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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, March 9th, 2018 - 7:00AM
Expires
Sat, March 10th, 2018 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

The avalanche danger is  HIGH  at all elevations where natural and human triggered avalanche 2-4+ feet deep are likely today. An avalanche from above could run farther and faster than expected and has the potential to reach valley bottoms. Travel in avalanche terrain is not recommended including being under the runout of larger slopes above. This will be extra important for the popular areas of Placer Valley, Turnagain Pass and Portage Lake.  

Special Announcements

**Dangerous avalanche conditions are region wide across the Kenai Peninsula due to heavy snowfall and strong winds. This includes Seward to Summit Lake. Expect avalanche danger to remain elevated through the weekend. A winter storm warning issued by the National Weather Service will remain in effect through 5pm today for the Eastern Kenai Mountains and the Seward Highway.  

Fri, March 9th, 2018
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
4 - High
Avalanche risk
Below Treeline
Below 1,000'
4 - High
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. Dangerous avalanche conditions. 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

Two feet of new snow and strong winds (up to 96mph) over the last 24 hours have created dangerous avalanche conditions for our entire region (Girdwood, Turnagain Pass, Portage, Placer Valley and Summit Lake.) Strong Easterly winds 20-40mph will continue through late afternoon and 5-10” of additional snow is expected today. Natural avalanches 2-4+ feet deep are likely and human triggered avalanches are very likely. Precipitation has remained snow to sea level, and cold snow means avalanches could run faster and farther than expected. Avoiding avalanche terrain is necessary today, which includes staying far away from runout zones of larger slopes above. Later today as winds decrease and snow stops falling be careful not to get tempted into steeper terrain. This new snow has fallen on hard surfaces and weak snow below and bonding will be poor. An avalanche from above could run into valley bottoms, especially in places like Seattle Ridge, Portage Lake and Placer Valley.   

Storm totals as of 6am:

  • Turnagain Pass: 20-26 inches (1.0” SWE)
  • Girdwood: 18-22 inches (1.3” SWE)
  • Portage: 20-24 inches (1.1” SWE)
  • Summit Lake: 10-14 inches ( 0.5” SWE)

Types of storm snow avalanches that are expected today:

–  Wind slabs:  Slabs up to 2-4 feet thick due to strong winds loading leeward aspects and terrain features
–  Storm slabs:  Soft slabs (2 feet deep) are expected in areas out of the wind due to rapid loading and poor bonding with the old surface
–  Cornice falls:  Cornice fall could trigger a wind slab or storm slab below

AKDOT snowstake at Turnagain Pass is showing around 2 feet of new snow in the last 24 hours. 

Yesterday at the beginning of the storm observers experienced strong winds even in the mid elevations. Photo taken at 12pm (3/8/18) at Sunburst around 1800′. 

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

A storm related avalanche could ’step down’ into a deeper weak layer causing a very large avalanche to send debris well into valley bottoms. Another reason to let the mountains sit as the storm moves out of our area. Areas on the South end of Turnagain Pass and the Summit Lake area are most suspect for having deeper weak layers release.

Several widespread persistent weak layers exist within our snowpack region wide including buried surface hoar and facets 1-2′ below the old surface. Don’t forget we haven’t had a storm like this in months. Lots of uncertainty exists around how our old weak snowpack will adjust to its new load. 

 

 

Weather
Fri, March 9th, 2018

Over the last 24 hours 18-26 inches of snow has fallen across our forecast zone. Yesterday Easterly winds increased into the 60’s mph in the afternoon with gusts into the 90’s mph. Overnight Easterly winds decreased into 20-40mph range. Temperatures at 3000′ have been in the low 20F’s and temperatures at sea level have been around 33F.

Today another 5-10 inches of snow (.5 € SWE) is expected through mid afternoon and scattered snow showers this evening. Southeast winds will continue to be strong (20-45mph) and decrease to 15-25mph by late-afternoon. Temperatures at 3000′ will be in the mid 20F’s and temperatures at sea level may increase into the mid 30F’s, but precipitation is expected to remain as snow at sea level.  

Snow showers are in the forecast through the weekend, but precipitation amounts remain uncertain. For Saturday winds are expected to be light from the East and temperatures in the upper elevations will be in the low to mid 20F’s and temps at sea level will be in the upper 20F’s to low 30F’s.  

*Temperatures at Center Ridge weather station have been reporting warmer temps compared with other weather stations at this same elevation band. There have been some issues with this temperatures sensor this winter.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) *31   26   1.0   93  
Summit Lake (1400′) 28   14   0.5   44  
Alyeska Mid (1700′) 27   18   1.36   77  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 18   ENE   37   96  
Seattle Ridge (2400′) 23   E   26   62  
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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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