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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, April 13th, 2021 - 7:00AM
Expires
Wed, April 14th, 2021 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE at all elevations today. With fluctuating sky cover, increasing temperatures and wind speeds, natural avalanches are still possible today. Above 1000′ triggering a storm slab avalanche is likely in steep terrain. In addition, triggering an avalanche that steps down to a buried weak layer remains a concern. Below 1000′ triggering a wet loose avalanche is likely and slides from above may run into this elevation band. Give cornices a wide berth and avoid travel under glide cracks. Pay attention to changing conditions and look for signs of instability. Cautious route-finding and conservative decision-making are essential.

PORTAGE VALLEY: There is potential for avalanche debris, from a slide occurring above, to run to low elevation terrain. Avoiding summer trails that run through avalanche runout zones, such as Byron Glacier trail, is recommended through this coming weekend.

Roof Avalanches:  Rain and warmer temperatures could cause roofs to shed their snow. Pay special attention to children, pets and where you park your car.

Special Announcements

From Alaska Department of Transportation and Public Facilities: There will be intermittent traffic delays for avalanche hazard reduction today, April 13, 2021 on the Seward Highway from mile post 88 to 83 south of Girdwood from 11:00 am to 3:00 pm. There will also be delays on the Portage Glacier Highway today, April 13, 2021 for avalanche hazard reduction near Mile Post 5 and Bear Valley from 12:00 pm to 4:00 pm. Motorists should expect delays of 45 minutes. Updates will be posted on the 511 system. http://511.alaska.gov/

CNFAIC End Of Season Operations:  Daily avalanche forecasts will continue through this weekend due to the weather forecast and elevated avalanche danger. We plan on switching to 4 days/week (Tues, Thur, Sat, Sun) beginning the Tuesday the 20th as our forecast season and funding winds down but that is still TBD. Stay tuned!

Tue, April 13th, 2021
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. 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
  • Avalanche hazard reduction along the Seward Hwy north of Girdwood produced avalanches, as did avalanche hazard reduction at Alyeska Resort.
  • There were a handful of natural storm slab avalanches observed in Portage Valley.
  • There was a skier triggered storm slab avalanche on Notch Mountain in Girdwood Valley that was 2-4′ deep and ran on the old wind crust. No one was caught or carried.
  • There were some small natural wet loose avalanches in Turnagain Pass when the sun poked out.

Skier triggered storm slab on Notch Mountain, 4.12.21.

 

Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • 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.
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

After the Sunday afternoon into Monday storm brought 12-20+ inches of snow to the forecast area, we have a pretty interesting and variable springtime avalanche situation (as the late DMX used to say) ‘Up in here. Up in here!’ As mentioned above in Recent Avalanches there were avalanches observed yesterday. There is the potential to trigger one or for a natural avalanche to occur today, despite the lull in the weather before the next storm comes in this evening. If you are thinking of heading out today there are a number of things to keep in mind.

  1. We still have a winter snowpack with cold snow and buried weak layers with some spring-like qualities such as buried sun crusts, warmer temperatures and longer days. This transitional time can bring dangerous avalanche conditions and make terrain selection more tricky.
  2. The mountains around Girdwood and Portage received higher amounts of precipitation. Expect storm/wind slabs to be deeper in upper elevation terrain in these areas.
  3. The storm started colder and ended warmer so it was ‘upside down’ with more storm slab potential.
  4. It rained to around 500′ and afternoon temperatures were in the high 30°F/low 40°Fs from sea level to close to 2000′. It was cloudy overnight and forecast to be warm again today. The warm temperatures could help the new snow bond to old snow surfaces at upper elevations and/or make the new snow behave even more like a slab if it doesn’t bond well (sorry it’s a little confusing). In steep low elevation terrain, wet loose avalanches could still be easily triggered on slopes with wet snow, especially if it is over an old crust.
  5. We are in the time of year where southerly vs. northerly aspects have different snowpacks. Prior to this storm most northerly slopes had a wind crust and southerly slopes had a sun crust (maybe with a bit of wind affected snow on top). The new snow may or may not bond to these different surfaces. In addition, northerly slopes are more suspect for harboring buried weak layers (Problem 2).

Pay attention to changing conditions. Are the winds increasing? Is it getting sunny for a bit? Is it really warm? Are you seeing signs of instability like cracking, collapsing or recent avalanches? Today is a day for choosing terrain very carefully.

Wind Slabs:  With the strong easterly winds that occurred during the storm expect more slab development in the higher terrain. Watching for cracks that shoot out from your skis or machine and stiffer snow over softer snow.

Cornices: Moist snow and strong winds are the perfect recipe for cornice formation and growth. Expect these to still be sensitive today. Give cornices a wide berth. Triggering a cornice fall could trigger an avalanche on the slope below.

New snow over a sun crust on a south aspect in a snow pit on Tincan yesterday, 4.12.21.

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • 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.
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

New snow, wind loading and warmer temperatures are all factors that can contribute to activating the buried weak layers in the top 3 feet of the snowpack that we have been talking about for long time. Triggering an avalanche in the new snow could step down to one of these layers and create a larger avalanche. The Crow Pass and Raggedtop area near Girdwood is a suspect region and has seen this occur over the past few weeks. Additionally, this is especially likely in Summit Lake south of Turnagain Pass, where there is an overall thinner snowpack with more advanced weak layers. Summit Lake did not see much snowfall during this storm, but the area did get strong winds and warming temperatures yesterday.

A layer of buried surface hoar and small facets was propagating in an Extended Column Test in a snow pit on Tincan yesterday, 4.12.21. This illustrates the potential for an avalanche to step down to a buried weak layer.

 

Weather
Tue, April 13th, 2021

Yesterday: Mostly cloudy skies with some pockets of sunshine. Snow/rain showers on and off throughout the day with precipitation favoring Girdwood and Portage. High temperatures in the mid 20°Fs in the alpine and 30°Fs and 40°Fs in the mid to low elevations. Winds were easterly, 20-30 mph with gusts into the 40s and 50s, decreasing in the afternoon. Overnight skies were cloudy and temperatures were in the mid 20°Fs to mid 30°Fs. Winds were mostly light and easterly.

Today: Mostly cloudy skies with a light precipitation during the day increasing tonight. Rain/snowline around 800′ today dropping back down to 300′ tonight. Snow could be heavy at times overnight. Temperatures today in the mid 20°Fs to low 40°Fs depending on elevation with southeast winds 5-15 mph with gusts into the 20s. East winds this evening, 15-25 mph with gusts into the 30s increasing overnight to 30-40 mph with gusts into the 70s, peaking early Wednesday morning.

Tomorrow: Snow and rain likely. Highs in the 30°Fs and 40°Fs (maybe the 50°Fs) with snow levels forecast to rise to as high as 1800′. Easterly winds 25-45 mph with gust into the 60s, decreasing as the day progresses. Precipitation eases up Wednesday night. The warm temperatures look like they stick around into the weekend with some sunshine in the forecast as well.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 32 0 0.3 110
Summit Lake (1400′) 33 0 0.1 46
Alyeska Mid (1700′) 32 3.5 0.58 129

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22 NE 16 59
Seattle Ridge (2400′) 26 E* 14* 27*

*Seattle Ridge wind sensor likely rimed over around 5 pm yesterday. All wind data is from prior to that time.

Observations
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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.
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