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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, April 22nd, 2018 - 7:00AM
Expires
Mon, April 23rd, 2018 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

A storm bringing rain, snow and wind is expected to increase the avalanche danger to CONSIDERABLE through the day. Slab avalanches composed of up to a foot of new snow are likely to be triggered and may release naturally at the high elevations above 2,500′. Wet snow avalanches are likely to be triggered on steep slopes below 2,500′ where rain is falling on an already saturated and wet snowpack. Cornice falls are also likely along ridgelines.

Portage Valley hikers:   Avalanche danger exists along  popular hiking areas such as those accessed from Byron Glacier trail.

MONDAY AVALANCHE OUTLOOK:
No avalanche forecast will be issued tomorrow, Monday April 23rd. Pay close attention to warming on Monday and the possibility for triggering a slab avalanche in the Alpine and/or a wet loose avalanche at Treeline.  

Special Announcements
  • CNFAIC is transitioning to springtime hours in preparation for the end of the season.  We are issuing forecasts 4 days/wk on Sat, Sun, Tue, Thur. The final forecast will be on April 26th with our springtime tips report on 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.
Sun, April 22nd, 2018
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.
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

Another spring storm is impacting Western Prince William Sound. Heavy precipitation has been seen in the Portage Pass zone and along coastal areas, with moderate amounts of precipitation in Girdwood and Turnagain. The storm is expected to intensify today and bring another 6-12″ of snow to the Alpine and .5-1″ of rain to Treeline and below. The rain/snow line is around 1,500′ this morning and should climb to 2,500′ by this afternoon. 

Estimated Storm Totals from yesterday through 6am today (Sunday):

Girdwood Valley:  ~6-10+” snow above 2,500′
Portage Valley:     ~16″ – 24″ snow above 2,500′  (twice as much precip seen in areas closest to the Sound compared to Turnagain/Girdwood)
Turnagain Pass:   ~6-10″ snow above 2,500′
Summit Lake:        0″ to a trace 

Some natural avalanche activity is expected today if the storm verifies due simply to ‘rapid loading’. In the Alpine there could be up to 2′ of snow in favored areas by this afternoon. In this case, storm slab and wind slab avalanches are likely to be triggered. Although bonding between the new and older snow surface should be relatively good, there is always a period of time needed for the new snow to stabilize. This could happen in several hours or 2 days. Quick hand pits are good ways to assess instability in the storm snow. Once again the avalanche danger will be directly related to the amount of new snow fallen. In areas seeing only 6″(ish) of new snow, slab avalanches are not expected.

Upper elevation North aspects: Shaded aspects in the Alpine had a thin layer of facets over a crust under last week’s new snow. This layer may still be lurking, which could result in slab avalanches 1-2+’ thick composed of last week’s new snow along with this weekend’s storm. 

A brief period of sun yesterday at Turnagain Pass. New moist snow falling on a warm springtime surface should bond fairly quickly after the initial stabilization period. However, once it warms up – avalanche activity will be on the rise again.

– 

 

 

 

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 will keep the chances for natural wet loose avalanches possible 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. 

Yesterday’s pre-storm snow surface and snowpack conditions at the mid-elevations. Rain at this elevation today will likely keep the snowpack punchy and wet, which means ripe for wet avalanches if the slopes are steep enough (greater than 35 degrees). 

–

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 several days of precipitation and warm temperatures. Give these a wide berth from above and limit exposure under them.

Weather
Sun, April 22nd, 2018

Mostly cloudy skies were over the region yesterday as another low-pressure system moved in. Light snow began to fall in the afternoon above 1,000′ and light rain below, the exception being heavy rain in the Portage area. Overnight, this first wave of the storm has added 5-12″ of moist snow above ~1,500′ and .5-1″ of rain below with Portage Valley seeing up to 2″ of rain at sea level. Ridgetop winds have been Easterly in the 20-30mph range. Temperatures along ridgelines have stayed in the 20’sF, while at 1,000′ sit in the mid 30’sF after warming to near 40F yesterday.

Today, Sunday, rain and snow will continue as this system looks to peak today. Another .5-1″ of rain is expected to fall below 2,500′ with 5-12″ of snow above this. Portage, and coastal zones, should continue to see twice as much precipitation as the Turnagain/Girdwood areas. Ridgetop winds will stay strong, in the 20-35mph range from the East. Temperatures also will remain in the 20’sF along ridgelines and in the mid 30’s at 1,000′.

For Monday, a break in the storm is expected before another round moves in Tuesday. Cloudy skies, light precipitation with a rain/snow line around 2,000-2,500′ and moderate Easterly winds should be seen Monday.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 35   6   0.5   69  
Summit Lake (1400′) 35   trace rain trace   20  
Alyeska Mid (1700′) 34   ~4 0.5   61  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24   NE   20   50  
Seattle Ridge (2400′) 29   ESE   18   46  
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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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