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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, March 20th, 2023 - 7:00AM
Expires
Tue, March 21st, 2023 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE at all elevations today. Conditions remain dangerous after heavy snowfall brought 3-6′ new snow to our advisory area over the weekend. Despite the break in the weather today, it remains likely that a person could trigger a very large avalanche with all of this new snow. It will be important to approach the mountains with a cautious mindset.

SUMMIT LAKE: This area has a thinner and weaker snowpack than our core advisory area. The new snow is adding stress to multiple weak layers in the snowpack, making larger avalanches likely.

SEWARD / LOST LAKE: It’s looking like the next round of stormy weather will impact this southern zone before it hits the Turnagain Pass area. Be aware of increasing danger as the weather picks up this afternoon around Seward.

Special Announcements

There will be intermittent traffic delays Monday March 20, 2023, on the Seward Highway and Portage Glacier Highway for Avalanche Hazard Reduction work. From mileposts 88 to 85 on the Seward Highway, South of Girdwood. Near mile Post 5 and Bear Valley on the Portage Glacier Highway. Motorists should expect delays of up to 45 minutes between 09:00 AM and 2:00 PM. Updates will be posted on the 511 system.  http://511.alaska.gov/

Transmission inspection using snowmachines:  Chugach Electric Association will be inspecting the transmission lines along the non-motorized side of the Turnagain Pass (between Tincan and Johnson Pass) using snowmachines on one day between Thursday 3/16 and Tuesday 3/21.

Mon, March 20th, 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
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. Dangerous avalanche conditions. 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

The clouds broke up enough in the afternoon for us to get a snapshot of some of the avalanche activity over the weekend. It is clear we had a significant natural cycle, with debris from multiple large natural avalanches along the front side of Seattle Ridge and on the non-motorized side of the highway.

Skier-triggered avalanche on a small roll in the Tincan Trees yesterday. This wouldn’t have been big enough to bury a person, but the size of the avalanche was mostly limited by the size of the terrain feature. 03.19.2023

Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

Stormy weather hung around a little longer than expected yesterday, bringing another 3-5″ snow with strong winds in the afternoon. We are looking at a break in the weather today before the next round of wind and snow starts to pick up this afternoon, but avalanche conditions remain dangerous at all elevations across our advisory area. We received 3-6′ snow over the weekend, and all of that new snow makes it likely that a person can trigger a very large avalanche. This storm snow fell on a variety of surfaces, including crusts on solar aspects and faceted snow on northerly aspects. Yesterday we saw encouraging signs that the snow is starting to bond well to those old surfaces in our snowpits (details here), but we also were still finding reactive storm slabs on small, steep terrain features. With strong winds picking up yesterday afternoon into last night, we are also concerned about the potential of triggering big avalanches where fresh wind slabs have formed.

This is one of the biggest storm events of the season so far, and it still needs time to heal. Traveling in the backcountry today requires a cautious mindset, avoiding traveling on steep slopes. It is looking like another system is moving in starting this afternoon through tonight, so conditions are expected to remain dangerous at least for the next few days.

Yesterday at Turnagain Pass we were seeing on average 3′ of settled snow from the recent storms. There is likely double that in some parts of the advisory area. This is a major load that will need a little time to heal. Photo from the Tincan Trees, 03.19.2023

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

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

We’ve been tracking a layer of buried facets in the southern end of our advisory area for over a month now. This is by far the biggest load that layer has experienced since it was buried, and it’s grabbed our attention. With such poor visibility since the storm started, uncertainty is high with how reactive this layer is. Until we get a better picture of how this layer has responded to this recent loading event, it is important to approach that southern terrain with extra caution.

You can click here to view the video below if it doesn’t load in your browser.

Weather
Mon, March 20th, 2023

Yesterday: The stormy weather hung around until early yesterday afternoon, bringing another 3-5” snow equaling 0.3-0.5” snow water equivalent (SWE) before skies started breaking up. Winds were out of the east at 10-25 mph with gusts of 25-50 mph. Winds were generally strongest yesterday afternoon through midnight, with the exception of the Max’s weather station, which has been blowing 20-25 mph since around 9 pm. High temperatures were in the upper teens F at high elevations, reaching the mid 30’ F at lower elevations. Lows were in the mid teens to mid 20’s F. Skies were overcast in the morning, with partly to mostly sunny skies in the afternoon.

Today: We should experience a break between periods of stormy weather today, with partly to mostly cloudy skies this morning. Things will start to pick up again later in the day, with clouds starting to build and precipitation starting again this afternoon. It is looking like most of the advisory area will only see a trace of new snow during the day, with heavier precipitation tonight into tomorrow. Easterly winds will start to increase ahead of the next round of snow, with average speeds of 15-30 mph and gusts of 20-40 mph. It is looking like the snow line should stay down to sea level for this next round of snow.

Tomorrow: Storm intensity is expected to pick up tonight through tomorrow morning before calming down tomorrow afternoon. With this next round, we are expecting to see 4-6” new snow in Girdwood and Turnagain Pass, with 12-18” possible in Portage and Placer. Winds will be steadily increasing tonight through tomorrow morning, blowing out of the east at 25-30 mph with gusts of 30-40 mph. Those easterly winds should back off mid day tomorrow, with average speeds of 5-10 mph likely by tomorrow afternoon. High temperatures will be in the mid 20’s to low 30’s F, with lows in the low to mid 20’s F tomorrow night. Snow line should stay down to sea level through tomorrow.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29 3 0.3 99
Summit Lake (1400′) 25 0 0 51
Alyeska Mid (1700′) 30 5 0.3 87
Bear Valley – Portage (132′) 34 1 0.5

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16 ENE 17 42
Seattle Ridge (2400′) 22 ESE 8 66
Observations
Recent Observations for Turnagain Pass
Date Region Location
11/16/23 Turnagain Observation: Tincan Common
11/14/23 Turnagain Observation: Sunburst/Magnum
11/14/23 Turnagain Observation: Seattle Ridge
11/13/23 Turnagain Avalanche: Tincan Trees
11/12/23 Turnagain Observation: Sunburst
11/12/23 Turnagain Avalanche: Goldpan – avalanche
11/11/23 Turnagain Observation: Tincan Common
11/11/23 Turnagain Observation: Taylor Pass – Sunburst
11/10/23 Turnagain Observation: Tincan
11/10/23 Turnagain Observation: Tincan Trees
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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.