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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, March 24th, 2013 - 7:00AM
Expires
Mon, March 25th, 2013 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

A couple reports of small to medium size skier triggered wind slabs will keep the danger rating MODERATE today.   Recent new snow is sitting on top of facets and sun crust, making for a poorly bonded interface.   With more snow in the forecast today and tonight we can expect the problem to stay the same or get worse today.  

So far all the avalanches reported to us have been relatively small.   The distribution is on specific wind loaded aspects and terrain features.   As storm snow accumulates we can expect the freqency and distribution to become more widespread.

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Sun, March 24th, 2013
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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
  • Wind Slabs
    Wind Slabs
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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

Since the recent wind event on Tuesday night we have gotten a number of people reporting small human triggered avalanches, including instances where people have been taken for a ride.  The trend continues as more snow gets deposited on the poor bonding surface of sun crusts and facets.  The photo below shows a pocket on Magnum yesterday where a skier easily popped this slab loose.  Another similar small avalanche was reported on the SW face of Eddies. 

The concept to keep in mind today is to stay in terrain where you can safely manage an avalanche of this size.  We are dealing with a completely different problem today that we were last weekend and terrain choices must be adjusted accordingly.  Small avalanches are likely, but they can be manageable in low consequence terrain.  Areas with cliffs below or large steep faces should be avoided.

skier triggered avalanche on lower Magnum.t

Avalanche Problem 2
  • 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

Snow is currently falling and by tomorrow morning it should be enough to blanket the mountains with a refresh on the surface.  It looks like low intensity during the daylight hours today, with an increase coming tonight. 

The storm snow by itself could peel off in steeper terrain on the new/old snow interface.  Expect some kind of unstable character when the depth reaches a critical mass.  We know there is already a tendency for loose snow to sluff in moderate volume and new snow will add to that volume.  Given the right combination of wind and temperature the storm snow could break with a slab character, making it a bigger problem than just loose snow alone. 

Weather
Sun, March 24th, 2013

By yesterday the new snow was ~5 inches with a little more in some areas and pockets of wind blown deposits reaching around a foot deep.   Temperatures are steady in the 20s and wind has dropped from the recent peak on Friday morning.  

Today we have 3-6 inches of snow in the forecast with light to moderate wind from the east.   Precipitation should stay as snow all the way to sea level.   Tonight the snowfall should increase slightly with another 4-7 inches expected.  

Snow remains the dominant weather type going into the work week, with a sunny break possible on Tuesday.  

Fitz will issue the next advisory tomorrow morning, March 25th.

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.
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