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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, March 21st, 2016 - 7:00AM
Expires
Tue, March 22nd, 2016 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

A CONSIDERABLE avalanche danger exists on all aspects at elevations above 1,000′. Human triggered avalanches are likely and natural avalanches are possible. A variety of avalanche problems will be seen today: storm slabs 8-14″ thick, wind slabs 1-2′ thick, loose snow avalanches and glide avalanches. If you are headed into the Turnagain Pass or surrounding areas very careful snowpack assessment and cautious route-finding will be necessary. If the sun comes out, it will act as a trigger for the types of avalanches listed above.

This is a day to have patience and let slopes steeper than 30 degrees have time to adjust. Safer places to recreate will be on mellow terrain, with nothing steep above you, or in the flats.

**Areas on the periphery, such as Portage Valley, that received more snow, a HIGH  avalanche danger could be seen.  If you are headed to the Summit Lake area don’t forget to check  Summit Lake Summary.

Special Announcements

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Mon, March 21st, 2016
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
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
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.
More info at Avalanche.org

Is it already the first day of spring? After rounding out the final week of winter with a stable snowpack, we head into spring with quite the opposite. As of this morning, a two and a half day storm is just finishing up. We have seen anywhere between 12 and 24″ of snow at the high elevations and this snow is unstable (more on that below). Several human triggered slabs and sluffs were triggered yesterday – see those reports HERE and HERE. One notable comment is the debris has been running much further than expected; likely due to a crust under the new snow.
 

STORM SLAB (upside-down storm):
We have a classic ‘storm slab’ avalanche problem in the backcountry. This basically means denser snow fell on top of lighter snow which creates a slab/weak layer combo. This set-up was quite sensitive to human triggers yesterday and is expected to be the same today. This combo also sits on a curst in many places which can allow debris to run longer than expected. Slab thicknesses depend on how much snow fell, roughly 8-14″. 

WIND SLAB: 
Although the Easterly ridgetop winds have died down, don’t forget they did blow very strong over the past 24-hours. Wind slabs are likely sitting on weak low density snow and could take a few days to adjust, which makes them likely to trigger today. Wind slabs are a similar beast to the storm slab, but they are thicker due to the wind-loading aspect, 1-2+ feet thick.

DRY SLUFFS (LOOSE SNOW AVALANCHES):
Areas above 2,500′ with drier snow and where a slab has not formed, dry sluffs will be a concern. On Southerly aspects these could become damp and easy to trigger with possible sunshine. Add to that, a sun crust sits under the new snow, which with allow these sluffs to run much further than you may think.

Yesterday’s conditions and how to do a quick hand pit for slabs that are relatively shallow: 

 

For the snow and weather geeks out there, below is a little annotated chart from the past few days showing how an upside-down storm creats ‘storm slabs’:

 

 

Avalanche Problem 2
  • Wet Loose
    Wet Loose
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.
More info at Avalanche.org

WET SLUFFS:
At elevations below 2,500′ watch for wet loose avalanche activity. The new snow at these lower and mid-elevations is damp to wet and easily pushed into a sluff. The crust sitting under the 6-10″ of wet snow is letting these sluffs run quite far and on mellow slopes as mentioned above. If the sun comes out today, it will act as a trigger we could see many wet loose snow avalanches occur.

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

With two days now of very limited visibility, it is hard to know if there has been recent glide activity or not. Nonetheless, continuing to avoid being under glide cracks, in the case they release and avalanche, is key! It’s just not worth it on slopes where the snowpack is falling apart and oozing down the mountain. Take a minute to work with your partners to map out glide cracks in the area you are recreating. 

Weather
Mon, March 21st, 2016

Yesterday’s up-side down storm system is winding down and coming to a close. During the past 24-hours we have seen around a inch of rain below 1,200′ with 6-8″ of wet snow above this. Some areas saw rain up to 1,700′. Ridgetop winds associated with the snowfall were strong, hourly averages as high as 40mph from the NE at the Sunburst weather station. Temperatures were warm, mid 30’s below 2,000′ and up to 30F at 2,500′.  

For today, we should see a break in precipitation with partly cloudy skies. There is a chance the sun could poke out here and there. Ridgetop winds are forecast to remain light to moderate, blowing in the 10-20mph range from the East. Temperatures continue to be warm with parking lots seeing upwards of 40F and up to the mid 30’s at 3,000′.  

Tomorrow, Tuesday, another weak disturbance will move through with increased winds. This is ahead of a stronger system (but hopefully a cooler one) slated for Wednesday. It seems spring is technically here but the long sunny days have yet to arrive.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 34   trace/rain   1.0   136  
Summit Lake (1400′) 35   rain   0.6   47  
Alyeska Mid (1700′) 34   trace/rain   1.1   112  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 27    NE 23   55  
Seattle Ridge (2400′) 29   SE   20   48  
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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.