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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, March 8th, 2019 - 7:00AM
Expires
Sat, March 9th, 2019 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

The avalanche danger has risen to CONSIDERABLE for Turnagain Pass, Girdwood, Placer Valley and the Seward zone due to strong winds and a mix of rain and snow. Above 1000′ triggering storm slabs up to 1′ thick is likely on slopes steeper than 35 degrees today.   Below 1000′ loose-wet avalanches are possible. Careful snowpack evaluation, conservative decision-making, and cautious route-finding are essential.

PORTAGE VALLEY: Natural avalanches are possible today in steep channeled terrain and could entrain wet snow in the lower elevations. Avoid places with significant avalanche terrain above like Byron Glacier Trail. In addition Portage Lake could be pretty dangerous this weekend due to thin ice, above freezing temperatures and rain.

JOHNSON PASS/SUMMIT LAKE ZONE: There is a shallow snowpack with a generally poor structure. Strong winds and new snow will be adding stress to the snowpack. In addition to storm slabs more uncertainty exists for triggering a deeper avalanche in an older layer of the snowpack.

Special Announcements
  • If you’re headed to Alyeska on Saturday, bring your avalanche beacon. Backcountry Access (BCA) and Alyeska Ski Resort will have a beacon training park set up all day on March 9, March 23, and April 6. Check in at any Ski Patrol station or visit the Snow Report for the location of the search park.
  • Heading to Hatcher Pass? Be sure to check out the Hatcher Pass Avalanche Center mid-week snow and avalanche summary  HERE.
Fri, March 8th, 2019
Alpine
Above 2,500'
3 - Considerable
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

After two weeks of high pressure the jet stream has shifted and the first of two big storms is here. Overnight 1-3“ of snow fell, favoring Girdwood so far, and an additional 6-10” is expect today across the region. Strong Easterly ridgetop winds 30-40mph will continue all day. Sunburst has already seen gusts in the 50s mph. Temperatures have also risen and rain/snow line should reach 1000’ by mid-day. As snow and winds continue storm slabs and wind slabs will be growing in size. Monitor slab depths and avoid slope angles greater than 35 degrees. Leeward aspects could have deeper more connected slabs due to strong winds transporting snow. Also don’t forget about the weak surface conditions this new snow is falling on. Surface hoar and near surface facets are sitting on hard surfaces (sun crusts and wind-packed snow.) Basically this set-up does not bode well for future bonding. As we get more snow and wind this weekend and slabs grow thicker the avalanche hazard will continue to increase.

Wet Snow – Below 1000’ where rain is falling on dry snow on Northern aspects, triggering a loose-wet avalanche will be possible. In Portage Valley where precipitation amounts will be greater, natural avalanches are possible in steep channeled terrain and may entrain wet snow into the lower elevations. Avoid traveling on trails or terrain directly under steep avalanche paths.

South of Turnagain in the Summit Lake and Silvertip zones: Similar surface conditions to Turnagain Pass exist and any new snow that falls today is not expected to bond well. In addition a variety of old weak layers (facets and buried surface hoar) sit in the mid and base of the snowpack and wind loading and new snow will be adding more stress to these layers. Although less snow is expected to fall in this zone today keep in mind uncertainty exists for triggering a deeper more dangerous avalanche.

Surface hoar was present on all elevations and aspects prior to todays storm and was capped by a trace of new snow a few days ago. 


Surface hoar sits on a sun crust on Southerly aspects (photo on left) and near surface facets exist on shaded aspects (photo on right.) In addition facets have been found under sun crusts and under thin wind slabs. We have plenty of weak surface snow issues to be concerned about. 

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

These types of avalanches are highly unpredictable and not associated with human triggers. A number of cracks are located in popular terrain like Magnum’s SW face, Corn Biscuit, Lipps, and Repeat Offender on Seattle Ridge. The last glide avalanches to release naturally were a week ago on Seattle Ridge and in Girdwood Valley on Penguin, Goat, and Raggedtop. The best way to manage this problem is to watch for and limit exposure time under glide cracks.

On the left a glide avalanche released on Seattle Ridge on March 1st. On the right a glide crack continues to open on Repeat Offender. Photos courtesy of Allen Dahl. 

Weather
Fri, March 8th, 2019

Yesterday: Skies were overcast. Easterly ridge top winds increased from light to moderate (15-25 mph) by the afternoon. Winds increased overnight with several gusts in the 50s mph this morning. Temperatures at ridge tops were in the low to mid-20s F and temperatures at sea level were in the mid-30s. Light flurries were observed late afternoon. Precipitation increased overnight and Alyeska midway station has seen 3 €œ new snow (.39″ SWE)   and 1″ of new snow was recorded at Center Ridge. Temperatures at sea level this morning are 37F and rain is falling to ~500′.

Today: Stormy weather will continue today and overnight. An additional 6-10 €œ of new snow (~0.5 € SWE) is expected above 1000′ and will fall as rain at lower elevations. Portage will see higher precip totals due to its coastal location. Winds will be strong 30-40mph from the East with gusts in the 50s mph. Temperatures will increase throughout the day with a high of 40F at sea level. Rain/snow line could reach 1000′. Overnight another 4-8″ of snow (.43″ SWE) is possible.    

Tomorrow: A second storm is tracking towards the Gulf of Alaska from the South and will arrive Saturday. This storm looks to have stronger winds, warmer temperatures and higher precip totals. Another 1-2′ of snow is expected in the upper elevations and rain below 1500′. Rain, snow and strong winds are expected to taper off Sunday afternoon.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 31   1   .1   57  
Summit Lake (1400′) 31   0   0   27
Alyeska Mid (1700′) 31 3   .39   53  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20   ENE   19   53  
Seattle Ridge (2400′) 26   SE   14   32  
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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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