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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, April 25th, 2019 - 7:00AM
Expires
Fri, April 26th, 2019 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 2500′ due to active wind loading and new snow. Human triggered wind slabs 1-2′ may be tender on leeward slopes 35 degrees and steeper. Triggering a slab 2-3′ in older snow on Northerly aspects above 2500′ is possible and could propagate across an entire slope. If the sun makes an appearance natural wet-loose avalanches on South aspects are possible in the afternoon. Careful snowpack evaluation and conservative terrain choices are essential. As always, give cornices a wide berth and limit travel under glide cracks.

Between 1000′ and 2500′ the avalanche danger is MODERATE where triggering dry loose snow is likely on steep features and wet loose avalanches will be possible in the afternoon if the sun come out.

PORTAGE VALLEY: Cornice fall and/or avalanches from above have the potential to send debris to valley bottoms. Traveling along summer hiking trails, such as the Byron Glacier Trail with steep slopes overhead is not recommended on rainy/snowy days or on sunny afternoons. Portage Valley received over 5′ of snow at upper elevations and cornices are looming large.

WHITTIER: Between 4-5 feet of new snow has fallen at the upper elevations in the Whittier Glacier region. Heads up as there is limited snowpack information for this area. Large human triggered slab avalanches are possible and extra caution is advised.

LOST LAKE / SEWARD: There is also limited snowpack information for this region as well. Ease into steeper terrain and look for signs of instability.

FRIDAY AVALANCHE OUTLOOK:
No avalanche forecast will be issued tomorrow. Clear and sunny skies are in the forecast tomorrow and into the weekend. Natural and human triggered wet avalanches are possible in the afternoon with warming from the sun. Evaluate surface conditions as you travel and be ready to avoid all South aspects later in the day. This includes traveling under the runout of steep South facing terrain.

Special Announcements

Advisories: Today is CNFAIC’s last avalanche advisory for the season. We will post our Springtime tips on Saturday morning. Keep in mind winter conditions still exist in the mountains. If you see any avalanche activity please send us an observation and we’ll continue to update social media with any pertinent observations in the coming weeks. A big thanks to all of our observers and the local avalanche professionals that share information with the CNFAIC.

Thu, April 25th, 2019
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
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.
Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • 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.
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

Winter is not over in the mountains surrounding Southcentral Alaska! Extra caution is advised at Turnagain Pass. A storm that favored Turnagain Pass overnight brought  9” of new snow. Ridgetop weather stations have reported winds in 15-45mph range with gusts in the 50-60’s mph. In areas that did not see new snow yesterday, 1-2’ of dry low-density snow was available for transport. Expect wind slabs to be 1-2’ in size on leeward terrain features. Shooting cracks and blowing snow today will be an obvious clue that wind slabs are tender.

DRY LOOSE: In areas protected from wind dry-loose surface snow may be easy to initiate and fast moving in steep terrain. Also keep in mind there may be a deeper more dangerous slab lurking under loose dry snow on Northern aspects in alpine.

WET LOOSE: Cold temperatures have caused light dry snow to fall in the mid and upper elevations over the last few days. When the sun appears this is the perfect type of snow for wet loose avalanches. There is potential for late afternoon sun. Monitor surface conditions as you travel and avoid South facing aspects if the surface become wet and sticky.

CORNICES: There are lots of large cornices on ridge tops. These features can heat up with daily warming making them easier to trigger. Give them lots of space.

FRIDAY: Tomorrow looks like the first clear sunny day following the storm. If winds are calm tomorrow and air temperatures increase into the 40Fs, natural wet loose avalanches are possible on South facings slopes.

Recent storm slab on CFR on Tincan yesterday seen just before winds started to increase. 

Dry loose sluffs seen on Lipps North facing terrain yesterday prior to winds increasing in the afternoon.

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

Over the last week 3’ of snow (3.0”SWE) has fallen incrementally in the Alpine of Turnagain Pass and Girdwood. On shaded (NE-N-NW) aspects above 2500’, weak snow may be lurking 2-3’ below the surface. Surface hoar was observed on shaded aspects prior to the last week of stormy weather and facets and buried surface hoar may still be lurking another foot below that layer. The reactivity of this weak snow is unknown, but keep in mind that a 2-3’ thick could have high consequences. Whumpfing and shooting cracks may not be present, and a person or machine may well onto a slope before triggering. Today with active wind loading expected to continue avoiding steep Northerly aspects in the alpine is a conservative choice. Looking into the weekend as snowpack starts to adjust, evaluate terrain for consequences and practice safe travel protocols – expose one person a time, watch partners and have an escape route planned.

* New snow totals vary across our region. Summit Lake has seen around 15” of new snow over the last week. The mountains surrounding Portage Valley and Whittier have seen up to 5’of new snow.

Additional Concern
  • Glide Avalanches
    Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
More info at Avalanche.org

Several new glide avalanche were observed in Turnagain Pass yesterday – one on a SE aspect of Seattle Ridge and the other on a SW aspect of Eddies. Glide cracks are continuing to creep open and still exist in many popular areas of Turnagain Pass. As always identify cracks and avoid traveling under their runout. They are unpredictable and can avalanche at any time.

Weather
Thu, April 25th, 2019

Yesterday: Skies were broken in the morning becoming obscure by early afternoon. Ridgetop winds were 10-45mph from the East and strongest overnight. Sunburst weather station recorded the strongest winds 30-40mph with gusts in the 50-60s mph. Turnagain Pass received 9″ of new snow in the last 24 hours. Temperatures were in the low to mid 20Fs in the Alpine. Near sea level temps were 30-40F. Rain/snow line was around between ~400′.

Today: Skies will be overcast with a possibility of broken skies in the afternoon. Scattered snow showers should diminish by late morning. Easterly ridgetop winds 15-25mph will start to decrease in the afternoon to 5-15mph and shift to a NW direction. Temperatures at sea level will be in the low-30Fs to mid-40Fs. Temperatures in the upper elevations will be in the 20Fs with a high near 30F. Overnight temperatures are expected to drop below freezing at all elevations.

Tomorrow: Clear and Sunny skies are in the forecast for Friday and through the weekend. No precipitation is expected. Northwest ridgetop winds are expected to be light, 5-10mph. Daytime  temperatures at sea level could reach the upper 40Fs to low 50Fs. Daytime temps in the alpine may reach the mid to upper-30Fs.   Freezing temperatures are expected overnight.

*Be aware that Seattle Ridge weather station may be under-reporting average wind speeds and gusts.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 30 9 0.7 81
Summit Lake (1400′) 33 0 0 19
Alyeska Mid (1700′) 30 trace  0.02 67

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 18   ENE   25   64  
Seattle Ridge (2400′) 26   ESE   *8   *21  
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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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