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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, February 6th, 2020 - 7:00AM
Expires
Fri, February 7th, 2020 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains CONSIDERABLE above 1,000′. Triggering a large and dangerous slab avalanche 2-3′ thick remains likely and continued cautious route-finding and conservative decision making is essential. These slabs are sitting on weak faceted snow and could be triggered from below, the side or along the ridge. They can be triggered on wind loaded slopes as well as slopes in the trees without wind loading. We are in a period that our guard needs to remain up!

*Roof Avalanches:  Warming temperatures are causing roofs to begin to shed their snow. Pay attention to children, pets and where you park your car.

Special Announcements

‏Chugach State Park:  An avalanche caught and carried a skier in the Eagle River area on Tuesday. That report and others sent in from this region can be found HERE.

TONIGHT!  Avalanche Gear in Review with CNFAIC at Black Diamond.  Swing by Black Diamond in Anchorage for a discussion on avalanche rescue gear. Thursday, Feb 6th from 6-7:30pm. Free! CNFAIC forecaster Ryan Van Luit will be there to discuss advances, common mishaps and answer any questions you may have about your own rescue gear. BYOG – Bring Your Own Gear! This is a great opportunity to test your gear and ask questions.

Thu, February 6th, 2020
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
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

Over the past two days we know of 6 skier and/or snowmachine triggered slab avalanches from Girdwood, Turnagain Pass and Summit Lake. They have all been around 2′ thick. One triggered yesterday in the upper Girdwood Valley was just over 3′ thick. No one has been caught to date and several have been triggered remotely. All of these are failing on weak faceted snow 2-3′ below the surface. Many are being triggered in the mid-elevation band around 2,000′ and outside of wind effect.


Slab avalanche triggered Tuesday, Feb 4th. This slide was triggered at the crown and below the common skin track when a skier veered below the track to perform a stability test. Photo taken the day after on 2.5.20 by Jennifer Dematteis.

 

Slab avalanche triggered by second person on the slope. This is an example of slabs that are releasing in mid-elevation terrain and in the trees. Location: Pete’s North. 2.4.20. Photo: Leif Mjos

Avalanche Problem 1
  • 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

Triggering a dangerous slab avalanche that is large enough to bury, injure and kill a person remains likely. These slabs continue to release the minute folks venture off the beaten track and especially into areas that were not heavily trafficed in January. There was a bump in westerly wind last night, which could create some small wind slab issues in isolated areas today, but the main concern still lies deeper in the pack.

That said, we can’t let the clearing skis today sway our decisions in the backcountry. We are dealing with an unstable snowpack in the lower/mid elevations as well as the upper. Widespread collapsing (whumpfing) and several recent avalanches are clues we can’t ignore. The problem is, various weak layers of facets and buried surface hoar sit between 2-3′ below the snow surface. The slab on top is composed of either wind packed snow (wind loaded slopes) or settling and condensing snow due to the very warm temperatures. Either way, the slab on top of the weak layer(s) is showing it’s prime to avalanche and all slopes are suspect.

Some things to remember with this kind of avalanche problem:

  • Tracks on a slope do not make it safe. It could be the 2nd or 10th person that triggers the slab.
  • Remote triggering is possible. This means trigging a slide from below, the side or from on top.
  • Signs of instability may not be present before a slab releases.
  • Small slopes in the trees may avalanche.
  • Consider the consequences if the slope does slide. Is there a terrain trap you could be caught in?

This pit from Eric Roberts around 2,500′ on Eddies shows a failure just over 2 feet below the surface on buried surface hoar. The failure near the top of the snowpack is a shallow wind slab. The deeper weakness is clearly the more concerning in our situation now.

Weather
Thu, February 6th, 2020

Yesterday:  Mostly cloudy skies were over the region yesterday. Temperatures were warm, in the mid 20’s°F along ridgetops and near 30°F in the mid and lower elevations. Ridgetop winds were light from the west (5mph) before picked up slightly overnight to the 10-15mph range.

Today:  Mostly clear skies are forecast before another system pushes in tonight. Ridgetop winds should continue to blow from the west in the 10-15mph range before turning easterly tonight and increasing to the 15-25mph range. Temperatures are expected to stay in the mid-20’s°F with lower elevations bumping up to near 30°F during the day. Snowfall overnight could accumulate to 1-2″ by tomorrow morning.

Tomorrow:  Cloudy skies, cooler temperatures and light snowfall is expected before a large storm system heads into the region Friday night and into Saturday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29 0 0 56
Summit Lake (1400′) 27 0 0 20
Alyeska Mid (1700′) 29 1 0.05 56

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23 W 7 27
Seattle Ridge (2400′) 26 *N/A *N/A *N/A

*Rime is covering parts of the Seattle Ridge wind sensor.

Observations
Recent Observations for Turnagain Pass
Date Region Location Observer
10/27/24 Turnagain Observation: Tincan Michael Kerst
10/21/24 Turnagain Observation: Turnagain Pass Road Observation Trevor Clayton
10/19/24 Turnagain Observation: Tincan – Below Todds Run Andy Moderow
10/18/24 Turnagain Observation: Taylor Pass Eli Neuffer
10/15/24 Turnagain Observation: Tincan Common John Sykes Forecaster
10/14/24 Turnagain Avalanche: Tincan CNFAC Staff
05/13/24 Turnagain Observation: Eddie’s, Sunburst, Seattle, Cornbiscuit, Pete’s South H Thamm
05/13/24 Turnagain Observation: Turnagain Pass non-motorized side Amy Holman
05/12/24 Turnagain Observation: Warm up Bowl Tony Naciuk
05/07/24 Turnagain Observation: Turnagain Pass Wet Slabs A S
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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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