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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, November 28th, 2023 - 7:00AM
Expires
Wed, November 29th, 2023 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 1000′ today. Strong winds will make it easy for a person to trigger a wind slab avalanche around a foot deep, with the odds of natural avalanches increasing as the wind picks up through the day. Be especially cautious on convex rolls, near ridgelines, and in cross-loaded gullies. Avalanche danger will be on the rise as the snow and wind ramps up this evening.

The danger is MODERATE below 1000′, where you may encounter some smaller wind slabs, but recent rain has left very little soft snow on the surface so these avalanches should stay small.

 

SNUG HARBOR/LOST LAKE/SEWARD: The approaching storm will impact these southern areas first, with heavy snowfall and strong winds expected to pick up this afternoon.

CHUGACH STATE PARK: The National Weather Service has issued a Special Weather Statement for the strong winds that are expected to impact the Front Range this afternoon. Expect avalanche danger to rise as the winds pick up today.

Special Announcements

New weekend outlook products:  Starting this Friday, Dec. 1, we will begin issuing Weekend Avalanche Outlooks for Chugach State Park, the Summit Lake/Central Kenai zone, and the Seward/Southern Kenai zone. These Outlook products will be published Friday evenings and will provide avalanche information for these three new areas for us.

Tue, November 28th, 2023
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
Wed, November 29th, 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
Wed, November 29th, 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

We triggered multiple avalanches above treeline on Tincan Ridge yesterday. These were all relatively small, failing on slopes that were actively being loaded by the strong winds. See more details in this observation.

One of the small wind slabs we triggered yesterday, on a convex roll just above treeline. 11.27.2023

 

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

A storm system is headed our way, and is expected to bring strong winds and heavy snowfall our forecast area today and tomorrow. It is looking like the wind will be the key player for the daylight hours today, and although winds are light this morning they should ramp up quickly. With sustained speeds of 20-30 mph with gusts of 30-50 mph by later this morning, natural and human-triggered avalanches will be likely. These will probably only be 6-12″ deep at treeline elevations because of the limited soft snow on the ground, but could get much deeper in the alpine. The wind has shifted from the southwest to the southeast, so you might encounter unstable conditions on multiple aspects today.

Wind slabs tend form on convex rolls, below ridgelines, and in cross-loaded gullies. Be skeptical around any of these terrain features today, and be extra cautious if you start to notice cracks shooting out from your ski tips or snowmachine- this is a clear sign that conditions are dangerous. Sometimes you can recognize them from a distance because the surface may have a different texture, but other times they may be tricky to identify until you are standing on a stiffer snow surface.

For the very highest elevations in our advisory area (around 3500-4000′ or so), there is still a small chance that an avalanche triggered near the surface may step down to weak snow buried at the ground. This is becoming less and less likely with time, but we still have very limited info from that elevation band so it is a good thing to keep in mind if you are trying to get up high in the alpine.

Winds were howling yesterday, and are expected to return today. Photo from Tincan Ridge, 11.27.2023

Weather
Tue, November 28th, 2023

Yesterday: Steady precipitation in the morning tapered off mid-day, with mixed precipitation at sea level and 2-4” snow at higher elevations. Winds were strong out of the southwest for most of the day, blowing 10-20 mph with gusts of 25-25 mph. Clouds broke up a little in the afternoon but skies remained mostly cloudy.

Today: A storm system is going to begin to impact our area today, with strong southeasterly winds ramping up this morning. Expect to see sustained speeds of 20-30 mph and gusts of 30-50 mph, with winds continuing to pick up tonight. Most areas will see only a trace of precipitation today, with rain levels rising to around 1000 feet as precipitation picks up tonight. Coastal areas like Seward and Portage could see around 0.5” precipitation today, equaling 1-4” snow. Skies will be mostly cloudy with high temperatures in the upper 20’s to low 30’s F and slightly warmer temperatures closer to the coast.

Tomorrow: We will see the bulk of this storm during the day tomorrow, with totals of around 10-12” in Girdwood and Summit Lake, and 18-24” possible in Turnagain Pass and Seward. Easterly winds are looking to remain strong throughout the storm, with sustained speeds of 25-35 mph and gusts of 40-55 mph. The rain line should hover between 300 and 800 feet, with temperatures in the mid 20’s to low 30’s F.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29 2 0.2 29
Summit Lake (1400′) 25 2 0.2 18
Alyeska Mid (1700′) 30 3 0.3 22
Bear Valley – Portage (132′) 31 0 0.3

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 21 WSW 12 41
Seattle Ridge (2400′) 25 S 7 20
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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.