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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, February 20th, 2020 - 7:00AM
Expires
Fri, February 21st, 2020 - 7:00AM
Forecaster
Aleph Johnston-Bloom
High Avalanche Danger
Travel in avalanche terrain is not recommended. Avoid being on or beneath all steep slopes.
Avalanche risk The Bottom Line

The avalanche danger remains HIGH today due to another day of heavy snowfall and strong winds in the mountains surrounding Turnagain Pass, Girdwood Valley, Portage Valley, and areas on the Kenai including Summit Lake and the Seward/Lost Lake zone. Dangerous avalanche conditions are expected on all slopes 30 degrees and steeper – including runout zones. Natural avalanches are likely and human triggered avalanches are very likely.  Travel in avalanche terrain is NOT recommended. Areas with steep slopes above should be avoided, such as the Byron Glacier Trail and the Seattle Ridge uptrack. Even small terrain features could act as deadly traps.

REGION-WIDE: Dangerous avalanche conditions extend north from our forecast area including Chugach State Park to Hatcher Pass.  Extra caution is advised.

*Roof Avalanches:  New snow/rain loading with warming temperatures could cause roofs to continue to shed their snow. Pay special attention to children, pets and where you park your car.

Special Announcements

UPDATED at 12 pm today.

From Alaska DOT & PF: There will be intermittent traffic delays Today February 20, 2020 on the Seward Highway for avalanche hazard reduction work from Mile post 82 to 89.South of Girdwood.  Motorists should expect delays of up to 45 minutes between 1:00 PM-3:00 PM. Updates will be posted on the 511 system. https://511.alaska.gov/

Thu, February 20th, 2020
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. 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

Avalanche hazard reduction work conducted by AK DOT and the Alaska Railroad produced a handful of avalanches yesterday including this very large avalanche in the Main Kern avalanche path south of Girdwood and one in the Door 4 avalanche path in Portage.

Main Kern avalanche path. The crown of this avalanche was estimated as 6-10′ deep. 2.19.20. Photo courtesy of Alaska Railroad.

The crown of an avalanche in the Door 4 avalanche path in Portage. This crown is estimated to be 8′ deep. 2.19.20. Photo courtesy of Alaska Railroad.

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

The next storm system has moved over the area after a quick pause during the day yesterday. The mountains have received another 5-15″ of snow overnight with Girdwood being favored again. Another 10-20″ of snow (close to another inch of water weight) is forecast to fall today with rain below 500′.  Winds also ramped up again overnight averaging in 30-50 mph with gusts into the 70s and 80s. Today due to the rapid loading occurring, travel in avalanche terrain is still not recommended, this includes runout areas as natural avalanches are likely during the storm.

Snow and water totals at mid elevation weather stations since February 17th:

  • Alyeska Mid Station (1700′): 46″ of snow, 3.96″ SWE
  • Turnagain Pass, Center Ridge Snotel (1880′): 26″ of snow, 2.4″ SWE
  • Summit Lake Snotel (1400′): 10″ of snow, 1.0″ SWE

Upper elevations have received even more snow and wind loading is most intense in the Alpine. Avalanches today could be large storm slabs that are deeper in wind effected areas. Cornices will continue to grow, could be very tender and natural cornice fall could trigger natural avalanches on the slopes below. Rain on snow a lower elevations could cause wet loose avalanches. To top it all off avalanches occurring in the upper snowpack could step down to weak snow buried deeper down (see Avalanche problem #2). Remember natural avalanches are likely and human triggered avalanches are very likely. All the avalanches have the potential to be very large and deadly. Please be patient. The mountains will need time to adjust to all the loading.

Rapid loading! The DOT snow stake at Turnagain Pass this morning. 2.20.20

 

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
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

Weak faceted snow that formed in January now sits anywhere from 3-6+ feet below the surface. Some of the avalanches that were triggered with avalanche hazard reduction work yesterday likely failed on this old weak snow. Crown were 6-10′ deep. More snow and wind loading today and/or an avalanche in the upper snowpack could initiate an avalanche that breaks down into the buried weak layers and result in very large, destructive avalanches that could run far into the terrain below. Not to sound like a broken record but this is a big reason travel in avalanche terrain is still not recommended today, including being in the runout of steep slopes.

 

 

 

 

Weather
Thu, February 20th, 2020

Yesterday: Skies were mostly cloudy with light snow showers. Temperatures were in the high teens at upper elevations and mid 20°Fs to low 30°Fs at low elevations. Winds were calm until late afternoon when they began to increase. Overnight easterly winds were 30-50 mph gusting as high as 80 mph. Snow picked up overnight with 5-15″ of accumulation and temperatures were in the 20°s and low 30°Fs.

Today: Snow is likely and could be heavy at times with rain falling at low elevations.  Rain/snowline is forecast to be around 500 ft. Temperatures will be in the mid 20°Fs to high 30°s. Precipitation will continue overnight and temperatures drop a few degrees. Easterly winds are still strong this morning averaging 30-50 mph, gusting into the 70s and 80s. Winds are forecast to decrease mid day and remain easterly 5-15 mph with gusts into the 20s, becoming calm overnight.

Tomorrow: Cloudy skies and light snow/rain showers, temperatures in the 20°Fs and 30°Fs. Calm winds. Similar weather overnight into Saturday with a clearing trend for the weekend.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28 8 0.8 80
Summit Lake (1400′) 27 2 0.2 30
Alyeska Mid (1700′) 28 13 0.7 88

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 18 NE 20 81
Seattle Ridge (2400′) 23 NA* NA* NA*

*Seattle Ridge wind sensor not reporting.

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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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