Skies were overcast with high temperatures in the upper 20s to 40 F and lows in the low to mid 20s F.

Today: The weather today is looking similar to yesterday, with warm temps, light precip, and strong easterly winds. Most of the area will see a trace to an inch of snow with rain up to 1700-1900’. Portage and Placer are expected to see significantly more, with up to 8-12” new snow at upper elevations by tonight. Easterly winds have calmed as of this morning, but are expected to pick up again through the day with average speeds of 15 to 35 mph and gusts of 25 to 45 mph. High temperatures will be in the low 30s F with lows in the mid 20s to 30 F. Skies are expected to be mostly cloudy.

Tomorrow: This stormy weather is expected to start to break up during the day tomorrow, giving way to spring-like weather for the first part of the week. It is looking like we should see another 2 to 4” new snow tonight before the storm passes, with the rain line dropping to around 300’ as the storm finishes. High temperatures should be in the mid 20s to low 30s F, with lows in the low to mid 20s F. Winds will be light out of the east, and skies are looking to be partly to mostly sunny by the end of the day.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28 0 0.1 106
Summit Lake (1400′) 31 0 0 47
Alyeska Mid (1700′) 29 2 0.22 108
Bear Valley – Portage (132′) 33 0 0.55
Grouse Ck (700′) 31 0 0. 72

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23 ENE 14 42
Seattle Ridge (2400′) 26 SE 12 24

 

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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.