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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, December 10th, 2023 - 7:00AM
Expires
Mon, December 11th, 2023 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE today, rapidly rising to HIGH around sunset as an intense storm moves in. During the day, strong winds will make it easy to trigger wind slab avalanches 1-2′ deep, with increasing chances of natural avalanches through the day. It’s looking like things are going to get wild later, so pay attention to changing conditions and increasing danger as the weather intensifies this afternoon, and be prepared to head back down if the storm arrives quicker than expected.

Special Announcements

The National Weather Service has issued a Winter Storm Warning for the approaching storm, which is expected to impact most of Southcentral Alaska.

Sun, December 10th, 2023
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
Avalanche risk
Mon, December 11th, 2023
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
4 - High
Avalanche risk
Below Treeline
Below 1,000'
4 - High
Avalanche risk
Mon, December 11th, 2023
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
4 - High
Avalanche risk
Below Treeline
Below 1,000'
4 - High
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

A skier triggered a small but deeper avalanche failing on a weak layer of facets just above the Thanksgiving crust yesterday on Eddie’s. The skier triggered the avalanche remotely and nobody was caught in the slide. The slab was roughly 2 feet deep and 50 feet wide. This is the first human-triggered avalanche failing on that Thanksgiving layer that we are aware of.

Skiers also triggered shallow wind slab avalanches on Eddie’s and Tincan yesterday.

Photo at the crown of the avalanche that was remotely triggered from 50′ away by a skier on Eddie’s. Photo: Andy Moderow, 12.09.2023

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

An intense storm is approaching today, and avalanche danger will rapidly be on the rise as the weather intensifies. For most of the day, the biggest factor will be the strong easterly winds that will be picking up starting this morning. We should see speeds around 30-50 mph this afternoon, picking up to 50-60 mph overnight. We are also expecting some snow today- around 2-4″ this afternoon- but the bulk of the storm is expected to hit us overnight tonight. We will likely see 2-3 feet of snow in Girdwood and Turnagain Pass, 1-1.5′ in Summit, and 3-4′ at higher elevations in Portage, Placer, and Seward. Most of this is expected to fall between 3 pm tonight and 3 am tomorrow. Expect HIGH avalanche danger as the storm intensifies, and plan on getting out of the backcountry before things get wild.

Since most of the snow is falling after sunset this evening, our main concern for the day will be the touchy wind slabs forming as the winds ramp up. We started seeing some of this yesterday, with skiers triggering shallow wind slab avalanches on Eddie’s and Tincan, and we are expecting to see more today. Slabs will be getting thicker, making avalanches bigger through the day, and the chances of natural avalanches will increase as the winds continue to blow. Given the strength of the forecasted winds, we’ll be on the lookout for winds moving snow into sensitive slabs down into the low elevations as well as in the alpine. To make things even more interesting, the new slabs forming at the surface will also be adding stress to a weak layer of snow buried deeper in the snowpack, increasing the chances of triggering a large avalanche (See the additional concerns for more on this). We’re entering a stretch of dangerous avalanche conditions, so travel cautiously and be prepared to get out of the backcountry as the storm picks up later in the day.

This is looking like a proper storm, with 12-hour totals exceeding 3 feet for many areas within our forecast zone. Graphic courtesy of NWS Anchorage, 12.10.2023

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

This round of intense snowfall will be applying a heavy load to the snowpack in a very short time, adding stress to the weak layer of facets above the Thanksgiving crust that we have been tracking for the past few weeks. There is a lot of uncertainty with this weak layer, and it is hard to say if the new snow will be enough to push that layer to its breaking point. Yesterday a skier triggered a small but deep avalanche failing on this weak layer in Eddie’s, and we also saw a natural avalanche that likely failed on this weak layer on the back side of Seattle Ridge sometime within the past week. If we do start seeing more activity on this weak layer it has the potential to produce very large avalanches, propagating wide distances over multiple terrain features. This storm will be a good test to see how close the layer is to its breaking point.

The layer of concern is currently buried about 2′ deep on average, but could be 5′ deep or deeper after the approaching storm. 12.10.2023

 

Weather
Sun, December 10th, 2023

Yesterday: We saw snow showers for most of the day yesterday, with clouds breaking up a bit in the afternoon. Weather stations picked up 2-6” snow equaling around 0.3” SWE. Winds were strongest in the first part of the day, blowing 5-15 mph out of the east with gusts of 20-30 mph before switching easterly and backing off overnight. High temperatures were in the mid teens to mid 20’s F, and have since dropped to the single digits F with Summit Lake showing temperatures just under 0 F this morning.

Today: It’s looking exciting for the next 24 hours. Winds will pick up today ahead of the next storm, which is looking to impact us overnight tonight. Easterly winds are expected to get up to 30-50 mph this afternoon, and keep increasing to 50-60 mph overnight, with gusts of over 80 mph possible. We might pick up 2-4” snow this afternoon, but that will intensify tonight. By tomorrow morning we are expecting 2-3 feet of snow for Girdwood and Turnagain Pass, 1-1.5’ in Summit, and 3-4’ at higher elevations in Portage, Placer, and Seward. This should start out as snow to sea level, and will hopefully stay that way, although we could see rain levels creep up to around 1000’ tomorrow morning. Temperatures will climb to the high teens to low 20’s F during the day, and continue to rise to the low to mid 30’s F overnight.

Tomorrow: The most intense weather should pass by sunrise tomorrow. Lingering showers could bring an additional 1-3” snow during the day under mostly cloudy skies, with winds backing down somewhat but maintaining at 15-30 mph out of the south. Temperatures will be in the mid to upper 20’s F during the day, with lows in the low to mid 20’s F.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 14 2 0.3 52
Summit Lake (1400′) 10 1 0.1 25
Alyeska Mid (1700′) 16 5 0.36 39
Bear Valley – Portage (132′) 17 6 0.65
Grouse Ck – Seward (700′) 17 2 0.3 28

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 8 E-W 7 32
Seattle Ridge (2400′) 11 SE-NW 4 15
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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.