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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, March 13th, 2014 - 7:00AM
Expires
Fri, March 14th, 2014 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

It’s another storm day!  8 inches has fallen overnight at Turnagain Pass and we could get another 20-30 inches in the next 24 hours.  

The avalanche danger will start this morning at  CONSIDERABLE  and rise to  HIGH as the day progresses and snow builds.  

Yesterday we saw very little natural avalanche evidence from our Monday storm, which dropped 2 feet at the Pass, but collapsing of the snowpack was frequent when breaking trail.  All the new snow from this week is poorly bonded to the February crust layer.  With a lot more snow forecasted today, avalanches could be breaking 4 feet deep to the crust, making them unmanageable and dangerous.  

Safe travel today will require staying away from all avalanche terrain, including runout zones.

Special Announcements

Turnagain Pass is  again  OPEN  to snowmachines.  The recent storms have provided enough snow cover to prevent resource damage.

Thu, March 13th, 2014
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.
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

There are 2 interfaces in today’s snowpack that may act as sliding layers.  They can be separated out by the 2 storms we’ve seen this week.

Storm snow that started falling last night may slide at the old surface layer we were skiing on yesterday.  Yesterday we had a sunny break in the weather that built sun crusts on southerly aspects.  With new snow building, this could become a sliding interface.  

This morning the new storm snow is less than a foot deep, but is expected to reach closer to 2 feet by the end of the day.

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
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.
More info at Avalanche.org

Monday’s storm fell on the old February crust, which has some buried surface hoar acting as a weak layer above the crust.  Yesterday we experienced frequent collapsing (whumphing) on this layer at elevations between 1000 and 2500 feet.   Some avalanche activity on Tuesday was reported up to 2 feet deep from this storm. 

A worst case scenario today would be avalanches that slide 3-4+ feet deep at the crust interface.  This would include snow from both storms this week.  

Weather
Thu, March 13th, 2014

The storm that ended Tuesday dropped about 2 feet of snow in Girdwood and Turnagain Pass.  That snow transitioned to rain at lower elevations at the end of the storm.  

This morning, Center ridge weather station has already recorded about 9 inches of snow and up to 0.8 inches of water equivalent.  Sunburst station saw wind gusts to 105mph overnight from the East.  

Snow/rain line is predicted to stay at sea level today, but temperatures are right on the threshold.  Some rain is possible at sea level.  Snow above 1000 feet is expected to reach 11-16 inches today and another 9-14 inches tonight.  An east wind will stay strong while this storm continues.  

Observations
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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.