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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, April 24th, 2018 - 7:00AM
Expires
Wed, April 25th, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger in the Alpine is  HIGH today due to snow, rain and strong winds impacting the area. Natural storm slab avalanches are very likely. Below 2500′ the danger is CONSIDERABLE,  wet snow avalanches are likely. In addition, watch for cornice falls along ridgelines.  

Portage Valley hikers:    Avalanche danger exists along  popular hiking areas such as those accessed from Byron Glacier trail. Over 3″ of rain has fallen in the past 48 hrs in Portage.  

WEDNESDAY AVALANCHE OUTLOOK:
No avalanche forecast will be issued tomorrow, Wednesday April 25th. Extra caution is advised. Triggering a slab avalanche in the Alpine and/or a wet loose avalanche at Treeline will remain likely.  

Special Announcements
  • CNFAIC has transitioned to springtime hours in preparation for the end of the season. We are forecasting today,  Tuesday and our final advisory will be Thursday, April 26th. We will publish our springtime tips on Saturday April 28th.  

  • The Chugach National Forest has closed Johnson Pass Trail, Resurrection Pass, and Summit Lake area to snowmachining for the season. Turnagain Pass, Carter Lake and Snug Harbor remain open. Keep an eye on the riding status at bottom of the page in the coming weeks as the snow continues to melt.
Tue, April 24th, 2018
Alpine
Above 2,500'
4 - High
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.
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

Rain, snow fall and East winds have intensified this morning as the next storm moves over the region. This storm is forecast to bring over a foot of snow to the upper elevations and close to an inch of rain at lower elevations. This is adding to the snow and rain that fell over the weekend. Rain/snow line is forecast to start around 1500′ and climb to 2200′ this afternoon. The strong winds will blow new snow around in the Alpine. Natural avalanche activity is expected today due to ‘rapid loading’.  Storm slab avalanches are very likely to be triggered by the weight of a skier or snowmachiner. 

Upper elevation North aspects: Shaded aspects in the Alpine had a thin layer of facets and/or surface hoar over a crust under last week’s new snow. This layer may still be lurking, which could result in slab avalanches 2+’ thick composed of couple of storm layers of snow. 

Water totals (inches of rain) from Sunday’s storm through 6 am this morning. This translates into 1-2 feet of snow at upper elevations with significantly more in and around Portage. 

Girdwood Valley:  1.5″
Portage Valley:  5″
Turnagain Pass:  1.1″
Summit Lake:  .2″

Snow falling on Sunday. Additional snow today will add to the slab depth. Photo: Ray Koleser

Avalanche Problem 2
  • Wet Loose
    Wet Loose
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
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

At the mid-elevations (below 2,500′) rain falling on a wet snowpack or rain falling on new snow will keep the chances for natural wet loose avalanches elevated on steep slopes. Make sure to avoid runout paths and gullies where even a small wet loose slide can send debris to valley floors. Triggering a wet avalanche will be likely if you find yourself on a steep slope with wet and unconsolidated snow. 

Recent wet loose avalanches along Seattle Ridge from the storm over the weekend. Expect larger and more widespread wet loose avalanches today.

Additional Concern
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Snow and wind continue to grow cornices along ridgelines. Cornice falls may trigger storm slab avalanches and/or wet loose avalanches today and looking ahead into the next few days of precipitation and warm temperatures. Give these a wide berth from above and limit exposure under them.

Weather
Tue, April 24th, 2018

Yesterday was mostly cloudy with light rain and snow showers. Temperatures were in the 30Fs to low 40Fs. Winds were easterly 10-20 mph with gusts into the 30s. Overnight temperatures were in the 30Fs and wind speeds increased with gusts into the 50s. Precipitation increased early this morning.  

Today snow and rain will be heavy at times with close to an inch of rain (a foot of snow above 2200′) forecast to fall. Winds will be easterly 20-30 mph with gusts into the 70s. Temperatures will be in the 30Fs and 40Fs. Overnight temperatures will drop slightly and the precipitation and winds will decrease.  

Tomorrow will be mostly cloudy with rain and snow showers. There is a possibility of some afternoon clearing. Temperatures will remain in the 30Fs-40Fs. East winds will be 5-15 mph. This unsettled weather pattern continues through the week with a chance of a break on Saturday.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  36  0  0.1  65
Summit Lake (1400′)  39  0  0.1  60
Alyeska Mid (1700′)  35  2  0.4  19

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  27  ENE  23 42  
Seattle Ridge (2400′)  31  ESE  20 52
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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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